Making accurate drop chart - who to trust?

[terryh]

dRb

 

Not necessarily 'Tenex' but a reasonable grade of target 22RF ammunition that your rifle likes i.e. give good repeatable small groups at say 50 yards.

 

If you are serious then also doing a simple batch by rim thickness of the ammo that you do find works helps at longer ranges.

 

If you want to see what people are achieving with the humble 22RF then have a read thru' this: http://forum.snipershide.com/snipers-hide-rimfire-section/207953-snipershide-rimfire-6-groups-5-30-rounds-challenge-thread.html

 

T

[gbal]

HI,this well intentioned exercise is becoming quite a saga-hopefully a learning curve,too.

 

I thing Oaken's advice is the way to go-you really can only base field shooting on field data-see where your bullets are impacting at the distances you wish to shoot-say,a 50/65 zero,and check targets at 10 yard out to say 100-getting a long shot for 22rf,except for a very experienced shooter.

Then you know where your bullet will strike at each of these distances,and can work out the clicks-check them.

While most 22 ammo is at least adequate to 50yards,rifles typically show preferences for accuracy,and beyond 50yards accuracy is at a premium-use quality ammo,that groups well.Your primary objective was practice,though you have to consider ethics of live target shooting.Accuracy matters,a central hit with match bullet will in general be more effective than a misplaced 'expanding' one-whether it expands or not.

Depending on your scope reticule,you might have markings that fit particular distances,otherwise a laser ranger is the way to go-there are very few scenarios where you can do better with any method.

 

OK-originalbrief-to get some experience that will transfer to 308 tactical shooting.The basic principles of transfer of skill are well established-the more similar the basics the better,and vice versa.For wind,all you will learn with a different cartridge is that wind has an effect-obviously the effects will differ very substantially,and at any distance.

For drop,ditto.

any actual drop/drift measures will just not aply,fairly obviously. If you want to get experience in ranging distant targets,as in tactical,then do that with the targets at distance-no need to shoot.Range shooting is easy-distances are known,and sighters get you on exactly-but this data is a great check on your field findings.

There was a fairly inconsequential discussion about practice with 'lesser' cartridges-generally,it will not help,as almost all the relevant parameters will be different (except for the least experienced of beginners)-indeed negative transfer is more likely-such practice will interfere,rather than help,shooting with a more powerful cartridge( again,complete novice excepted).

 

By all means shoot 22rf,and enjoy it. Study the trajectory drop/drift of your larger cartridge rifle,get that sorted on the scope/clicks. If variable ranges are to be common,a laser rangefinder may be an investment.(first focal plane scope works for some-but only if you know target size?).

As you are finding,there are a whole lot of things in reality/physics that affect bullet trajectory,and most inputs to programs are incomplete/inaccurate for the actual conditions of shooting-nor are all programs necessarily optimal-G1/G7 is just the well known BC issue.There are others.(see how eg Berger bullet design-hybrids- have improved long range performance-what G are they,and yes ,there are G6,G5 etc!)

Back to where we came in-holes on paper are the truth,and there is no substitute for using that data directly;have fun trying to replicate your program to reality,and a good fit is very useful in getting you 'on paper' if you will,but any deviation from that reality means misses."Shoot and see"! Good luck.

Gbal

[dRb]

HI,this well intentioned exercise is becoming quite a saga-hopefully a learning curve,too.

 

I thing Oaken's advice is the way to go-you really can only base field shooting on field data-see where your bullets are impacting at the distances you wish to shoot-say,a 50/65 zero,and check targets at 10 yard out to say 100-getting a long shot for 22rf,except for a very experienced shooter.

Then you know where your bullet will strike at each of these distances,and can work out the clicks-check them.

While most 22 ammo is at least adequate to 50yards,rifles typically show preferences for accuracy,and beyond 50yards accuracy is at a premium-use quality ammo,that groups well.Your primary objective was practice,though you have to consider ethics of live target shooting.Accuracy matters,a central hit with match bullet will in general be more effective than a misplaced 'expanding' one-whether it expands or not.

Depending on your scope reticule,you might have markings that fit particular distances,otherwise a laser ranger is the way to go-there are very few scenarios where you can do better with any method.

 

OK-originalbrief-to get some experience that will transfer to 308 tactical shooting.The basic principles of transfer of skill are well established-the more similar the basics the better,and vice versa.For wind,all you will learn with a different cartridge is that wind has an effect-obviously the effects will differ very substantially,and at any distance.

