standard load suddenly making pressure

[wolfman]

All of a sudden my standard .308 F/TR load has started showing significant ejector and extractor marks. Brass, powder and primers are all the same lot as last year. I even had some ammo left from last year, and it is now showing pressure. 43.3g H4895, BR-2, 185 BT, seated 0.015 off, in Lapua cases. Barnard S action/ 1-11" barrel. I do not do anything specfically to address carbon in my cleaning routine- just Butch's boreshine. Could this be a carbon ring issue? The really weird thing, is accuracy has improved significantly!

 

[Jason Boersma]

Sounds like it is a carbon ring, you need to get a carbon remover and a bore polish like iosso

 

[lpreddick]

If you have a carbon ring in the very end of the chamber it will likely be as hard as baked on carbon can be...VERY hard. A borescope will be the next step. You can see the ring and follow your attempts at it's removal. Abrasives alone may do little. A bronze brush with solvent rotated in the neck portion of the chamber may be necessary if a very stiff nylon brush with jb doesn't get it out. An established carbon ring can crimp the neck of a case and pressures go up and accuracy goes away. Measuring your chamber will tell you the length of the gap beyond the case lip to chamber end. Tony boyer's book has nice diagrams and advice re cleaning this ring after each shooting session.

 

[Macropod]

How long are your cases? If they have grown too long you will get pressure spikes.


Edit: If you are getting to the problem with older known cases then maybe not.

 

[wolfman]

Case length double checked, and they're fine. I don't have access to a bore scope, but I can't help but think a carbon ring could be the problem.

 

[jlow]

Look for scratches on the outside of the neck for carbon ring? Not an absolute as the pinch can be on the bullet. Also try chambering a dummy round by hand (i.e. with on bolt), if you push it in does it get stuck? Since you are 15 thousands off, it should not get stuck. If it does, could be carbon ring or even COAL problem...

 

[Lapua40X]

Truth is, you can make a hundred guesses and never identify what's going on in that chamber without a bore scope - and guessing will only identify the real issue if you're incredibly lucky.
If you don't own a bore scope and don't want to invest in one and can't find someone at the range who has a bore scope you could befriend and get a look at the chamber, It'd be worth the price of a gunsmith's time to look at your rifle and tell you what he sees. IMO, analysing pressure issues using a Ouija board approach wastes time and ammunition.

 

[wolfman]

I think you are absolutly right George. I'm going to park this one until I can get a look inside witha borescope. Now I've got a week to develope a load for the other tube, and then load 300. No rest for the wicked!

 

[savagedasher]

Pressure signs are common . Many of powders don't like temp changes . If your load was on the hot side in cold summer temp can put you over pressure. Then the carbon ring changes because you shoot more.
In Florida where I shoot Cold is 50 and hot is 90 . The load that works here can be different then where you have elevation and cold weather. Larry

 

[Laurie]

Truth is, you can make a hundred guesses and never identify what's going on in that chamber without a bore scope - and guessing will only identify the real issue if you're incredibly lucky.
If you don't own a bore scope and don't want to invest in one and can't find someone at the range who has a bore scope you could befriend and get a look at the chamber, It'd be worth the price of a gunsmith's time to look at your rifle and tell you what he sees. IMO, analysing pressure issues using a Ouija board approach wastes time and ammunition.

I agree 100% with this. Put component lot changes aside and there is no obvious temperature change that affected pressures, you can still get quite inexplicable occurrences if you're running close to maximum pressure for the rifle / barrel. Oftentimes, a barrel's relationship to a bullet changes with use / round-count and it's nothing to do with a carbon ring. A friend who is a gunsmith, BR competitor, obsessive cleaner and bore-scoper has had this with a 6XC in a Benchmark barrel seeing a substantial MV increase alongside; I'm getting it with a 6.5X47L where a previously happy load is now wrecking half the cases put through the rifle despite nothing at all having been changed and shots taken in cool even cold weather. If it starts, the barrel often coppers quicker too, and that increases pressures further producing a self-reinforcing cycle.

 

[hogan]

This is a Perplexing Situation....


Carbon ring seems far fetched as does fouling and gilding metal build-up in barrel; but you clean with Barnes CR-10 until no more discoloration is seen to be sure. Really comes down to analysis and isolation of the variables from here on. If you have some loaded rounds from the batch of ammo that gave the pressure signs, might pull some bullets and be sure didn't have another powder type or weight in those loads. Might even use a bullet comparator to be sure bullet design didn't change ogive position from prior batch of bullets. Ogive moved forward could have some pressure effects; but unlikely.

Temps are not going to produce this kind of pressure variation; not unless you leave ammo in direct sun, like on dashboard with windows up. Heating ammo to 140f or more could be problematic; but unlikely to have happened.

If you have new brass to work with, and same lot of powder, primer and bullets your orig good loads used, might just load some new rounds and see if the pressure signs persist. Load to your last 'known good' specs and carefully fire couple rounds as diagnostics.

Good luck!

 

[barefooter56]

wolfman,
In some cases as the powder gets some age on it ( especially if its a like N550 that uses nitro to enhance power) it dries out. As it dries out the nitro gets closer to the surface of the powder and changes the burn rate ( faster). I had this issue with my PALMA load and it took Kevin Thomas of LAPUA/VHITVOURI to make a call to Finland to find out what was up. A load ( all the same lot ) that worked great got a little hotter every year until it was blowing primers out of LAPUA brass ( at Camp Perry of course). I thought it was my powder charge and went nuts checking my scales. Lesson learned. Back off a grain or two and work back up every year just to be sure.

 

[wolfman]

Thanks for all the input gents. I'll let you know how it all shakes out once I figure it out after Eastern Regionals next week.

 

[TikkaSporter]

Sounds like a classic carbon ring issue. I fought this for a year with one of my rigs... Kept reducing powder charge, only to temporarily resolve the issue and have it come back. In my case accuracy suffered.

The way I found it was when I re-measured COAL to the lands and found that it had gotten shorter, no longer as would be expected. I was also seeing scratches on the ogive of the bullet if I removed it from the chamber a couple times without firing.

I was aggressive removing the carbon ring since I didn't have a bore scope to confirm that it was removed... Shouldn't even say what I did , but it worked...

Cut a cheap brass cleaning rod off short with a hack saw, Added a chamber brush, chucked it in a drill, and carefully spun it in the chamber and beginning of the throat. Followed that by short stroking an undersized brush with a patch wrapped around it, treated with kroil and JB paste. Short stroked the first 6-inches of the chamber about 10 - 15 times.

Pressure signs went away, measured COAL went back to what would be expected (longer than original measurement indicating some normal throat erosion), went back to original powder charge with no pressure issues and original accuracy returned.

BTW, certain powders are more prone to causing this than others. In my case it was Varget.