Pressure question.

[ARIZONA_F_CLASS]

I have been shooting 190 SMK's @ 2,750 and have some looser primer pockets but they still work without gas leakage. So far I have had only one fail. And during testing blew one primer out of 1,500 rounds fired. The cause was not enough neck clearance in the chamber. .003 I am using .005 now and am thinking I may need to turn them down a little more.

One thing I have noticed after Raton was I reloaded a box of 50 that I had shot there. Only about 8 rounds were marked as being loose. After I reloaded them I found out that all 50 were loose. Could that be the elevation? But They still shot fine last weekend with no gas leakage here in Arizona.

My question is. If it takes around 5 firings for the brass pocket to become a little loose ( not falling out and still functioning) How close to the edge do you think I am?

I have tested all the way up to 45 grains of Varget with no pressure problems. And I tested with .02 increments and 5 shot groups. The load I am using is 43.8 of Varget. I get 1/2" vertical at 500 yards.

 

[ARIZONA_F_CLASS]

Also I have been told that the Lapua Palma brass (SR) is much stronger in the primer pockets. Is that true?

 

[bheadboy]

not knowing the cartridge you are using i can offer no comment on the load, i am not convinced neck clearance has anything to do with it as long as you clear, the instant of ignition the case expands to the chamber/neck area and only if the neck has a squeeze on the bullet will this cause a problem

however, i have shot 10's of 1000's rounds and only when pressure was too high did i discover loose pockets that soon, with the exception of very soft brass (rws,etc.), never r/p, win, lc, gi, lapau

changes in temp can alter the pressure of a load, being safe at one temp and hot at a higher temp.


Bob

 

[ARIZONA_F_CLASS]

Sorry forgot .308

 

[bheadboy]

Sierra shows 41.5 gr max at 2500fps with a 26 inch bbl,with the smk's
Hornady with Varget shows 44grmax compressed but with a Hor btsp, at 2500,

i think you are on the high side and would consider backing off a bit.
bob

 

[Steve Blair]

Also I have been told that the Lapua Palma brass (SR) is much stronger in the primer pockets. Is that true?

[br]Yes. Nothing except a drastic overload will loosen those pockets. I have some with 14 firings and no discernible loosening.

 

[TonyR]

with the exception of very soft brass (rws,etc.),

I'd be interested to know what RWS brass you found to be soft. Norma maybe but every RWS case I've worked with has been tougher than anything else I've tried of the same cartridge. Just my experience, but I use it whenever I can just because the primer pockets have held up so well compared to others I've tried. I hope we will be able to get it again here in the USA.

As an example and to address the OP's question about pressure, I used both Norma and RWS with a 7mm/300WSM I have and the RWS required an additional 1.5 grains of powder to get the same extractor groove expansion on the first firing compared to Norma. I believe that extractor groove expansion is one very good way to judge the pressure of a load. It increases very quickly after the first half thousandth as the charge increases.

 

[bheadboy]

300wm was the worst.

Bob

 

[TonyR]

300wm was the worst.

Bob

Interesting. I've got a bunch of that but haven't built anything with it yet. My next project uses the 8X68 case.

 

[Keith Glasscock]

Rocky,

I only use .0025 neck clearance in my 308 f/tr rifle. I can run some pretty scary velocities without pressure problems. That is, until I get a carbon ring in the neck or throat.

I'd look at it this way: if it is loosening the pockets, but not blowing primers or leaking gas, you are as hot as you can go. I'd say that primers loosening in 5 firings is indicative of a max load.

If you are bothered by the short case life, you could change bullets. The 185 jugg should give you better bc with lower pressure and equal velocities.

Just a thought.

 

[XTR]

One thing I have noticed after Raton was I reloaded a box of 50 that I had shot there. Only about 8 rounds were marked as being loose. After I reloaded them I found out that all 50 were loose. Could that be the elevation? But They still shot fine last weekend with no gas leakage here in Arizona.

It's not the elevation it's the heat. I don't see elevation having effects on internal ballistics. Temperature on the other hand...

I think that pretty much every day we shot the temp on the line went into the mid 90s, and unlike at my home range my stuff wasn't in the shade between relays. Most all of us left our kit in the sun (there is no shade). The point is that with no shade and the sun baking on the dirt everything got up to temperature. If you had a bullet made with a thermocouple inside I'm betting that most of us at home rarely see the core temps get to 90, and I'd wager that if you weren't keeping your bullets in a cooler (I did for the FCW) you were getting temperature effects. My loads for the FCW held good vertical, my loads for the FCN were pittiful. I didn't ice them, just kept them on a big cold pack in a small soft sided cooler that started the day at whatever temperature it got to being left open in the truck at night. Bullets no get hot and worked better.

 

[ARIZONA_F_CLASS]

One thing I have noticed after Raton was I reloaded a box of 50 that I had shot there. Only about 8 rounds were marked as being loose. After I reloaded them I found out that all 50 were loose. Could that be the elevation? But They still shot fine last weekend with no gas leakage here in Arizona.

It's not the elevation it's the heat. I don't see elevation having effects on internal ballistics. Temperature on the other hand...

I think that pretty much every day we shot the temp on the line went into the mid 90s, and unlike at my home range my stuff wasn't in the shade between relays. Most all of us left our kit in the sun (there is no shade). The point is that with no shade and the sun baking on the dirt everything got up to temperature. If you had a bullet made with a thermocouple inside I'm betting that most of us at home rarely see the core temps get to 90, and I'd wager that if you weren't keeping your bullets in a cooler (I did for the FCW) you were getting temperature effects. My loads for the FCW held good vertical, my loads for the FCN were pittiful. I didn't ice them, just kept them on a big cold pack in a small soft sided cooler that started the day at whatever temperature it got to being left open in the truck at night. Bullets no get hot and worked better.

It was a bit toasty there. And I'm from Arizona. It was not what I expected or had hoped for. lol