Pressure Problem Ruined Bolt Please help!

[Glocksig]

Looking for some help diagnosing this one. I'm shooting a custom Remington 700,M40A1 Copy)with a 6mm Remington match chamber,PTG). The load is 80 g Bergers over 41.5 g 4064 in new Remington brass and Fed 210M primers. It's a one hole shooter and I don't see any pressure signs from this load.

Took the rifle to Nebraska for a praire dog trip, worked flawlessly for 2 days, first shot of the day on day three blew a primer, I thought I might have felt some lube in the chamber so I set it aside until the afternoon. I was able to gun scrubber it out and get it back in the field. The next two shots were fine and then boom, shot three blew gas out the relief vent and I could hardly budge the bolt. Got it open and of course the case was firmly affixed to the bolt face. Not being able to budge it by hand,out in a field in Nebraska mind you,) I smacked the casing against a plastic ammo box and proceeded to break a sliver of steel clean off the bolt face. I tried diagnosing everything, load, seating depth, bore obstruction, chamber obstruction and the best I can come up with is that the neck is so tight,.273 per the reamer blueprint) in this gun that on some rounds there just isn't enough room for the neck to expand and release the bullet properly. Contacted the folks who built it for me and the fellow there suggested it may be a bad lot of powder. He's shooting a .272 neck and not turning either with no problems. I never had any trouble chambering and got through about 450 rds before the failure.

The first round I ever fired in the gun was a 100 g Federal soft point factory load. It blew the primer on that which I attributred to the neck at the time. Never had problems with my handloads so I didn't think much of it until this happened.

I was planning on having the neck opened up when I have the bolt replaced but if that's not the problem I won't bother. That being said I'm not neck turning thousands of varmint rounds.

I looked at the rounds that caused the failure and can't detect any difference between them and a batch that fired fine. What am I missing here?

 

[LHSmith]

I would have begun my diagnosis much sooner,...say after shot #1. A blanked primer on the rifles maiden shot,with a factory load none-the-less) is never a good sign that all is well.
What did your loaded rounds measure at the neck? The cartridge diagram section on this site lists .276 as the neck O.D. on the 6mm Rem. case

I don't see how you got any rounds to chamber.
Match chambers can cause a lot of headaches.
Getting the proper sizing die might be difficult.

 

[Glocksig]

They measure at .271 to .272. There's variation as most of this is factory new brass and we all know how that goes. I've got a Redding bushing die with a .271 bushing. Maybe neck sizing all the brass first would do it. At the least it will get rid of some of those oval necks you tend to see in Remington brass.

I've never had a gun with a match chamber so this is new to me..

Here's a pic of the chamber:

 

[Ackman]

If just the bolt's face has been damaged, it can be fixed easily with a bushing. Same procedure and the same thing as having the firing pin bushed.

I'm not real clear about which ammunition caused the mishap. Was it a Federal factory load? Brass spec's differently from one maker to another. Measure a loaded round. My .243AI reamer has a minimal web dia. and .273' neck......some headstamps are fine, but Win. needs to be neckturned. The Lapua web is a few thou. larger than others and won't fit the chamber by about .001'. My 6mmAI has a .272' nk and unturned Win. brass is about perfect. Don't have any Rem. or Federal. And even within the same headstamp, brass can vary from lot to lot. With custom chambers you need to check this stuff and know what's going into them. However, a neck has to expand only far enough to release the bullet. And if it were only just a few ten-thou larger than the chamber neck to begin with, you'd definitely feel a press fit if the round would even chamber.

Another thing too, it gets hot out there. Was your ammunition sitting in the sun? The hood of my dark colored truck can really cook ammunition, enough to where loads that are normally fine will show overpressure on a hot day. Same with chambering a round when the gun is hot, then letting it sit while finding another target. I've also come home with lots of blown primer pockets and the bolt of a 17MIV locked shut from a fairly conservative load that had been sitting on the dashboard under hot Nevada sun.

 

[cliffy]

Regardless of ammo source, a blown primer signifies too much MOXY for the given application. I fight this problem relentlessly, since I load to maximum power regularily. Ambient Temperature and Altitude are major culprits regarding this problem. The hotter the weather, the lighter the load required or acceptable. I estimate reductions of 1/10 grain per every 10 degree of temperature rise. Texas is a prime example of powder reduction requirements according to degrees of temperature rise. Michigan weather does not present so drastic of powder grain extremes required, since its always rather brisk here. Factory loads are a good guide, since they have no clue as to where you fire your cartridge, and under what circumstances regarding heat and quality of your firearm. Handloading allows for most conditions and condition of a firearm, so load appropriate to your conditions! cliffy

 

[Winchester69]

Mic the necks of your remaining ammo. If you're getting good neck tension sizing with a 0.271 bushing, you probably don't have adequate chamber clearance for the necks. Your loaded ammo will tell what your chamber size needs to be.
.

 

[LHSmith]

Is that small sliver possibly the Remington extractor?
Is the .271/.272 the brass neck dimension of a loaded round? The loaded round dim. is the one to focus on.
I recommend having the shop run a standard reamer in to avoid situations like this.
As was said, custom chambers require a lot of scrutiny to get everything to function without problems.
Also, off-the-shelf dies are not designed to properly size brass from match chambers.....generally speaking.

 

[alf]

I'm afraid that sliver of steel is what holds the extractor in.

 

[Glocksig]

I guess it's really the rim of the bolt:

Definitely not a temperature issues as the rounds came right from my bag after a 15 minute ride. Cool to the touch. Temperature was about the same as back home although the elevation was higher. Since I had fired so many rounds before the failure I don't think that was the culprit.

