Pressure Issues

[zrodwyo]

Hi all,

I’m struggling with a frustrating issue and need help diagnosing what’s happening with my rifle.

I have a 6.5 SAUM barrel on a Bighorn TL3 action, which was once the most accurate barrel I’ve owned. It consistently shot bug-hole groups with any bullet and powder combination. I use it strictly for hunting, so it has fewer than 300 rounds through it.

Recently, I started using a new box of ADG brass and mounted a new Nightforce scope. Since then, the rifle’s accuracy has dropped significantly, producing groups just over 2 MOA, compared to its previous capability of 1 MOA for a 20-round group. I confirmed the scope is not the issue by testing it on another rifle, where it performed fine.

The main problem is that once-fired cases are getting stuck in the chamber during the second firing. I verified that the sizing die is fully sizing the brass to SAAMI specifications, and the new brass also measures within spec.

I’m observing signs of excessive bolt thrust. There are pressure signs on the brass, including loose primer pockets and excessive case head expansion, despite no increase in velocity. I’m seeing these issues with a load 4 grains lighter than my usual “go-to” load. The brass shows scuffing above the case head where it sticks in the chamber.

I’ve pulled the barrel, thoroughly cleaned it, and bore-scoped it to confirm there’s no carbon ring or physical damage. I also verified that the brass has no residual lube and the chamber is clean and dry.

Normally, I’d replace the barrel by now, but given its past performance, I’m hesitant to give up on it. Could something be wrong with the action causing this issue? Any ideas?

 

[Jason Snyder]

How far are you moving the shoulder back?

 

[ResearchinStuff]

What's the measured case capacity difference between your brass now and your old brass?

 

[MontyMontana]

The two above posts are asking the same but... My guess is you are bumping the shoulder back too far and basically what happens is the firing pin slams the cartridge forward then its immediately slammed back into the bolt face. Need YOUR chamber size.. and a comparator. Measure a fired piece of brass and only set the shoulder back 2 thousands. See what that does.

 

[MontyMontana]

I should add.. In both cartridges i use ADG brass in (its actually both 7SAUM brass with an improved shoulder, 1 necked up to 30 cal the other down to 6) the brass ALWAYS shows ejector marks with no other pressure signs.. even when fireforming with a light load. That's over several different lots of brass.. just my experience.

 

[Jason Snyder]

The two above posts are asking the same but...

Kind of. The ADG capacity could be different enough the loads won’t cross over safely. Also, as you clarified, there could be a self induced headspace issue.

 

[SDHNTR]

What brass were you using before when it shot well? New, unfired brass changes pretty much everything. You’ll need to start over with load development.

 

[wind gypsy]

What is wrong with the brass you were previously using in this barrel? Can you still use it and return to prior performance? How accurate was the virgin firing of the new brass?

I’m observing signs of excessive bolt thrust. There are pressure signs on the brass, including loose primer pockets and excessive case head expansion, despite no increase in velocity. I’m seeing these issues with a load 4 grains lighter than my usual “go-to” load. The brass shows scuffing above the case head where it sticks in the chamber.

What is the velocity with 4 grains less powder compared to your prior load? Assuming prior load used ADG brass as well?

You have a few things going on it sounds like. One, clickers. Do you have a Saami or aw2 chamber? If Saami, the base is too tight and you need to ream it to aw2. Easy fix.

What brass were you using before when it shot well. New, unfired brass changes pretty much everything. You’ll need to start over with load development.

There are AW2 SAUM chambers? A 4 grain drop in charge should overcome any variance in case capacity I'd think?

 

[BBob]

There are AW2 SAUM chambers?

Someone's confused and it's not you

 

[SDHNTR]

What is wrong with the brass you were previously using in this barrel? Can you still use it and return to prior performance? How accurate was the virgin firing of the new brass?

What is the velocity with 4 grains less powder compared to your prior load? Assuming prior load used ADG brass as well?



There are AW2 SAUM chambers? A 4 grain drop in charge should overcome any variance in case capacity I'd think?

Whoops. I thought I read prc. Disregard, will edit post.

 

[TaperPin]

Different lots of powder can cause big differences in pressure without additional velocity. Brass might be smaller internal capacity. I’d check the neck diameter of loaded rounds in case you have a lot of wonky brass with extra thick necks. Soft thick brass can get you thinking all the rifle’s problems are do to pressures, when only the pressure issues are and the accuracy is something else. It’s good you checked the scope, but I’d swap a different scope in until the problem is resolved in case it’s an intermittent scope issue like a loose lens that seems ok, but really isn’t. Adding a shim to firm up bedding cut off a birthday card can sometimes surprise you. Even good bedding shrinks over time, and you might find some debris from the last time it was taken apart. Look carefully at the crown in case there’s a new ding in it.

If I was in your shoes I’d buy a new pound of powder and a box of new brass to see if the problem goes away. Ask a friend who is used to bore scoping take a look just in case you missed something - some flaking carbon falling down he bore from a can get ironed into the neck of the chamber and be missed. Unscrewing your sizing die a full turn to only neck size for a few loadings will take excess headspace out of the equation. It sounds stupid, but I’d also triple check my load data and actual bullet weights in case of brain fart.

Edit: If using an electronic scale, maybe switch to a mechanical while sorting this out in case you are having intermittent issues.

 

[zrodwyo]

Different lots of powder can cause big differences in pressure without additional velocity. Brass might be smaller internal capacity. I’d check the neck diameter of loaded rounds in case you have a lot of wonky brass with extra thick necks. Soft thick brass can get you thinking all the rifle’s problems are do to pressures, when only the pressure issues are and the accuracy is something else. It’s good you checked the scope, but I’d swap a different scope in until the problem is resolved in case it’s an intermittent scope issue like a loose lens that seems ok, but really isn’t. Adding a shim to firm up bedding cut off a birthday card can sometimes surprise you. Even good bedding shrinks over time, and you might find some debris from the last time it was taken apart. Look carefully at the crown in case there’s a new ding in it.

If I was in your shoes I’d buy a new pound of powder and a box of new brass to see if the problem goes away. Ask a friend who is used to bore scoping take a look just in case you missed something - some flaking carbon falling down he bore from a can get ironed into the neck of the chamber and be missed. Unscrewing your sizing die a full turn to only neck size for a few loadings will take excess headspace out of the equation. It sounds stupid, but I’d also triple check my load data and actual bullet weights in case of brain fart.

Edit: If using an electronic scale, maybe switch to a mechanical while sorting this out in case you are having intermittent issues.

I have basically a lifetime supply of h1000 and have been using the same powder for years. No issue with my reloading. Everything is spot on and I started double checking all my powder charges with an accurate beam scale. I’ve also tried backing the die out to not touch the shoulder.

I’ve used my old brass that I never had issues with and it also gets stuck in the chamber now.