MV/pressure variation

[TonyH]

I wonder if any of our more clued-up members have thoughts on this. It seems to me that a 3x-fired case, neck-sized only, ought if anything to produce fractionally lower internal pressure (= lower MV) compared with otherwise identical cases that have been fired only twice. I've just been trying to shoot bunnies, until the light faded so badly I couldn't distingusih them any longer. I missed a few, at ranges between 320 and 380 yards, and I'm positive my shots went high. Yes, it was slightly downhill, but although I clicked up my scope according to settings that to date have seemed reliable, and aimed low as well to compensate for the slope, I was still shooting high... These were 3x fired Lapua cases with the same load as 2x-fired cases I used last week which connected with rabbits more consistently. What do you think?

Tony

I did get some bunnies too, out to 315...

[Chris-NZ]

.... and aimed low as well to compensate for the slope...

 

I would be focussing on the comment above Tony.

You've made an assumption of the effect of slope and then further guessed as to where that aiming point should be.

 

I wouldn't read a single thing into the number of reloads

 

Chris-NZ

[TonyH]

I would be focussing on the comment above Tony.

You've made an assumption of the effect of slope and then further guessed as to where that aiming point should be.

 

I wouldn't read a single thing into the number of reloads

 

Chris-NZ

 

Chris, that's a very common-sense opinion and I dare say you're right. But you know how it is - missing is very frustrating when you're taking care to do everything just right! It wasn't much of a slope at all, just a slight variation in height across 300+ yards, and by dialling my scope for distance and aiming low as well I should have hit the buggers - but on three successive shots I went high. My ammo is loaded round by round, powder trickled in by the grain for each case, and I am very fussy about priming and bullet seating. I started the session with an easy fouling shot on a rabbit at 120 yards across level ground, dialled down the theoretical 8 clicks from my zero and hit it exactly on the crosshairs point - then my next was on the (slightly) sloping place at 380, dialled in 20 clicks up from zero, shot went just over...

It's possible my click-chart needs subtle refining: right now I'm suspecting it's on for ranges less than my zero but that the trajectory is slightly flatter than Exbal suggests for my 20Tac load (already very flat) so I need to tune for 300 yards plus.

Thanks for your input. I'll try not to be so nerdy about minute differences in case capacity...

Tony

[duey]

Hi Tony

could also be a possibility that the higher ambient temperatures are also effecting MV which will have an obvious elevation effect, coupled with slope dope changes, could reflect in your misses

just something else to throw in the mix

 

Ian

[1967spud]

you need to take clicks off for both uphill and down hill shots Tony

 

do you think you just added instead of taking off

[1967spud]

beinng nerdy about case capacity in my eyes is a valid variable

 

think about it as you say 3 x shot cases internally would have less carbon lining the walls compared to a 4 x shot cases

 

thus leading on from that smaller space inside should increase pressure resulting in elevation differances

 

 

thats another good reason to ultrasonic clean

[TonyH]

Hi Tony

could also be a possibility that the higher ambient temperatures are also effecting MV which will have an obvious elevation effect, coupled with slope dope changes, could reflect in your misses

just something else to throw in the mix

 

Ian

Thanks Ian - I'd have thought a considerable temp variation would be required to have a significant effect on the sort of shots I was making, though it's a valid point. And I don't know where you live, but last night it was distinctly cool here! Certainly cooler than when I last went out a week previously.

I emphasise, there wasn't much of a slope at all, I was shooting downhill but (at a guess) no more than 3-4 degrees at most.

spud:

No, I'm certain I didn't get mixed up with my clicks! I sometimes feel my brain is softening but I'm very careful with elevation clicks. And yes, I do use an ultrasonic cleaner, plus a tumbler with corncob media plus a bit of chrome polish... When I inspect my cases the insides are shiny.

I probably just haven't finished tarting up my clickchart for this load. But thanks for the suggestions, guys.

Tony

[duey]

Hi tony

your probably correct as it would take about a 15degree change to accomplish probably no more than a 1 click change in elevation but perhaps a combination of small factors effected the shots maybe even an unseen vertical wind!!!

 

having been shooting on a field range in scotland where all the figures known and observations made by several of us should have resulted in hits but didnt!! turned out an unseen vertical wind caused by the lie of the land was lifting shots by upto a minuet at 500yds

 

probably never know

 

good luck at getting your real world data transcriped into your charts

 

Ian

[eldon]

Tony, may I suggest you would have been better off just popping up a paper target and shooting a 3 shot group in the same circumstances actually at the time. Obviously if the light was going then next day or whenever. This way any mysteries would have shown up straight away such as an unseen wind and the correct correction could be applied for the angle and this then dismissed as the cause.

