Weird stuff going on, Sudden over pressure signs after a few shots

[AKdan]

Hi all. I was out shooting my 308 win the other day with a buddy and had the strangest thing happen. After about 8 rounds (over 10-15 min) i got sudden severe over pressure signs on multiple rounds( significant ejector marks, stiff bolt lift, etc). This is a load that has never shown pressure signs(400+ down the tube) actually quite below where i saw pressure signs during development. I double weigh all charges on 2 scales. All rounds were loaded at the same time. I measure all loads for seat depth. I have shot long strings of this load without issue. I clean the rifle about every 50-60 shots. Seems to like this range. I am loading the berger 185 VLD with 42 gr Varget. Have any of you seen this before this is a new one for me. Temps were in the 30's.

Thanks,
Dan

 

[jomiba]

Check case length. Maybe too long and is crimping when you close the bolt.

 

[codyadams]

Check case length. Maybe too long and is crimping when you close the bolt.

+1.

If that's not it, then it could be possible you got some magnum primers mixed in on accident, or possibly the manufacturer accidentally put some mag primers in your pack.

 

[Corey Schwanz]

Check the case length like mentioned and maybe check for neck tension. The brass may not be holding the bullet as tightly and you are getting some bullet shift when you chamber it?

 

[el matador]

Another vote for case length. They will stretch after each firing and eventually will cause problems if not trimmed.

Different brands of brass have different case capacities as well. You could get pressure with some brands but not others.

 

[MudRunner2005]

Check case length. Maybe too long and is crimping when you close the bolt.

This was the first thing that went through my mind when I read the OP's post.

 

[AKdan]

Thank for the replies. I went through my brass and measured fired and unfired brass. All dimensions are as to be expected. Brass length averages 2.005" (SAAMI =2.015) These are Nosler brass (300 pcs) They are about the 4th firing on them. I have not trimmed them only prepped case mouths They came from nosler this length. The length to shoulder is per SAMMI spec. Pre-fired cases are within -0.002 of saami specifications. Fired brass are only a few thousands larger in diameter but as far as length everything appears to be uniform. I use a bushing neck sizer and a body die to size for smooth feeding and consistency. I checked the neck tension and neck thickness. No variation there. I gave the rifle a thorough cleaning and inspection. I took apart several loads and checked powder charge. All charges were within 0.1 gr. I also rechecked to make sure I was not jammed into the lands. Seat depths were within 0.003 at 0.020 off the lands. I took the rifle out behind the house after I cleaned it and remotely fired a string of 5 shots. The first two show no pressure signs. #3 started showing faint ejector marks, #4 was a little more noticeable, #5 had made a full circle ejector mark and slightly deflected rim. I stopped there. The shots were 2 min apart. This is a heavy barrel 308. It is a shilen barrel chambered with a match reamer. The brass was used in another barrel for the first 2.5 firings then full length sized when the new barrel was put on. total of 500+ round count on the new barrel. It didn't like the 168 so I switched to the 185 VLDs. It has a 1:10 twist. I am using 42 grns varget and the bullets are loaded a COAL ~2.984 (I us a Comparator for seat depth). Pressure signs started about 43.5 gr. MV is a tame 2570 fps. I may take it to my smith to scope the chamber.

Again thanks for the replies you guys are great!

 

[jomiba]

Are you single feeding or using a magazine box? Annealing? Shot in the dark here but perhaps bullet is being setback on recoil and chambering if your rifle is a repeater. If youre not annealing, using the same neck bushing everytime will not get you the same neck tension do to work hardening after repeated firings and sizing causing there to be insufficient neck tension. Its odd that it happens after a couple shots which makes me think something is changing during your course of fire and thats the only thing i can think of.

 

[AKdan]

The rifle is a repeater, but I have been single feeding all the rounds. I usually only magazine feed while hunting. Annealing is the next thing that I need to learn how to do. I think I may pick up a box of cheap ammo to see if I can repeat the issue. I guess it is to the old eliminate variables until I find the culprit.

Thanks

 

[el matador]

So you're allowing a couple of minutes between shots for cooling. At what point are you loading the next cartridge? If it's sitting in the hot chamber for those 2 minutes that could be the problem.

 

[ShootnMathews]

So you're allowing a couple of minutes between shots for cooling. At what point are you loading the next cartridge? If it's sitting in the hot chamber for those 2 minutes that could be the problem.

+1. If the cartridge is sitting in the hot chamber a while you'd be heating the powder up considerably. Although in my experience Varget is very temp stable

 

[MNbogboy]

Was it raining or snowing?....wet cartridge cases can show wild signs of over pressure....Several discussions have been done on this subject here and on other forums....Personally I have had this happen and it was scary..

Just a thought,
Randy

 

[AKdan]

Thanks guys. The quandry still looms. The rounds were in the chamber just long enough to close the bolt step away and pull the trigger. Rounds were kept outside to keep me cool and ambient air temp was approx 15 degrees F. I have not seen much change temperature wise with varget. This is the reason I use it. I use the gun for smaller large game and winter predator hunting. It was dry out during the test. I have seen moisture problems on rounds before with different firearms. I pulled all the bullets and dumped the powder into a separate container. All projectiles weighed correct and powder charges were within 0.1 all the way across. I am curious if I got a hot batch of varget???. I had only been able to find and purchase in 1 lb containers in the last few years locally. I've heard stories about hot batches of powder, but never experienced it. I am going to try some ladder tests when I get a chance to retest the load. I may test that batch of powder against another to see if there is a difference. Then again, Maybe just maybe, if I can convince my wife the gun is unsafe maybe I can get a fancier new barrel and maybe a change of caliber to some new fancy 6.5 ..... Not likely but it's worth a shot

thanks guys you have been loads of help. Appreciate the feed back and helping me narrow things down.

 

[cowboy]

If that was my rifle/situation I would be sending the next test rounds through a chronograph to see what my velocity is doing.