Please help: Question regarding pressure signs, barrel breaking in?

[Bassnbow]

Background: I have a newly re-barreled Christensen Arms 280AI with approximately 145 rounds through it. I completed the 50 round barrel break-in as they recommend. I have 100 pieces of Pettersen brass and fireformed all of it using older Hornady bullets (half was used for the break-in). Having all brass once fired in new barrel, I began load testing. I loaded 4 different bullets (4 groups of 3 each) with powder increases of half grain between groups. Fortunately of the 16 groups, every one was sub MOA with 5 being sub .5. I had no signs of pressure and velocity was about what I expected. Obviously I was very please with these results and the next time out I loaded up more groups with the most promising from the first session. I also moved up in powder chares hoping to maximize velocity. The bullets I was using were all mono's ranging from 140-152 (yes, I live in the commie state). The two powders were H4350 and RL23. The next trip to the range was surprising, the rifle was still shooting very good groups, but with the same loads as the week earlier, several started showing signs of pressure and velocity was up between 55-95 fps. I was unable to try several of the hotter loads as I was hitting pressure.

The outside temp was actually 10-15 degrees cooler than the first trip out and nothing else was changed with the loads. The brass was also all once fired?????? I'm guessing the barrel must be smoothing out and getting broke-in. What do you guys think, am I missing something? Obviously I'm going to back things off and avoid pressure, will this get better or worse as the brass gets more firings in this barrel? For info the pressure signs were very slight bolt lift and slight ejector marks. Neither were significant, but I don't like pushing it. Any advise appreciated

 

[Bob Wright]

I had problems with donuts forming in the neck shoulder junction. Also, Peterson brass is thicker and required neck turning. See if a new bullet passes thru the spent brass at the neck. If it requires a bit of effort, alignment, wiggling to get it started, that would be something to look at anyway. I had to turn all my nosler brass, then no doubt the Peterson definitely was thicker than Nosler.

 

[Bret GRAVELINE Graveline]

Were the exact same bullets used in the two shooting sessions?

 

[26Reload]

but with the same loads as the week earlier,

Are we talking sameexact loads or a
'new batch of loads' on another fired round....cause your brass should have been fired formed by 2nd.....meaning 3rd round should have less forming of brass resulting in higher speeds and more pressures....
Reduce powder in fired brass to achieve the desired fps.....be happy with .5......

 

[Bassnbow]

Were the exact same bullets used in the two shooting sessions?

yes everything was same

 

[Bassnbow]

Are we talking sameexact loads or a
'new batch of loads' on another fired round....cause your brass should have been fired formed by 2nd.....meaning 3rd round should have less forming of brass resulting in higher speeds and more pressures....
Reduce powder in fired brass to achieve the desired fps.....be happy with .5......

exact same load, and same brass (once fired), and yes I am happy with with .5

 

[Mikecr]

I can't figure out how many times you've fired the brass with pressure issues.
You should not work up loads until brass is fully fire formed(~3 firings), as pressure can rise with fire formed cases.
You need brass stability for load workup.

 

[GLTaylor]

Just curious. Are you forming AI brass from standard brass?
Also, even if just firing to form to your chamber -
2 things:
Some of your pressure is being used to form your brass to it's new dimensions (whether AI or not) in the first/second firing. After forming to your chamber, subsequent firings may show more pressure as indicated above.
Donuts are real and can be formed in the firing process. I've also had to turn Peterson brass due to slightly thicker necks (above comments).
In one wildcat (25/284), i had to inside ream to remove donuts that formed (test as recommended above).
Keep us posted.

 

[Greyfox]

I would also check your barrels throat to be sure you have not developed a carbon ring which can also produce high pressure/sticking cases, without signs of accuracy deterioration. A carbon ring can be hard to detect without the use of a bore-scope, and the used, Lilly White cleaning patches are not an indicator that you have removed the "hard" carbon deposit.

 

[Seabeeken]

You need brass stability for load workup.

^^^
This

 

[ButterBean]

I had problems with donuts forming in the neck shoulder junction. Also, Peterson brass is thicker and required neck turning. See if a new bullet passes thru the spent brass at the neck. If it requires a bit of effort, alignment, wiggling to get it started, that would be something to look at anyway. I had to turn all my nosler brass, then no doubt the Peterson definitely was thicker than Nosler.

Just curious. Are you forming AI brass from standard brass?
Also, even if just firing to form to your chamber -
2 things:
Some of your pressure is being used to form your brass to it's new dimensions (whether AI or not) in the first/second firing. After forming to your chamber, subsequent firings may show more pressure as indicated above.
Donuts are real and can be formed in the firing process. I've also had to turn Peterson brass due to slightly thicker necks (above comments).
In one wildcat (25/284), i had to inside ream to remove donuts that formed (test as recommended above).
Keep us posted.

These two Gentlemen are in your corner and 3X fired Peterson brass is a must

 

[Okanogan]

I had a similar experience when breaking in my 6.5 SAUM. I chrono graphed all my shooting for the first 200+/- rounds. As the barrel broke in, the velocities and pressure signs increased. Everything seemed to stabilize after about 150 rounds. I backed my charge weights down to the velocities that had originally worked well and achieved the same groups I had early in my load development.

It's hard to do load development while your barrel or your brass are changing. It is also worth noting that as you continue to shoot it is possible that the throat can wear and change the distance to the lands.

 

[Carey Farmer]

I've seen a number of my magnum-chambered barrels do this at 100 - 150 rounds. Seems to coincide with the lands starting to show fire cracking. I interpret this as an increase in the chamber pressure needed for the bullet to start engraving into the rifling.

Increasing starting pressure cascades to an increase in maximum pressure. When I model this in QuickLoad, increasing the assumed initiation pressure yields model results that match pretty well with observed velocities and pressure signs.

This phenomenon is more pronounced with mono's and heavy-jacketed hunting bullets.

If your bullets will shoot with big jumps, that will help bring pressure back down. I often end up with jumps of 0.120" to 0.160" with the bullets I shoot. I typically don't get quite as good accuracy as with shorter jumps, but still sub 0.5 moa.

 

[Bassnbow]

Thanks to everyone trying to help. Just to clarify these loads were exactly the same with brass with the same number of firings. I originally fire formed my 100 pieces of Pettersen brass 1 time, then annealed and sized bumping shoulder back just enough to chamber smoothly. I then loaded and fired the original mentioned groups that were very accurate with the first half of that brass. I then loaded the second set of rounds using some of the remaining brass (still once fired). This was when I saw slight pressure signs and increased velocities from the same loads as the last session. I did purposely load some of the rounds hotter than the first session, but I didn't end up shooting those because of the pressure signs.

This was one of the reasons I was so surprised to see pressure/velocity increases, because the brass was the same in both sessions. Obviously as some of you noted it is likely the barrel which is changing/stabilizing that is causing the increase.

This brass is 280AI brass and I was aware that it really isn't completely fireformed until fired 2-3 times, but with components what they are I was hoping to conserve.

I did not neck turn the the brass, but it feels very consistent when seating bullets. I also have a bore scope and will check for a carbon ring and some of you noted. I think I'm very fortunate the barrel is obviously a shooter and I won't have any trouble finding a sub .5 load when barrel and brass settle in. Guess I need more patience, LOL