Nosler BT 40gr 20-cal BC figure?

[TonyH]

I wonder if anyone has done a careful study of Nosler's BC figures and modified any of them from experience. Yesterday evening I took out my 20Tac with Minox ZA5 6-20-x50 and did some scope/click-chart checking: from a known 200-yard zero I clicked up for three rounds at 250 and the group was fractionally low but nice and tight; I went back to a measured 417 yards and fired another 3 rounds, clicking up again accordingly. This group too was pleasingly tight (two rounds under 3/4", the third - which I'd pulled - 1.5" away) but overall 3.5" lower than expected...

Fiddling around in Exbal (the best ballistics programme I've used out of several during 25 years) I tried adjusting the factory BC figure of .239 to get a chart that matched my results, but I'd have had to change it drastically; instead I used the handy "Trajectory Validation" option to derive a fresh drop chart, which gives what it thinks is an appropriately modified MV figure to account for the actual results. So I now have a drop chart that looks as if it will avoid further surprises - but to get there, I'm using a nominal MV figure of 3489 instead of the 3850 my chrono delivers... And I trust my chrono, had it for years.

It wasn't cold when i was shooting, and in any case I haven't found N130 particularly sensitive to temperature changes.

Hence my query about the accuracy of Nosler's BC figure. Ah, just remembered other key data: my click-chart figures for windage proved to match exactly the results obtained, with a 3mph wind blowing from my 5 o'clock... One can go round in circles with this stuff! Your thoughts would be welcome.

Tony

[6.5shooter]

Is the scope height to the centre of the bore entered correct?

[TonyH]

Is the scope height to the centre of the bore entered correct?

 

Good question. Yes, to the best of my ability to measure it (which I did after fitting this scope recently), it is pretty much exactly 1.5" axis to axis. The bell end just clears the barrel.

Regards, Tony

[AnthonyR]

Personally I think that 3489 fps is a far more realistic velocity than 3850.

 

I use a max load of RL10X and get a very simmilar velocity using 39gr BK's.

 

How long is your barrel?

[achosenman]

Tony, I’ve been playing with ballistic programs a long time so like you, have seen the good the bad and the ugly.

 

I learned never use claimed BC’s. I’ve banged on about this before since IMO this is all about SISO. You can’t really expect to use G1 modelling for a G5 bullet and expect to get meaningful data at long range. The G1 BC for the Nosler bullet is .239, the G5 is .147, remembering that this is claimed BC. If you must use a fudge to avoid surprises then ok. I would rather pick the right drag model for the bullet shape and go from there. I fire across the chronograph to get an average MV and then do it at 100m + to get a downrange average. Those two parameters go into the program along with station data etc to generate a true BC from your rifle and generate a dope chart.

 

If your ballistics program will not allow this, your only option is to fudge it.

 

ATB

[TonyH]

Personally I think that 3489 fps is a far more realistic velocity than 3850.

 

I use a max load of RL10X and get a very simmilar velocity using 39gr BK's.

 

How long is your barrel?

 

Well I dunno, Anthony... I've been chrono'ing loads for a long time, and used several models; my CED Millennium has been used to chrono multiple loads developed in several rifles, mainly 22-250 and .223 prior to this 20Tac, and I always got believable results that worked out in the field. The MV figures I've had using various bullets and a couple of powders, during 20Tac load development, match very closely the sort of figures reported by other users of this calibre - and I emphasise that I never go for the highest MV possible, but the optimum combo of good MV with best accuracy. Some of the figures I got during load development were very close to 4000fps...

But I hear what you say, and I'm not ruling out a lower MV than I'd thought - whether in general or just in cool winter conditions. My barrel is 22".

Regards, Tony

[TonyH]

Tony, I've been playing with ballistic programs a long time so like you, have seen the good the bad and the ugly.

 

I learned never use claimed BC's. I've banged on about this before since IMO this is all about SISO. You can't really expect to use G1 modelling for a G5 bullet and expect to get meaningful data at long range. The G1 BC for the Nosler bullet is .239, the G5 is .147, remembering that this is claimed BC. If you must use a fudge to avoid surprises then ok. I would rather pick the right drag model for the bullet shape and go from there. I fire across the chronograph to get an average MV and then do it at 100m + to get a downrange average. Those two parameters go into the program along with station data etc to generate a true BC from your rifle and generate a dope chart.

 

If your ballistics program will not allow this, your only option is to fudge it.

