Blown Primers

[LVLAaron]

Was verifying a load I worked up on a 70 degree day. Wanted to double check velocity on a hot (90 degree) day.

This is the load I'm checking.
Alpha SRP 308 brass
Berger 200.20x
Varget 44.1
Federal small rifle match primers

Loaded 5 charges,
43.3
43.5
43.7
43.9
44.1

Lost a couple of primers. I know it wasn't smart to continue so please withhold those comments.

43.7 got me to the right velocity (2690)

Gun is a Savage 12 action. New bartlein barrel w/ about 200 rounds through it. Got the barrel up to temp with ~20 rounds of my usual Juggernaut load, shot great. Barrel temp was up to 120 degrees when these groups were fired, same temp as the sample group in the first photo. 120-130 is where the barrel normalized on a hot day shooting a match so that's where I test.

Anyway, I'm stumped. I've fired much hotter rounds with this setup minus the barrel and the first pressure signs I saw was the notable "circle" indent left in the brass from the ejector. I've seen blown primers before, but the blowout was around the circumference of the primer.

Same lot of Varget I've been using for two years, very little neck tension, SD under 10 on all of the groups today even with the blowout.

Thoughts?

 

[rwj]

Looks more like primer flow around the firing pin into the bolt face hole. It may simply be due to excessive clearance. Did you observe any gas blow by?

 

[LVLAaron]

Looks more like primer flow around the firing pin into the bolt face hole. It may simply be due to excessive clearance. Did you observe any gas blow by?

You looking at the 3 pieces with the center blown out?

 

[NorCalMikie]

Brass snug in the chamber OR, too short and being blown back on firing and primers being punched? Just thinking out loud.

 

[243winxb]

High pressure caused by to much powder.

 

[rwj]

You looking at the 3 pieces with the center blown out?

Yes... there’s a limit to metal deformation... why it was asked if any gas blow by was observed...

 

[beemanjohn]

Those craters are not unusual in a stock Savage. The hole for the firing pin is just WAY too large for the firing pin. Do yourself a huge favor and contact @Grimstod And have him bush the bolt for you. He is famous for his work on Savage bolts. (he now does other bolts too.) He has very fast turn around time and a very reasonable price. Your gun will be safer and shoot better to boot. (no more pieced primers) I have had him do two of them for me, and I am just thrilled with the results.

 

[Dusty Stevens]

I see a blown primer and an ejector mark right in the center of the box. Going from 70 degrees to 90 is way off on a load. Those blanked primers and ejector marks are both good signs of too much powder. A good load at 70deg will be hot at 80 much less 90. No matter what you read there is no such thing as a load that is optimum over that far of a temp swing. It may work at each but is not going to be right

 

[243winxb]

ejector mark

2.

 

[LVLAaron]

I see a blown primer and an ejector mark right in the center of the box. Going from 70 degrees to 90 is way off on a load. Those blanked primers and ejector marks are both good signs of too much powder. A good load at 70deg will be hot at 80 much less 90. No matter what you read there is no such thing as a load that is optimum over that far of a temp swing. It may work at each but is not going to be right

Completely agree on the temperature. That's why I retested with lower charges in higher temps!

Also, I know there are ejector marks on the brass, they are not from this session. This brass has been fired 4x times and I've used it in the past for similar load development on a previous barrel. It's really hard to tell from an iphone photo, but not hard to tell in person, if the ejector marks are new, or from a previous firings.

 

[Grimstod]

Check for case head expansion using some micrometers. They will give you better and more accurate numbers then calipers will. I am not familiar with Alpha brass. If the primer pockets are excessively tight the primers will not flatten before you start to get case head expansion.

 

[David Christian]

@LVLAaron, I think you found your answer. You are probably targeting the high node, yet you might need to back off your powder and find your barrel’s middle node such that a 90 degree day does not blow your primers. I have never found the fastest node to be the most accurate.

