Had an over pressure round ........ Why??

[Dave A]

This may come across as a dumb subject line, but let me explain.

This winter I built a 6mm Norma BR around a new Remington 700 SA with a Bartlein 28" one in eight twist barrel. All the components were new and the action was trued up by the guy who chambered and mounted the barrel. The chamber was reamed to SAMMI specs with a .114 throat and it ended up a bit on the tight side after all was finished. I bought 200 new Lapua cases and started shooting the first 50.

After I had reloaded the first 50 cases 7 or 8 times, I broke out 50 new ones and alternated them. This barrel always seemed to shoot most accurately at 27.5 grains of Varget with Berger 105 Hybrids, CCI BR4 primers seated at 1.665" which was about .015 to .020 off the lands. I had tried 28.5, 29.5 and 30.0 grains but none of those were as good.

Two weeks ago I laddered some loads at 28.8, 29.3, 29.8, 30.0 (reference) 30.3 and 30.5. On the third shot of five at 30.0 grains of Varget I had a "can't lift the bolt moment" after firing the round. After cooling for about five minutes I was able to force the bolt open. The case looked OK to the naked eye, with no primer flattening, but a dark ring was noted around the edge of the primer hole.

I went ahead and shot the next two rounds with no problems and then shot five at 30.3 and five at 30.5 with no major issues, except two rounds at each level had some bolt resistance, but nothing like the one at 30.0 grains.

Later at home I measured the offending case at the diameter at the base was .478, which was about .007 larger than it should have been. The other cases that showed some bolt lift resistance measured .473, but there were several others that measured the same but extracted with no problems at all.

It seems clear the somehow I got too much powder in that one round. I use an RCBS digital dispenser and double check each charge with a Pact digital scale. For the life of me I can't figure out how it happened, except it did. Opinions welcome.

Yes I have retired that entire batch of cases. They measured from .471 to .472 for the most part at the base and might have been salvaged but why take chances.

 

[dkhunt14]

Maybe some powder hung up in funnel and when you went to the next one it went in. Matt

 

[fredhorace77]

is it possible a carbon ring developed at "old case length" and the new cases are cramming their way into it causing the higher pressure.

 

[swampshooter]

I ran a test check on fifty rounds dispensed and weighed with my RCBS electric powder dispenser and RCBS digital electronic scale against my AND .02 Laboratory scale. It has been my understanding that the RCBS unit is made by PACT. I had several that varied by as much as .7 grains when weighed on the Laboratory grade .02 scale. IMHO your attempts to accurately measure your powder charges are exceeding the abilities of your equipment.

Bill Batson

 

[jlow]

It could be powder weight but the OP did double check each charge with his Pact digital scale so I don’t think it is a powder weight issue. It also seem like the first 50 cases have been fired 7-8x with no problem with the same setup.

My question is did the OP weight sort the cases? I use Lapua brass but even with this more expensive brass, there is a weight distribution. If you do a QuickLoad simulation, you will see that a significantly smaller case volume i.e. a heavier case can significantly affect pressure. So a combination of a hot load with a new case that is significantly heavier/smaller volume may be what pushed your load over the edge.

Since you have all the cases and know which is the offending case, weight it and the rest of your cases to see if it is in fact significantly heavier.

 

[dkhunt14]

I don't believe the capacity of the case would be enough to make a .007 expansion of the case head. A one thousandth expansion is a hot load and it was way over that. Is it a new lot of powder? Do you have a carbon buildup in your barrel or throat? What is the neck diameter of the reamer and the loaded round diameter? Did you check trim length? Matt

 

[CaptainMal]

Don't discount Fred's "carbon ring" comment. When I had trouble it started with one or two cases and just got worse.

You use Varget. It's a hideous carbon fouler. Clean thoroughly with carbon cleaner like CR-4 and use some Iosso. If you don't, or have not, expect what you described.

 

[jlow]

The point is not to discount any theories but to put out possibilities. OP need to check each one out.

 

[jlow]

I don't believe the capacity of the case would be enough to make a .007 expansion of the case head. A one thousandth expansion is a hot load and it was way over that. Is it a new lot of powder? Do you have a carbon buildup in your barrel or throat? What is the neck diameter of the reamer and the loaded round diameter? Did you check trim length? Matt

Matt, without knowing the case capacity in question, I think it is hard to say it cannot be. This is one reason to weight cases just in case something out of the ordinary comes down the line even if one does not believe case weigh has anything to do with case volume. Time and time again I have seen odd ducks come down the line even with the best of components. For the OP, this is especially true if you are on the edge when it comes to powder weight.

On the point of 0.007 expansion, that does seem like a lot and to be honest, I am not sure I believe it can be done without a catastrophic even. Not being nasty just saying what I think.

 

[Waskawood]

He stated a black ring around the primer, but at .007'' case head expansion wouldn't the primer fall completely out? .007" seems quite excessive and serious to me, but I do admit to not being an expert.

 

[Jet]

Do you clean your cases with corncob media? If so, could a clump of the stuff have gotten clogged in the case?

Good Shooting

Rich

 

[fguffey]

After I had reloaded the first 50 cases 7 or 8 times,

Later at home I measured the offending case at the diameter at the base was .478, which was about .007 larger than it should have been. The other cases that showed some bolt lift resistance measured .473, but there were several others that measured the same but extracted with no problems at all.

