Velocity went up on 5.56 2 yr. old reloads - Why???

[MMH]

I have some 5.56 reloads that are shooting faster than they did 2 yrs. ago. Here are the load details:
Brass: LC15
Bullet Hornady 75 gr. BTHP
Powder: 23.5 gr. of 8208 XBR

Average velocity (22" barrel) is 2894 fps & ES (5 shots) is 9 fps
This is about 100 fps faster than the velocity that I had 2 years ago.

I did wet tumble the brass w/ SS pins. I have been reading that this may cause bullets to stick - could this be the cause? When I recently checked the velocity it was 70F outside. Don't remember what it was 2 years ago but guessing that it was colder - probably 40F or so. Is 8208 temperature sensitive?

Anyway, happy w/ extreme spread just confused why the velocity went up and with that velocity not getting close to transonic at 1000 yards.

 

[PinesAndProjectiles]

It could be the bullets sticking in the case.

Take a few and seat them .010" deeper and shoot them.

If velocity drops, you'll know that's the culprit.

 

[Mr Tibbs]

Could be the way the rounds were stored ?

 

[sinister]

23.4 / 23.5 is about right for 8208 and an H75.

My experience (spotting for another shooter with an 18-inch SPR) is the Hornady 75s hit trans-sonic around 850-875 yards, south of Las Vegas in April at around 70 degrees F.

ALL powders show some temperature sensitivity. A thirty-degree temperature and density altitude change may or may not favor your exterior ballistics.

 

[Doom]

I suspect that the wet tumbling with the pins is the source of the issue. I doubt if its the powder. While I haven’t run any test I don’t see a significant POI shift @300yds with it going from 30 to 100F. A 3fps/degree shift is also huge.

 

[Bmghunter]

Maybe a combination of factors contributed, including temp sensitivity of powder, cold welding, and one not mentioned yet is, was it a relatively new barrel 2 years ago? I see lots of comments that barrels speed up and after the first hundred rounds or so it settles as the higher speed.

 

[KZP]

Try sticking your ammo in a ziplock, then pack it in a lunch cooler with some ice for a bit. See if you can drop the FPS by chilling the powder.

 

[918v]

I would break the cold weld by seating .010” deeper as was suggested by MM first and go from there.

 

[nutation]

Is 8208 temperature sensitive?

"This short grain extruded rifle powder exhibits exceptional stability with virtually no change in velocity at temperatures ranging from -40 degrees F to 165+ degrees F."

 

[MMH]

So I seated the bullets 0.010" deeper and now have:
Average velocity is 2862 fps & ES (5 shots) is 30 fps.

Reshot the original version (i.e. bullets not reseated) and got:
Average velocity is 2870 fps & ES (5 shots) is 30 fps.

So, I do not believe that the bullet was stuck/cold welded. Avg. vel. for 15 rounds is 2875 which is 75 fps faster than what it was 2 years ago. Will throw 5 rounds in the freezer to check temp stability.

Maybe the barrel got faster - it was fairly new 2 years ago and now has 400 rounds through it. It is a Bartlein 1/7 4 groove SS barrel.

 

[Milo 2.5]

So I seated the bullets 0.010" deeper and now have:
Average velocity is 2862 fps & ES (5 shots) is 30 fps.

Reshot the original version (i.e. bullets not reseated) and got:
Average velocity is 2870 fps & ES (5 shots) is 30 fps.

So, I do not believe that the bullet was stuck/cold welded. Avg. vel. for 15 rounds is 2875 which is 75 fps faster than what it was 2 years ago. Will throw 5 rounds in the freezer to check temp stability.

Maybe the barrel got faster - it was fairly new 2 years ago and now has 400 rounds through it. It is a Bartlein 1/7 4 groove SS barrel.

Is your barrel as clean as when the original test was ran? Same chrono?
I myself do not buy that 8208 is that stable of a powder. You may have a combinations of circumstances messing with it.

 

[MMH]

Is your barrel as clean as when the original test was ran? Same chrono?
I myself do not buy that 8208 is that stable of a powder. You may have a combinations of circumstances messing with it.

Yes & yes. I too believe that there are multiple factors and trying to determine what they are so I can minimize my future surprises.

 

[sinister]

Do you anneal your cases? Work-hardening can also add to your neck tension.

 

[MMH]

Went back out to the range today. Verified my velocity for the third time. Shooting at 2875 fps, which is about 75 fps faster than when the barrel was new. Also, started using a G7 = .183 instead of the G1 = .395. Shot from 300 to 1000 yards and predicted drop was close to calculations, but not perfect. However, wind was significant, varying from a standstill to 12 mph and going all the way around the clock. Mostly either at 12 or 6 o'clock, but sidewind was significant as it wandered around. Rifle shot good to 900 yards, but at 1000, my vertical spread was over 3'. Thinking that wind combined with bullet going subsonic is making 1000 yds. a challenge. I may only have a 900 yard rifle. I guess that's OK as I'm only a 600 yard shooter...