New Rifle - Muzzle Velocity Expectations - When Do They Settle Down?

[Monk Medic]

So I have a new AL AXMC .338LM that I started gathting data on Saturday.

Did my standard process. Installed scope, zeroed rifle, and after about 45 rounds downrange, I allowed the barrel to cool down, and attached my Magentospeed to gather data, fired 5 more.

Using factory ammo, Lapua Scenar 250g.
Results are below. D/A captured on my Kestral.

Today, I cleaned the rifle this morning, using the AI cleaning kit, brass brush, bore solvent for 10 minutes, followed by running patches though until clean.

Same ammo, brought to the range, checked zero, and fired a string of 20 rounds, allowed barrel to cool, then fired 5 for the Magnetospeed. MV was lower, S/D was better. M/V difference surprised me, thought if anything it might go up after some rounds down the pipe. Was happy to see S/D number lower.

D/A was different, but I wouldn't exepct that to affect the MV at the barrel end where the Magnetospeed sensor sits.

Questions below.

Aug 24, 2019	Aug 25, 2019
1 - 2933 2 - 2906 3 - 2945 4 - 2953 5 - 2899 Max: 2953 Min: 2899 Avg: 2927 S/D: 23.7 D/A : 950	1 - 2876 2 - 2863 3 - 2865 4 - 2887 5 - 2863 Max: 2887 Min: 2863 Avg: 2870 S/D: 10.5 D/A : -450

Questions:

1) D/A will not effect MV measurements at barrel end, true or false?

2) Reason for change in MV? Ambient temp today was in the 60s, yestreday was about 70.

3) Which number to I use for my A/B in the Kestral?

4) Will the MV increase as the barrel is broken in?

Thanks!

 

[plidenbrock]

One person's take:

1) True: DA as a whole will NOT affect MV - but temperature WILL.

2) Increasing temp of ammo increases activation energy of many powders, resulting in higher MV. You caught it yourself that the temp yesterday was higher, and so the avg m/v was higher. Can only speculate that there was some sunlight on the ammo involved as well. Also putting ammo into a warm chamber and letting it sit will increase temp even more, resulting in even higher MV.

3) Depends on which Kestrel you have: on 5700 Elite there is a temperature/MV curve generator that you should use. You input matched temp/MV readings, and it will then adjust the MV to match the measured temp. Only prob is you have to make sure your ammo approximates ambient temp, and make sure you take several readings at a wider range of temperatures. I think NK recommends 30*F or more difference for temp/MV curve.

4) For majority of barrels in my limited experience, MV will increase for a little bit as you break in barrel, will level off, and then settle in a tad lower. That's independent of temperature effects, meaning if you take all at 70*F, the MV will go up a shade, level off, then settle in for 70*F.

Don't underestimate the effect ambient temperature (ammo temp) has on MV. The ammo temp and ambient temp are not always the same. Have quite a few instances myself where i had to run calculations for ammo temp to get proper 1st round hits, especially in ultra-cold conditions where ammo is quite a bit warmer than ambient. When they are drastically different and I don't run separate solutions, well...results vary.

And personally, I tend to shy away from hard scrubbing of the barrel. Sparse carbon fouling cleans and otherwise leave it alone.

But that's all strictly personal opinion.

 

[Steel head]

1-Pressure no, temp yes.

2-50 FPS for a 10 degree change? I think not.

3-I’d run the slower number and see how it does at distance, true it up and monitor it as barrel gets going.

4-should at least a little bit.

 

[LawnMM]

Anything prior to 150-200rds on the barrel is supposition at best.

 

[Monk Medic]

One person's take:

1) True: DA as a whole will NOT affect MV - but temperature WILL.

2) Increasing temp of ammo increases activation energy of many powders, resulting in higher MV. You caught it yourself that the temp yesterday was higher, and so the avg m/v was higher. Can only speculate that there was some sunlight on the ammo involved as well. Also putting ammo into a warm chamber and letting it sit will increase temp even more, resulting in even higher MV.

3) Depends on which Kestrel you have: on 5700 Elite there is a temperature/MV curve generator that you should use. You input matched temp/MV readings, and it will then adjust the MV to match the measured temp. Only prob is you have to make sure your ammo approximates ambient temp, and make sure you take several readings at a wider range of temperatures. I think NK recommends 30*F or more difference for temp/MV curve.

4) For majority of barrels in my limited experience, MV will increase for a little bit as you break in barrel, will level off, and then settle in a tad lower. That's independent of temperature effects, meaning if you take all at 70*F, the MV will go up a shade, level off, then settle in for 70*F.

Don't underestimate the effect ambient temperature (ammo temp) has on MV. The ammo temp and ambient temp are not always the same. Have quite a few instances myself where i had to run calculations for ammo temp to get proper 1st round hits, especially in ultra-cold conditions where ammo is quite a bit warmer than ambient. When they are drastically different and I don't run separate solutions, well...results vary.

And personally, I tend to shy away from hard scrubbing of the barrel. Sparse carbon fouling cleans and otherwise leave it alone.

But that's all strictly personal opinion.

Ah, this is excellent, and something that I had not considered, knew about, but had not considered, yes, powder temp was in the high 50 or 60 degrees. Thank you!

I do have a 5700, and that's a great point, have never filled out the MV curve, will look into that.

I have the same rifle, and it breaks over 3,000, course it has 750 rounds down the pipe.

Agree in the scrubbing, I usually do that once with a new barrel and follow a very light cleaning protocl there forward.

Thank you very much for your time, and insight!

 

[Monk Medic]

Anything prior to 150-200rds on the barrel is supposition at best.

Good to know. At$6 per round, that’s a fair investment to break in the barrel.

I’ll continue to monitor and see what trend develops.
Thanks!