in need of experienced help

[pfinney]

so, i have a 6mmbr that was custom made. Although i do reload, for this particular rifle i cannot better Lapua factory loads (105g Moly), so while i reload for another 6BR, for this one, i just shoot factory loads.

I have put maybe 600 shots through it without issue, but, over the last 100 or so i have noticed the primers begin to act strangely. The 'dent' has a crater around it that sticks out. This to me is a sign of pressure, but being factory loads i assumed that i maybe had a hot box or lot number. Anyway, this issue has been getting worse and worse, just a little each new box of ammo (different lot numbers). Then primers started to pop and the odd one or 2 per box had clean holes in the primer.

So, i made enquiries and Kelbly suggested it could be a build up of moly and i needed to JB paste the gun for a solid 2 hours. Now ive done this, and used a bore scope, the throat and barrel literally look new, as in never used. So, back to the range, and while the primers are not popping, they are still cratering. It seems to be pressure related, but why? The same ammo shoots fine in other 6BR guns so why has my rifle become sensitive to pressure now when it used to be fine?

Frazzle

 

[cmillard]

it could be that the firing pin is oversized. What kind of barrel do you have. i have read in the past that broughton 5C barrels, after getting apx. 100-200 rounds down the tube, will start to show pressure signs on brass that did not show before. run your rounds through a chrony and see if you see any durastic changes.

 

[pfinney]

none of the components are 'known' brands, but the rifle shoots very well, with regular 5 shot groups being 1/4 MOA (200m) and on a bad day its never over 1/2MOA (200m).

The pin could be large (whats normal?) but it hasnt grown over the last 600 rounds. I think there is little doubt that my rifle has developed a pressure issue, but before i can fix it, i need to know why it was caused. Im at a complete loss....

Its worth pointing out that when it pops the odd primer, that bullet hits exactly where the rest did (ones that the primers didnt pop), so i assume there was no jump in velocity or the bullet would not have the same POA?

Frazzle

 

[DickE]

Send the bolt and firing pin to Gre-Tan. Fast turn-around. High pressure and large firing pins are very common causes of this kind of problem. He will bush the firing pin hole and reduce the size of the firing pin to fit to very close tolerances. Standard procedure on a BR or Dasher build. End of problem.

 

[jb1000br]

Wait, you scrubbed the bore with JB paste for 2 HOURS!?!?! ???

Increasing pressure over time with the same load/lot/temperature range has to be either moly or carbon buildup IMO

 

[camac]

The other issue that can cause pressure jumps is freebore or jump. (although I agree with JB as most likely cause is barrel fouling) but as a barrel ages, the freebore can increase (would suggest yours is not really old enough to see much effect here though and normally asociated with a "looser bore" unless accompanied by fouling). Also sometimes factories change seating depth on bullets. If your freebore has increased this can cause a "spike" in pressure as the projectile contacts the lands. Most of the time this is visible with a chrony as reflected in change in velocity but the increase in pressure can sometimes be more than the change in velocity. The firing pin crater still can (and should) be fixed by a bushing and nice neat fit with smaller firing pin as suggested.
Are there any other signs - case head growth, extractor marks, tougher extractions etc. etc.??

 

[pfinney]

Guys

thanks for your input so far.

JB, yes, i did scrub for 2 hours, though not in 1 sitting, i did this over 4 days. I used a borescope between scrubs and it took this long to remove all the moly and carbon.
This did help the primer issue (no more holes!!!) but the primers still show a sign of pressure by having a small lifted crater on each. I would explain this as a sign the its on the limit, and usually, if it were a reload, i would back of the powder a little, but these are factory loads that shoot fine in other rifles.

The firing pin is not bushed, so thanks for that tip. The barrel is now literally brand new (after the scrubbing with JB Paste), so its not a build up of fowling that is causing the craters.

Would an oversize firing pin do this?


Frazzle

 

[NorCalMikie]

If you've fired close to 600 rounds, I would suspect that the "distance to the lands" has changed from when it was new. After 200 rounds the "distance to the lands" in my barrels has increased by .010 to .015 so what you might be getting is your bullets now have to "jump" to the lands and when the barrel was new the rounds were at a jam.
And one thing that was mentioned was the "pressure spike" that develops when the round is fired. I would suggest that you measure the OAL to the lands of the chamber and compare it to your loaded ammo. You should be able to pull your bullets a bit and reset the OAL to match the chamber measurements.
On my rifles, a bullet set to "jump" to the lands will show pressure signs but with that same exact load with the bullets set at a "jam" into the lands, the pressure signs go away. And, I can increase that load if I want with no pressure problems. Just something else to consider. HTH, Mike.

 

[brxbrad]

Have you considered posible head space?...Do you full length size?...are you bumping the shoulder back?...Have you change primers?...The reason I ask is, I experienced the same problems with my brx. I experienced cratering and flow back and popping holes and even Case head separation. I was using CCI 400's and then descovered this article..http://www.jamescalhoon.com/primers_and_pressure.php. I have moved to Wolf
Small Rifle Magnum. I also discovered I didnt need to full length size and because I had my die set too deep, this bumped the shoulders back and inadvertantly increased the head space which lead case failure. I thought my loads were to heavy, I wondered if my gun was going south, and was looking for everything but the obvious... well I was wrong. After choosing the right primer with the right cup thickness and adjusting my sizing die, my problems disapeared.......Just a thought....

Good Luck

 

[brxbrad]

The link I attached is bad .....here is one that works http://www.jamescalhoon.com/primers_and_pressure.php
Sorry

 

[gambleone]

I had the same problem. Sent my bolt to Gre-Tan, problem solved. I was even able to go up in charge about half a grain without any problem. I also thought it was my headspace, but it wasn't.

 

[pfinney]

To BRXBAND

Thanks for replying with your very thorough comments, but the only ammo used in this rifle are Lapua fatory loads. When the rifle was new, i tried several reloads and could not get close to the accuracy that factory loads give me (unusual, and this is the only rifle i own where this is the case), so unless Lapua have changed primers, i can confirm that its none of the points you have made.

Im beginning to wonder if it is the firing pin, but i dont understand why it would take 1000 or so shots to develop, surely a firing pin (and hole) doesnt change shape over time?

Frazzle

 

[brxbrad]

You are welcome sir....If all you shoot is factory, this would lead me to agree with gambleone and maybe your bolt needs to be looked at. I would disassemble the bolt and look for obvious signs of wear on the firing pin and the bolt at the point where the two meet. Have you talked to the people who made your gun and asked their opinion? This might be a job for your gun doc. One other thought, Did it start having this problem with this box of ammo? Maybe try another.....