For drop,ditto.

any actual drop/drift measures will just not aply,fairly obviously. If you want to get experience in ranging distant targets,as in tactical,then do that with the targets at distance-no need to shoot.Range shooting is easy-distances are known,and sighters get you on exactly-but this data is a great check on your field findings.

There was a fairly inconsequential discussion about practice with 'lesser' cartridges-generally,it will not help,as almost all the relevant parameters will be different (except for the least experienced of beginners)-indeed negative transfer is more likely-such practice will interfere,rather than help,shooting with a more powerful cartridge( again,complete novice excepted).

 

By all means shoot 22rf,and enjoy it. Study the trajectory drop/drift of your larger cartridge rifle,get that sorted on the scope/clicks. If variable ranges are to be common,a laser rangefinder may be an investment.(first focal plane scope works for some-but only if you know target size?).

As you are finding,there are a whole lot of things in reality/physics that affect bullet trajectory,and most inputs to programs are incomplete/inaccurate for the actual conditions of shooting-nor are all programs necessarily optimal-G1/G7 is just the well known BC issue.There are others.(see how eg Berger bullet design-hybrids- have improved long range performance-what G are they,and yes ,there are G6,G5 etc!)

Back to where we came in-holes on paper are the truth,and there is no substitute for using that data directly;have fun trying to replicate your program to reality,and a good fit is very useful in getting you 'on paper' if you will,but any deviation from that reality means misses."Shoot and see"! Good luck.

Gbal

 

100% see the limitations in transferring direct experience from .22 to .308; however I can shoot hundreds of rounds of subsonic .22 a day where I work, but could probably only get away with 5-10rds of suppressed .308 before it became a noise issue!

 

My questions re target ammo and vermin was more to do with shooting over normal .22 vermin ranges (typically up to 80m where I shoot as a max), once I had the scope setup and the ammo in stock - ie can I use it whilst my setup is optimised for the long range practice/plinking, or do I need to re-zero and go back to HP's.

 

I'd bet that the person who pulls a trigger hundreds of times a week on 'tactical' type target shooting will still shoot their .308 better than someone who only fires their .308 once every couple of weeks on a range when it comes to events like Tiff's.

 

Finally...the issues surrounding the quarry got me thinking that a series of subsonic moderated tactical comps using .22's could be easier to get off the ground, hopefully leading to future options for full bore shooting on the same land once the owners new more about the niceness and sensibleness of the people shooting?

[dRb]

I'd like to say too that I'm not ignoring Rup's excellent advice re shooting and clicking to make a chart of drops at ranges, but I am after a simulation so that I can then look at ranges I've not tested and have a good (better) chance of scoring first round hits.

 

The other thing is that ballistics are not impossible to calculate, so there is a part of me that is not happy to accept that it won't fit the model given the right inputs!

 

If it is annoying for me to log this all here I will stop by the way - my hope is that either others will be able to help, or that this will be able to help others.

[gbal]

I'd like to say too that I'm not ignoring Rup's excellent advice re shooting and clicking to make a chart of drops at ranges, but I am after a simulation so that I can then look at ranges I've not tested and have a good (better) chance of scoring first round hits.

 

The other thing is that ballistics are not impossible to calculate, so there is a part of me that is not happy to accept that it won't fit the model given the right inputs!

 

If it is annoying for me to log this all here I will stop by the way - my hope is that either others will be able to help, or that this will be able to help others.

Hi-I didn't mean to give the impression that what you were doing wasn't interesting,but more that -for 22rf ranges and typical performance-it looses precision fairly rapidly at distance-the shoot and see is maybe better-and if your data/program output continues to be out of synch,I'd go with the holes on paper.

It's pretty easy to ignore posts,and my intention was to give helpful advice-but also advise that simulations can cause mischief-because of input being mismatched to the programs assumptions.(cr** in/out,as it is popularly referred to-though that misses the inbuilt assumptions).

Ditto comments an bullet choice,and especially 'Practice/transfer"....practice yes,though as close to real use as possible.