Expect case head expansion to be .00025" with factory loading. Logic leads reloaders to believe a case fired 4 times would increase in diameter .001". There is case head work hardening. Measure case head diameter before firing and again after firing if the case head diameter is going to be used when deterring the onset of pressure. Then there is the flash hole, there is a tool that measured the diameter of the flash hole. If the case head expanded the primer pocket increased in diameter and the case head got shorter from the of the cup above the web to the case head.

F. Guffey

 

[CatShooter]

This may come across as a dumb subject line, but let me explain.

This winter I built a 6mm Norma BR around a new Remington 700 SA with a Bartlein 28" one in eight twist barrel. All the components were new and the action was trued up by the guy who chambered and mounted the barrel. The chamber was reamed to SAMMI specs with a .114 throat and it ended up a bit on the tight side after all was finished. I bought 200 new Lapua cases and started shooting the first 50.

After I had reloaded the first 50 cases 7 or 8 times, I broke out 50 new ones and alternated them. This barrel always seemed to shoot most accurately at 27.5 grains of Varget with Berger 105 Hybrids, CCI BR4 primers seated at 1.665" which was about .015 to .020 off the lands. I had tried 28.5, 29.5 and 30.0 grains but none of those were as good.

Two weeks ago I laddered some loads at 28.8, 29.3, 29.8, 30.0 (reference) 30.3 and 30.5. On the third shot of five at 30.0 grains of Varget I had a "can't lift the bolt moment" after firing the round. After cooling for about five minutes I was able to force the bolt open. The case looked OK to the naked eye, with no primer flattening, but a dark ring was noted around the edge of the primer hole.

I went ahead and shot the next two rounds with no problems and then shot five at 30.3 and five at 30.5 with no major issues, except two rounds at each level had some bolt resistance, but nothing like the one at 30.0 grains.

Later at home I measured the offending case at the diameter at the base was .478, which was about .007 larger than it should have been. The other cases that showed some bolt lift resistance measured .473, but there were several others that measured the same but extracted with no problems at all.

It seems clear the somehow I got too much powder in that one round. I use an RCBS digital dispenser and double check each charge with a Pact digital scale. For the life of me I can't figure out how it happened, except it did. Opinions welcome.

Yes I have retired that entire batch of cases. They measured from .471 to .472 for the most part at the base and might have been salvaged but why take chances.

Right off the top... there is no such thing as SAAMI SPEC for any of the BR family of cartridges.

It sounds like the head on that case is a bit soft. It happens.

If you would like, I will run a hardness test on it if you send me that case, and 3 or 4 others in the same batch (for a base line)...

 

[snowpro440]

This may come across as a dumb subject line, but let me explain.

This winter I built a 6mm Norma BR around a new Remington 700 SA with a Bartlein 28" one in eight twist barrel. All the components were new and the action was trued up by the guy who chambered and mounted the barrel. The chamber was reamed to SAMMI specs with a .114 throat and it ended up a bit on the tight side after all was finished

Can you explain " tight side " after all was finished ?

 

[Dave A]

Thanks for all of your thoughts.

The issue of a carbon ring is one that had crossed my mind and I admit to never having specifically cleaned for that condition. Will perform that specific cleaning operation today. I keep cases trimmed to within about .002 on length and I don't have to trim often at the normal load of 27.5 grains. The barrel has 1,560 rounds through it and depending on how it shoots this Friday in our last varmint match it may be retired and another installed.

I have checked powder weights periodically with an RCBS balance beam scale just to verify the Pact is telling me the truth. I have never found any charge to be off by more than about one tenth of a grain, but then again I don't do that on every loading.

The offending case was resized just to see what would happen. Needless to say the die pushed metal to the rear of the case and left a small lip. Then I cut it in half to see if there was impending separation, but there was no evidence of any cracking problem inside the case. It was clear that metal had been pushed out as the variation on the inside of the case wall could be seen using the end of a metal ruler as a straight edge.

I may very well take up the offer to have the hardness checked.

I should add that the older cases that I started with are all still consistent when measuring the bottom of the case at pretty close to .471. These cases now have been loaded about 12 times.

 

[Dave A]

Just noticed I did not respond to the question on "tight side".

I found that at around 1,100 founds some cases were becoming a bit sticky on extraction. Not a problem with bolt lift, just pulling from the chamber. Those cases exhibited some longitudinal rub marks from the middle portion toward the base. The barrel was pulled and the reamer used by hand and a small amount of metal was found on the cutting edges. Then the chamber was briefly sanded in the lathe with a fine grit paper and the problem went away. Extraction after that was normal again until this event.

 

[LoganDon]

As a new benchrest shooter, 6ppc, I read all the replies with great interest. At 71 with a great shooting and hunting background had I known there were so many variables in benchrest, I doubt I would have gotten in to it, but it is really fun.

Dean Parker of Utah (see SD records) built my gun and is my mentor. Anytime I shoot a .25" group he tells me "don't embarrass the gun." It is absolutely amazing when I can shoot a .15" group.

No advice but do not give let the problem get to you - with the skill of others on this site, we all learn something every day.

Don

 

[Jet]

A small base die should take care of your problem. Send a couple of fired cases to Harrells and they will send a die to you that should correct the sticky extraction.

Good Shooting

Rich