Measuring loaded rounds gives me .271 to .272 as you rotate the case in the calipers. Strangely enough what I just found out is that if I measure the very edge of the neck,basically so the neck edge is half way into the caliper) I'm getting variation all the way up to .275 when I rotate the case. It appears that there's a little edge there kind of like the rolled edge you'll get from the factory, not the clean edge you'll get after trimming. That's obviously too thick, but the batch that fired fine has the same variation,I still have 5 unfired rounds from that batch). How could that possibly be? If it was too think wouldn't it pinch that round each an every time?

The lack of consistency is what is really throwing me off. Everytime I think I have it figured I disprove myself..

 

[uncleduke]

just my 2cents, but have you checked to see if a carbon donut has built up in your neck? I had that problem once before I got serious about proper cleaning. Match chambers require a lot more attention to this area.

 

[Glocksig]

I have not checked as this is all new brass, that being said I would have never thought to check that anyway.

 

[uncleduke]

I meant carbon in the rifles chamber. A donut can form in the neck that if not removed will restrict the brass.

 

[Glocksig]

My bad, I misread your post. I managed to get a bore light down there and I don't see anything visible. I use wipeout foam to clean the chamber and gun scrubber to make sure it's dry, looks pretty clean to the eye.

 

[jdajr]

Check your case length also. Sounds alot like problem my father was having just recently. He was using new brass and it was too long for the chamber. Don't take much to pinch the bullet and spike up the pressure.

JDA

 

[AckleymanII]

41.5g of 4064 is a warm load, I shoot the exact same load in my 700's, velocity is 3475 fps.

Part of the problem is that it is warm weather. No doubt that the ammo was out in the sun a little, increasing the pressure.

So, you had two big things working against you, heat and sunlight.

Bottom line, you are shooting a cool weather load in hot weather.

First time this happened to me, I beat the bolt open on the bumper of the truck.

I dropped the load back to 39.0-39.5g for warm weather.

One other thought, when you shoot one dog, then go for another, the round is cooking in the chamber. In the heat of battle, it is hard not to be cocked and locked.

 

[Glocksig]

What confused me was that it was the first round of the day, ammo came right out of the air conditioned truck..Well no matter the gun has been sent off and will be rechambered in with a standard chamber. I'll work that load up again and go lighter on the charge, it shot through one .243 hole at 38 g so we'll see where the balance between accuracy and velocity comes when the rifle is returned..

 

[ChuckMiller]

In reading your posts and replys I'm not convinced that heat had anything to do with it, especially after your last post. Any minumum spec chamber raises the issue of too tight of a tolerance and requires closer attention. A 273 nk shooting 271-272nk loads definitely raises that as a possiblilty, you might want to take a chamber cast to verify the neck dia. Because of the amount of rounds you have fired through this rifle with no problem I would look for something that affected the load integrity when loading. I've had 2 instances with friends that mirror your experience. First one had used 'Mothers' rim polish in his cleaning media to polish cases,learned that right here on 6mmBR), actually he was too cheap to buy the good stuff. He had media that clumped inside his cases about 1 in 10 would have this crap in it. He was loading one case at a time through an electronic powder drop so didn't eyeball it. Boom...instant blown primer and trip to smith to hammer open bolt. The other had some lubricant inside his cases, he also was using an electrionic powder drop and about every 10-15 cases if his load was in question he would dump and reload, instant extra 5-8gr's that were sticking to the inside of the case. Boom...you know the rest. On both we didn't determine what it was until we pulled about 20-30 rounds and inspected them, not one or two. Ruling out the heat or barrel obstruction your basicaly left with either a tolerance issue, media in the case, or too much powder to create excessive pressure. The damage to the bolt looks like it was caused by the method used to remove the case or a manufacturer defect that surfaced because of the pressure. Good luck and glad you didn't get hurt.

 

[rogn]

I had abit of a problem like this at one time with a 22 br, didnt check case oal after forming cases from Lapua brass. Barrel was a take off that I was messing with. Didnt take the time to check the actual chambering. A couple of cases were abt 0.010 over lenght, which was enough to blow and I mean blow a primer--never did that before in 50+ yrs of loading. Checked lenght, cast chamber and trimmed cases--- no more prob. Dont know if this is your issue, but just wantd to add this to check list.

 

[Glocksig]

Talked to the smith today, he suggested fixing the bolt, installing a Sako extractor and doing some more troubleshooting. In his experience he feels that recutting the chamber isn't the answer especially becuase he's shooting a tighter necked match chamber than I am without turning.

The data I got from Berger shows the load over max although the other 3 manuals I checked for 80 g showed it to be in line. Of course that doesn't mean anything. I do wish Berger would release a manual though. I think I still have the cases from when I first laddered the load so I want to go back and look at them again.

The bolt is really misleading because as someone previously mentioned, I busted it extracting the case. I never thought brass would effect steel like that but I got a quick lesson in physics.

I want to go back and check some of this brass. I'm thinking about neck sizing everything with the bushing so I'm at least at a uniform diameter all around. No more oval readings I'm getting off the caliper bouncing around from .271,.273 .271, .

What kind of clearance should I be looking for between the loaded round and the neck?

 

[LHSmith]

For a varmint rifle, no tighter than .002.
The loaded round measurement should be done with a micrometer.
I have a loaded .244 Norma case that measures .276.
The cartridge drawings in all my manuals lists the 6mm Remington neck OD as .276 before a bullet is inserted.
I have trouble understanding how you end up with such low numbers.
If your calipers read up to .273 on a loaded round, then you are probably swaging the case neck into the bullet jacket.
What brass does your smith use?
Are you seating bullets into the rifling giving no room for error pressure-wise?
That first shot fired, a factory round that blanked is the key.
You are on the ragged edge, either neck turn for more clearance, or re-do in a more foregiving SAAMI chambering.