Don't stop,we all learn/have learned,sometimes the hard way.It's likely that you will have not dissimilar experiences with a full bore

cartridge,and It's a given that few have the chance to practice at multiple targets at varying long ranges,at their convenience,so a drop chart from real data is very valuable-and a good simulation from a ballistics program is a good aid-though of course has nothing to say about eg bullet dispersion (precision-see Bryan Litz model that shows a 1/2 moa rifle in 243 has a high probability 'hit zone' of 6 inches at 250 yards,depending on some real world variables(mostly shooter's wind misjudgement...but also eg small fps variations in ammo,etc etc)-one reason 3 shot 'groups' are less reliable than 5 shot ones-better yet 10 shot ones-there will be 'errant' shots-when all the parameters go against the shooter,and these cumulatively increase miss probability! However,that's for the future-we want your shots POI to be where you expect then to be,at least within the cartridges normal envelop.

If you have a good ,reliable chronoscope,use its data-though it would be odd indeed if its muzzle readings were much out from published data,if barrel is similar length.Program has to fit the muzzle velocity ,and actual shot drop values to the ranges you have these-thereafter it's extrapolation and may not fit actual shooting results,when you get them....(let alone the 1/2 moa rifle that is sometimes shooting some shots way out of 'expected' group).

Books have been written...or at least many many bits of advice offered-as on here-we are all on the side of good performance,and reducing the need to reinvent the (crooked) wheel! Keep the faith!

Gbal

[The_Dogge]

Attached is a chart showing the first 80m of flight (slowing down and falling) as measured today - the Series 1 line is my results, the Series 2 line is the absolute best fit I could get by basically messing with the MV, scope height, and fractional changes in the POI at the zero range (15m).

 

As you can see it fits 5/8 points on the curve, but my result clearly fall short in the 40m, 50m, 60m areas before 'catching up' on 70m and 80m.

 

My worry here is that the continuation of these lines will show that the line I've forced to fit from the calculator will drop away.

 

I've had to take my measured MV of 1016fps (Chrony F3, new battery, 20 shot string) down to 965fps on the JBM settings!

 

It is frustrating, even worse, you go out on a different day, different temperature, and it is all different again!

 

One thing, there are no error bars on your graph, your measurements of group mean POI will have an error associated, and they will never all fall exactly on the curve, I'd use the standard deviation of the height data for the group, though this is probably a minimum error.

Also, small errors in the zero at a really close range like 15m will translate into big errors downrange, I'd zero at 50m at least, maybe 100 if you want to shoot really long range with it, reduce the distance you are extrapolating the result from.

 

The way I'd do it from experimental data only would be put it in excel, like you have, add a trendline, the data should be cubic, but it is close enough to a square curve to fit well, then use the eqution of that curve to extrapolate to ranges you didnt shoot. don't extrapolate far beyond the furthest you shot as that wont work exactly.

Of course this doesnt allow you to compensate for conditions like a software solution would, the drag model shouldn't be a problem at subsonic speeds, but you may have to adjust the BC "a fair bit", as the diffetn drag curves dont agree very well on what the subsonic drag coeficient is. I would be wary of solutions that have an "unrealistic" velocity as the input, but there will of course be an error associated with the V measurement as well, it doesn't have that much effect though as you have accounted for it when zeroing, have a play with chairgun or whatever to do a sensetivity analysis on it.

 

Good luck, don't go nuts! :-)

[dRb]

Some of the groups on that snioercentral link are mental!

[tackb]

you could always do what I did before software? 8x4 sheet of plywood on end with a target at the top then dress back in 10m intervals from 50m to as far as required shooting groups as you go then dress forward and mpi each group and measure your drops , you will also get a feel for how your groups get worse as the distance increases, I got to a shade over 200m before I ran out of drop space on the board.

[dRb]

 

It is frustrating, even worse, you go out on a different day, different temperature, and it is all different again!

 

One thing, there are no error bars on your graph, your measurements of group mean POI will have an error associated, and they will never all fall exactly on the curve, I'd use the standard deviation of the height data for the group, though this is probably a minimum error.

Also, small errors in the zero at a really close range like 15m will translate into big errors downrange, I'd zero at 50m at least, maybe 100 if you want to shoot really long range with it, reduce the distance you are extrapolating the result from.

 

The way I'd do it from experimental data only would be put it in excel, like you have, add a trendline, the data should be cubic, but it is close enough to a square curve to fit well, then use the eqution of that curve to extrapolate to ranges you didnt shoot. don't extrapolate far beyond the furthest you shot as that wont work exactly.

