POI and Velocities Changes with 1-Fired Brass

[AM_Hunter]

Attention Reloaders of Rokslide!

Is it typical to see a POI shift and Velocity increase going from virgin brass to once-fired brass?

I was out shootings and testing velocities on my .308 the other day and had about 13 loaded round with brand new hornady brass. The remainder of my rounds were all once-fired, this is from the same gun, same load data, bullet, everything.

For the once-fired brass I full length sized it, cleaned it, chamfer and deburred, I did NOT however trim or anneal this brass. Dont know if that would make a major difference. I had been crimping all my .308, new brass and once-fired, because I did not know any better but I dont plan to crimp anymore.

First 5-round group virgin brass
Average: 2605.4 FPS
Ext Spread: 25.9 FPS
STD Dev: 8.5 FPS

First 5-round group once-fired brass
Average: 2613.5 FPS
Ext Spread: 115.2 FPS
STD Dev: 41.3 FPS

POI shift right about 1"

Second 8-round group virgin brass
Average: 2602.1 FPS
Ext Spread: 31.3 FPS
STD Dev: 9.7 FPS

Second 5-round group once-fired brass
Average: 2635.0 FPS
Ext Spread: 22.7 FPS
STD Dev: 8.0 FPS

POI shift right about 0.75" - 1"


As you can see theres a slight increase in velocity on the first round of shots but the once-fired in that group had a pretty crazy spread, going from 2533 to 2648. The second set of groups the spread and STD deviation narrowed down but its still 33+ fps higher.

Is this normal or to be expected when going from virgin brass to used brass? Should I do anything different in my reloading process aside from not crimping? Could it be a neck tension issue possibly from inconsistent crimps? Any input and ideas will be greatly appreciated.

I still have another 55 rounds of the once-fired brass to test, we just ran out of time that day.

Thank you!

 

[TaperPin]

A really smart guy, probably an engineer, explained the increase in velocity as equaling the amount of energy required to stretch the brass to fit the chamber. Once it’s fire formed, that energy gets put into a little extra velocity.

 

[Koda_]

Ive never measued any difference in POI or velocity between virgin and fireformed brass.
But i would guess not annealing and trimming could affect it.

 

[Harvey_NW]

What kind of chronograph were you using? I've seen POI shift wildly from bayonet style like Magnetospeed.

Velocity/pressure increase on 1x fired with proper shoulder bump vs new, yes. POI shift, not that I've ever noticed from that alone. It's possible you're seeing some small sample noise. I would also suspect the velocity variability could be from inconsistent crimp, as you mentioned.

I would load up 13-15 for test/zero with no crimp, and shoot a 10 shot group. Adjust zero, confirm with the remaining 3-5, and roll on.

 

[AM_Hunter]

What kind of chronograph were you using? I've seen POI shift wildly from bayonet style like Magnetospeed.

Velocity/pressure increase on 1x fired with proper shoulder bump vs new, yes. POI shift, not that I've ever noticed from that alone. It's possible you're seeing some small sample noise. I would also suspect the velocity variability could be from inconsistent crimp, as you mentioned.

I would load up 13-15 for test/zero with no crimp, and shoot a 10 shot group. Adjust zero, confirm with the remaining 3-5, and roll on.

I was using a Garmin Xero C1 for the chronoing. I agree it could be small sample size, since I still have plenty of those rounds loaded, I will do another comparison with 10 of those and I will load up the remaining once-fired brass I have with no crimp, and a case trim as well, and see how the two compare with a 10-shot group.