 

ATB

 

Indeed, I know manufacturers' claimed BC figures can be unreliable - there's an article in the latest Varmint Hunter magazine giving the results of yet another test and Nosler don't come out of it very well for BC reliability... In the past I've looked at ballistics theory in a superficial way (most of it is beyond my understanding) and decided I really didn't want to get into different BC models/formulae. I know what you mean about getting the BC right, and in an ideal world I would do what you suggest: I recall once I did book the range on my own and stuck a chrono 100 yards away, but the results didn't help me enormously. I agree that technically speaking it's a "fudge" to fiddle the MV figure when it is quite possibly a mischosen BC instead, but to be honest I'll just use any form of calculating a click chart that gives me reliable real-world figures in the field! I just checked, and Exbal doesn't appear to have BC model options beyond G1 - where did you find that G5 figure for the NBT 40gr 20...?

regards, Tony

[achosenman]

Tony if you give me your station data and scope click resolution zero range etc I’ll run it through my program. Let’s see if we can’t find a similar drop to what you see using the correct MV etc.

 

ATB

 

Edit to answer.

Tony I use something called RSI shooting lab. It is far more than just a ballistics program. Don’t expect to just install and start using though.

 

In answer to your question, I put the claimed BC in and let it convert it to a G5 equivalent. It’s not the best way but it will do at a pinch.

 

ATB

[AnthonyR]

I fully understand that Quickload is not the be all and end all and the values can be taken with a pinch of salt, but I have entered your load data into it and the results were interesting.

 

You would have to use 25.8 gr of N130 to reach 3850 fps out of a 22" barrel, this gives the pressure as over 88,000 psi with the listed maximum for that calibre as 55,000 psi. You might well be using that amount of powder but it seems a bit OTT to me. Todd Kindlers recomended loads for the 20Tac are notoriously OTT.

[TonyH]

Tony if you give me your station data and scope click resolution zero range etc I'll run it through my program. Let's see if we can't find a similar drop to what you see using the correct MV etc.

 

ATB

 

Edit to answer.

Tony I use something called RSI shooting lab. It is far more than just a ballistics program. Don't expect to just install and start using though.

 

In answer to your question, I put the claimed BC in and let it convert it to a G5 equivalent. It's not the best way but it will do at a pinch.

 

ATB

Thanks for the generous offer, which I accept gratefully. Station data? Presumably you mean altitude est 500' above sea level, temperature 10 degrees C, relative humidity est. 90%, wind 3mph from my 5 0'clock. Rifle/load data: MV chrono'd at 3850 actual, bullet you know, scope has 1/8"/100 yard clicks, zeroed for 200 yards, scope height above bore 1.5". Shooting was on level ground or very nearly. The group at 250 yards was around 0.5" lower than the 2 clicks up it should have been; the group at 417 yards was 3.5" lower than the 29 clicks calculated previously as correct...

Will this do?

Thanks again, Tony

[TonyH]

I fully understand that Quickload is not the be all and end all and the values can be taken with a pinch of salt, but I have entered your load data into it and the results were interesting.

 

You would have to use 25.8 gr of N130 to reach 3850 fps out of a 22" barrel, this gives the pressure as over 88,000 psi with the listed maximum for that calibre as 55,000 psi. You might well be using that amount of powder but it seems a bit OTT to me. Todd Kindlers recomended loads for the 20Tac are notoriously OTT.

Wow! I've never used data from Todd Kindler. Yes, i've heard his figures are somewhat hot... My actual load is 23.7 grains of N130 plus CCI BR4 primer in once-fired neck-sized Lapua brass, Nosler BT bullet seated about ten thou off. Thanks for your interesting input! I must say, i've seen MV figures from a variety of sources, UK as well as US, which always seemed to make my chrono figures seem entirely believable; but I'm prepared to be proved wrong of course. I never use super-hot rounds! My load does not exhibit any danger signs.

Regards, Tony

[AnthonyR]

Wow! I've never used data from Todd Kindler. Yes, i've heard his figures are somewhat hot... My actual load is 23.7 grains of N130 plus CCI BR4 primer in once-fired neck-sized Lapua brass, Nosler BT bullet seated about ten thou off. Thanks for your interesting input! I must say, i've seen MV figures from a variety of sources, UK as well as US, which always seemed to make my chrono figures seem entirely believable; but I'm prepared to be proved wrong of course. I never use super-hot rounds! My load does not exhibit any danger signs.

Regards, Tony

 

 

23.7 gr of N130 gives a pressure of 66,200 psi and should give a velocity of 3596 fps. I have so far always found QL to be within 50 fps of my chrono across all 3 of my rifles, I am not saying mine is right that's just what I have found.