Of course this doesnt allow you to compensate for conditions like a software solution would, the drag model shouldn't be a problem at subsonic speeds, but you may have to adjust the BC "a fair bit", as the diffetn drag curves dont agree very well on what the subsonic drag coeficient is. I would be wary of solutions that have an "unrealistic" velocity as the input, but there will of course be an error associated with the V measurement as well, it doesn't have that much effect though as you have accounted for it when zeroing, have a play with chairgun or whatever to do a sensetivity analysis on it.

 

Good luck, don't go nuts! :-)

Got to go a little nuts to get the required drive and determination to beat the problems

 

You're right re errors. As a quick and very dirty solution I dropped the top and bottom of the 5 shots and used the mean of the middle three. Poor science, but it was applied evenly!

 

My hope is that I can get a model for ranges well beyond my tested ranges, hence trying to get JBM to fit - it seems to work well for my .308, and Rex (youtube long rangist) seems to male it work too.

 

I'm going to shoot my 10m to 80m again with target ammo, 5 round groups, this time also clicking a solution on the paper for each range too for validation of software dopes/scope accuracy testing.

 

Do I bother with a chrony? I was taking it as a pretty accurate data set, but I seem to have to fiddle it to make models fit.

[dRb]

you could always do what I did before software? 8x4 sheet of plywood on end with a target at the top then dress back in 10m intervals from 50m to as far as required shooting groups as you go then dress forward and mpi each group and measure your drops , you will also get a feel for how your groups get worse as the distance increases, I got to a shade over 200m before I ran out of drop space on the board.

This is the obvious thing to do (in some format or other), however the thing I am after is being able to calculate any range as closely as possible.

 

For example; should I happen come across a wild 8"x8" steel plate here on the farm, I want to be able to range it, look up a firing solution, and then stand as good a chance as possible of putting the first round on the target.

 

I 100% need to shoot known ranges and measure the drop I find in the field, but then I want to use that to validate some form of predicted trajectory to allow me to pick points at ranges I've not tested with as much reliability as possible.

[dRb]

dRb

 

Not necessarily 'Tenex' but a reasonable grade of target 22RF ammunition that your rifle likes i.e. give good repeatable small groups at say 50 yards.

 

If you are serious then also doing a simple batch by rim thickness of the ammo that you do find works helps at longer ranges.

 

If you want to see what people are achieving with the humble 22RF then have a read thru' this: http://forum.snipershide.com/snipers-hide-rimfire-section/207953-snipershide-rimfire-6-groups-5-30-rounds-challenge-thread.html

 

T

Bought 500 Eley Club to play with

[The_Dogge]

Do I bother with a chrony? I was taking it as a pretty accurate data set, but I seem to have to fiddle it to make models fit.

 

I see your point, if you're going to ignore the data anyway.... I would want to know the ballpark figure to check I wasn't using an unrealistic value as an input, but you already have that data.

 

I just dug out some old data I had (I haven't shot unknown range with a .22 for a while, HMR made my .22s mostly a range only thing!) and I found it didnt take much massaging to get Strelok on my phone to match the measured values to 100m well within margins of error. BC=0.15, V0=305m/s which is close enough to what they should be.

 

I find that although real world results should take precedence over calculated values, when the real world results disagree they often turn out not to be repeatable, just the result of a bad day at the range for whatever reason and consequently some bad measurements.

[gbal]

I see your point, if you're going to ignore the data anyway.... I would want to know the ballpark figure to check I wasn't using an unrealistic value as an input, but you already have that data.

 

I just dug out some old data I had (I haven't shot unknown range with a .22 for a while, HMR made my .22s mostly a range only thing!) and I found it didnt take much massaging to get Strelok on my phone to match the measured values to 100m well within margins of error. BC=0.15, V0=305m/s which is close enough to what they should be.

 

I find that although real world results should take precedence over calculated values, when the real world results disagree they often ' not to be repeatable, just the result of a bad day at the range for whatever reason and consequently some bad measurements.

Good point about the human source of variability on the ,rubbish in / out issue.Conditions too can vary on different days.The basic issue is having a representative sample to work with,just like more than two shots on a group!

Something like ten shot groups at zero/50 yards,and ten shot groups at ten to fifteen yard intervals to around 100 yards,on each of at least three separate days.The same beyond 100 yards if that is where you want to shoot.That is less than 200 rounds for the hundred yard data-if that is too expensive then you might as well quit.