 

[varget204]

Your new barrel may have a tighter chamber,anytime around 90 or above ,one can get changes,have other loads cratered out of new barrel,on a hot day ,shells sitting in sun can cause problems.

 

[T-shooter]

The load is probably too hot. I used the 200.20x bullets in Hornady Match brass in a .308. I expanded a case head and blew out a primer at 43.7 grains of Varget. 43.5g was showing signs of overpressure. These cases were about 7% smaller in volume than my older Hornady cases that I retired. I got the best groups with them at about 43.3 grains with a .005" jump with a velocity of 2,540.

 

[Ned Ludd]

That load is simply too hot for the conditions for several reasons. First, you're trying to hit the high node with the 200.20X bullet and Varget. In Lapua Palma brass, that means pressures predicted to be well above SAAMI max for a .308, probably in the 63-64K psi range, or even higher. According to the folks at Alpha Munitions, their SRP brass has an average water capacity of 54.8 gr, which is more than a full grain lower than Lapua brass runs on average. Add in the elevated temps and excessively large firing pin hole on a Savage rifle, and what you observed shouldn't be too surprising. If you dial that load back to the 2640-2660 fps node for that brass/powder/bullet combination, it may not be necessary to have the firing pin hole bushed immediately, even though it's probably a good idea to have it done anyhow. Typical Varget charge weights to hit the lower node with Lapua brass and the 200.20X usually end up somewhere in the 43.2 to 43.6 gr range.

 

[David247]

If the pressure is too much, why are the primer shoulders still so round? It seems like every time I have seen signs of over pressure, the primers become flattened.

 

[brxbrad]

I've been shooting Savage actions in competition for 13 yrs now and never had any primer problems. I've had 6mmBR, 6BRX, 284 Win, 7RSAUM on both my Model 10 and 110 actions. I purchased a new barrel and chambered it for BRX. Shot lights out but kept blowing primers. A bench rest shooter friend told me that my firing pin spring was probably getting weak. He informed me that primers are blown because the pressure against the strike zone at the firing pin pushes back towards the bolt and the piece that disappears from the primer actually blows backwards and will sometimes get stuck in the bolt. That is what was happening on my gun. I replaced both springs in both guns with "new and improved" ones. Same problem with the same gun......Found out my barrel had a tight bore. I normally shoot 33.4 grs of Varget and 105's in my BRX's so I dropped my charges by .1 all the way to 31.4. It still blew 1 out of 10 primers. I put a new barrel on the gun (same caliber, same brand, same twist) and all my problems disappeared. The old barrel is now my fire forming barrel. You can get new springs through Midway. If your charge is not within acceptable limits, I would start with dropping the charge, then the firing pin spring, then the barrel.

Just my two cents....

 

[Drop Port]

The first thing that jumps out to me is SRP and upper node which often = problems in a factory action. If I'm running SRP and upper node I am always running a bushed bolt or custom with an .062" tight fit pin. From there accessing how much is slop and how much is pressure is much easier.

On another note, if I plan on working up a load I might shoot year round (hunting ) I always work it up at the highest temp it will be shot at.

The simple answer for your components and load is, it is too hot a load.

 

[Webster]

Those craters are not unusual in a stock Savage. The hole for the firing pin is just WAY too large for the firing pin. Do yourself a huge favor and contact @Grimstod And have him bush the bolt for you. He is famous for his work on Savage bolts. (he now does other bolts too.) He has very fast turn around time and a very reasonable price. Your gun will be safer and shoot better to boot. (no more pieced primers) I have had him do two of them for me, and I am just thrilled with the results.

Bushing the firing pin hole just hides the signs of pressure. The loads are hot regardless of fps.

 

[LVLAaron]

Check for case head expansion using some micrometers.

Checked my whole batch, they're all .4700 (Mitutoyo Vernier)

@LVLAaron, find your barrel’s middle node

Thought about that. I wanted a little more advantage over the Juggernauts if I'm going to put the time/energy into making the 20x work.


What should my firing pin protrusion be? At maximum poke it's about 65 thou - seems like a lot.