 

Maybe something to think about next time.

 

I personally don't think the x2 or x3 fired case would make that much difference.

[taylor]

if the shoulders need bumped back on a case then the shot always goes high that why i full length size my brass ,try stripping bolt down then insert fired case if thiers any resistance then theirs a increase in chamber pressure vs sized brass ,could be up to .5moa at 1 k

also wind direction gives vertical its more likley the root cause.

what direction was the wind coming from ,was it bright or overcast ,angle of shot

all play a part in elevation errors

[TonyH]

....perhaps a combination of small factors effected the shots maybe even an unseen vertical wind!!!

 

having been shooting on a field range in scotland where all the figures known and observations made by several of us should have resulted in hits but didnt!! turned out an unseen vertical wind caused by the lie of the land was lifting shots by upto a minuet at 500yds....

Ian

 

Ian, thanks - the concept of "vertical wind" is not something I'd thought consciously about, ever! It could indeed have been a factor - but there was very little wind, so slight I didn't aim off or correct my scope for windage, didn't measure it but it really was negligible. However, when shooting in significant wind I shall certainly bear this in mind, especially since I tend to shoot on very uneven ground, little hills and steep little coombes.

 

Tony, may I suggest you would have been better off just popping up a paper target and shooting a 3 shot group in the same circumstances actually at the time....

Good idea Eldon, and I certainly need more field target time to help resolve this. It would not have been practicable at the time since dusk was close, I hadn't a target with me, to get to the right spot would have meant a half-mile round trip uphill and down, etc!

 

if the shoulders need bumped back on a case then the shot always goes high that why i full length size my brass ,try stripping bolt down then insert fired case if thiers any resistance then theirs a increase in chamber pressure vs sized brass ,could be up to .5moa at 1 k

also wind direction gives vertical its more likley the root cause.

what direction was the wind coming from ,was it bright or overcast ,angle of shot

all play a part in elevation errors

Again, very little wind so probably not a factor. I keep a close watch on my cases and these 20Tacs after 2-3 firings still don't require body-bumping though I think they will very soon. Not sure what you mean by "stripping bolt down" - ? Not sure why "bright or overcast" might make a difference, care to elaborate? It was approacvhing dusk but still enough light to place my crosshairs precisely through the Bushnell Elite 6-24x40, just - at high power this is not an especially bright scope even though it performs very well.

 

Thanks for your input chaps, all helps a lot.

Regards, Tony

[taylor]

you need to strip your bolt spring or you will only feel the firing pin spring cocking and wont feel cases touch the shoulder headspace

also light has a huge impact with no wind its called mirage a displacement of the target and can play havoc on your bullet placment especialy when their is no wind , and you could be even be zerod on a higher impact ,the day you zero make sure its overcast ,not bright and no wind or you could be off zero wich is giving you a higher impact, theirs a ton of things that could of caused high shot

[taylor]

i would re-chack your zero and velocity again to confirm trajectory, work out angle of shot and recheck zero at that disance on a paper target were the rabbits were

[TonyH]

you need to strip your bolt spring or you will only feel the firing pin spring cocking and wont feel cases touch the shoulder headspace

also light has a huge impact with no wind its called mirage a displacement of the target and can play havoc on your bullet placment especialy when their is no wind , and you could be even be zerod on a higher impact ,the day you zero make sure its overcast ,not bright and no wind or you could be off zero wich is giving you a higher impact, theirs a ton of things that could of caused high shot

 

Thanks. I'm confident of my zero and my MV: I only settled on this load after extensive load-development at the range. As I understand it, mirage (which I know about) tends to be a problem only in conditions of unusual brightness & warmth, which rarely applies in S.Devon... I've checked my zero in moderately bright/sunny and in overcast conditions too. I mentioned the estimated angle of the shots where I was missing, and it's not really enough to make a difference; I certainly will do some more field target tests. Stripping my bolt in the way you describe seems a bit extreme! I keep a careful check on my cases and measure their expansion: in any case, i've found that even a small case like the .223/20Tac will show excess expansion very clearly with increased bolt-lift pressure, at which point I bump the shoulder back one or two thou.

Thanks for your input.

Tony