That data plus muzzle velocity and BC should give most Ballistic programs a good chance of useable predictions,based on your actual field performance, unless conditions change radically

A diesel car needs both diesel,not petrol,and enough diesel to take you where you want to go.Same with simulations-the right data and enough of it,to remove any noticeable effects of variations not intended (including minor human error).

good shooting

Gbal

[dRb]

Ok, so a change of tactics...

 

As mentioned above, I bought a load of Eley Club ammo, so have cleaned the bore on the rifle (for the first time in a LONG time), and then set off to the intermediate range area on the farm to play.

 

Zeroed at 60m (measured on flat ground using a long tape), and after about 80 shots the groups started to tighten up to be a bit better than I was getting from the HP's at the same range yesterday. Annoyingly I was still finding occasional fliers, or getting odd vertical strings (?).

 

As I have Chairgun on my phone thought I'd give it another go and plumbed in the standard numbers, including the measured MV of the HP's from yesterday - this may seem an odd choice, but they advertise them being the same sort of speed, and by using the data form yesterday I had a scaled effect on my gun from advertised performance. Ish.

 

Moved out to 80m and dialed the clicks as per Chairgun....missed high.

 

Cursed stupid scope.

 

Went back to zero and held over by the recommended mils....spot on!

 

Mils worked for 50m too, and 30m.

 

At 50m I checked for ranging too, and the mils came up as a tiny bit short of 4 mil for the width of the A4 paper, which is a tad under 200mm, and 200mm @ 4 mil would be 50m, so I was happy with that.

 

So I now wrapped my self up in waterproofs and went off to lie in the long (wet) grass hunting steel. I didn't have to look too hard as I knew where I'd just put the steel, but sometimes it is fun to pretend!

 

The steel I'm shooting at is a 140mm round disc, which came up as almost exactly 1 mil from the point I was firing from. 140mm/1mil = 140m. Check chairgun and dial in the drop of 118 1/8th"@100yrds clicks; missed. Tried dialing 14.2MOA (1/8th MOA rather than 1/8"); missed. Tried reading the clicks as 1/4"; missed. Held over the 4.1 mils; hit pretty much bang centre.

 

Cursed stupid scope.

 

Walked it in on the turret and found the actual solution to be 88 clicks.

 

Cursed stupid scope.

 

Walked back a bit and re-ranged to 150m, looked up the drop in mils (4.7), put the rather handy 4.75mil hash on the steel; hit pretty much bang centre!

 

So basically I can hold over on my target, but not dial in.

 

Since the turrets are angular, and a mil is angular, is it linear to say that since 88clicks hits the same place as 4.1 mils, one click is 4.1/88 mils?

[dRb]

ok...

 

So if 1mil = 3.6" @ 100yrds (pretty much), 4.1mil = 14.76".

 

14.76"/88 clicks = 0.16" per click @ 100yrds.

 

Not 1/8" being 0.125" per click.

 

This needs further investigation...

[gbal]

What are the clicks on the scope 'supposed' to be....one click to 1/8 inch at 100y,or one click to !/4 at 100y,or metric ( i click is 1cm at 1100) etc..I don't think you ever said.....or it's got lost in all the 'linear' arithmetic.

Published speeds for 22 rf is usually fairly ok (ish!).

Better quality target ammo isn't hugely better than the next grade down for field use-it's about one point out of the hundred in old fashioned 25 yard target competitions-that of course matters for such competition- eg in a good batch Tenex shoots about 99.9 over many thousands,so there will be the very occasional 9,not 10 scored.Club grade-or whatever it is now,will be more like 97/8-not something field shooting a box or two will readily show,given the considerable premium price for Tenex.(or Midas gold etc)

Are there not pretty good trajectory tables for most bullet/speeds for 22rf- eg : Eley match/Tenex 40g@1085fps,BC .150 :

25y +2"; 50y +3.6"; 75y +2.9"; 100y 0"; 125y -5.4"; 150y -13.3"; 175y -24"; 200y -37.5"

 

Gbal

[dRb]

What are the clicks on the scope 'supposed' to be....one click to 1/8 inch at 100y,or one click to !/4 at 100y,or metric ( i click is 1cm at 1100) etc..I don't think you ever said.....or it's got lost in all the 'linear' arithmetic.

Published speeds for 22 rf is usually fairly ok (ish!).

Better quality target ammo isn't hugely better than the next grade down for field use-it's about one point out of the hundred in old fashioned 25 yard target competitions-that of course matters for such competition- eg in a good batch Tenex shoots about 99.9 over many thousands,so there will be the very occasional 9,not 10 scored.Club grade-or whatever it is now,will be more like 97/8-not something field shooting a box or two will readily show,given the considerable premium price for Tenex.(or Midas gold etc)

Are there not pretty good trajectory tables for most bullet/speeds for 22rf- eg : Eley match/Tenex 40g@1085fps,BC .150 :

25y +2"; 50y +3.6"; 75y +2.9"; 100y 0"; 125y -5.4"; 150y -13.3"; 175y -24"; 200y -37.5"

 

Gbal

Cheers

 

The scope is supposed to be 1/8th inch at 100yrds...

 

I need to re-test, but if it is out I'll be in touch with the manufacturer to see what they think - part of taking it off my .308 was due to wanting matching ret/turrets anyway, so it's sort of on a last warning anyway.

[dRb]

Part of the overall plan is to makea drop chart, so there isn't any resl issue in having a hidden calculation adjusting the clicks - I'll list mil and clicks anyway as I find holding over fairly instinctive anyway, and it saves getting lost in the turret turns! :/

[Swamp Donkey]

Do a box test.

[dRb]

Do a box test.

Yep.

 

Tomorrow, and at exactly 50 yards to measure the change in poi vs clicks too.

[gbal]

What are the clicks on the scope 'supposed' to be....one click to 1/8 inch at 100y,or one click to !/4 at 100y,or metric ( i click is 1cm at 100m) etc..I don't think you ever said.....or it's got lost in all the 'linear' arithmetic.

Published speeds for 22 rf is usually fairly ok (ish!).

Better quality target ammo isn't hugely better than the next grade down for field use-it's about one point out of the hundred in old fashioned 25 yard target competitions-that of course matters for such competition- eg in a good batch Tenex shoots about 99.9 over many thousands,so there will be the very occasional 9,not 10 scored.Club grade-or whatever it is now,will be more like 97/8-not something field shooting a box or two will readily show,given the considerable premium price for Tenex.(or Midas gold etc)

Are there not pretty good trajectory tables for most bullet/speeds for 22rf- eg : Eley match/Tenex 40g@1085fps,BC .150 :

25y +2"; 50y +3.6"; 75y +2.9"; 100y 0"; 125y -5.4"; 150y -13.3"; 175y -24"; 200y -37.5"

Gbal

[oaken]

Try using your 50 yard (or are you working in meters?) as your zero and you will significantly reduce most of the errors that are wildly exaggerated by using a 15yd zero.

[gbal]

+1 on Oaken's recommendation for a realistic zero (say 50y)-I gave figures for a 100y zero,just to indicate that the drop at distance even so ,is very considerable-at one point you mentioned 2-300 yards/metres,and 'instinctive' holdover-well,good luck,but a hold over of more than 3 feet is getting a challenge,goodness knows(well,a good program would tell us) what it is at 300-but I don't think simple holdover would be very feasible.Good fun ,maybe,and it's certainly been done with clicks or mil dots/scope markings.Don't use a petit pois tin can,get a big one! ( subs,of course,are even worse.....and accuracy will be long gone...).Enjoy the curve!

Gbal

[The_Dogge]

Odd fliers and vertical stringing? Yep that will be a .22 then!

 

Also, I suspected from what you wrote but it's only now clear, you don't have a rangefinder. To shoot long range with a .22 you NEED a rangefinder. Ranging optically with the scope is ok, but given the rate of drop, especially for making a drop chart, you need precision, errors in the measured range of the target will throw everything out.

It doesn't need to be a fancy one, for .22 ranges a £100 one will do fine.

 

When I was shooting live stuff with a .22 I set my maximum range as where I could not range precisely enough to guarantee a headshot, even with the laser this was about 80m. When the gradient of the trajectory was more than 5mm/m.

 

At 150m the gradient is more like 15mm/m, so an error on the range of your 140mm steel plate of 5m (3%!) and you miss even with a perfect shot! I seriously doubt many people can measure range optically to 3% in field conditions.

[Swamp Donkey]

Just as Gareth says. Buy a bloody range finder. You know where there's one going cheap....