Question about flattened primers?

[MSLRHunter]

I have a Rem.700 5R in 300 winmag that I am working up some loads with. Today, I was shooting 210 Bergers, cci 250 primers, 2x fired Winchester brass, and varying loads of Retumbo. The max load of Retumbo for this bullet is 77.2 according to the folks at Berger. After shooting, I noticed that the primers with the 77.0 charge were flattened, however there was no cratering on the primer, and I didn't notice a sticky bolt lift. Also there were no ejector marks on the case. So, if this is all the pressure signs I have, should I slowly work up to the max of 77.2? Or is this where I should stop?

 

[boomtube]

You probably don't have a pressure problem, more commonly it's a FL sizing problem from setting the shoulder back too far.

Normal case stretch during firing pushes the case back over the primer but the chamber pressure can prevent it from sliding back in. So, the top gets squashed a little. FL sizing correctly controls case stretch and your primers won't pop out and then get "flattened" with normal pressures.

Do a test. FL size a case exactly as you normally do. Prime and fire it, then examine the fired primer. If it's standing proud, out higher than the case head, you need to back your sizer up that far; measure it with your caliper. A 1/8th turn moves the die 9 thou.

 

[MSLRHunter]

I do FL size my cases, and I set the shoulder back 3 thousandths from my fired brass, measured using my Hornady headspace gauge, so I dont think improper sizing is to blame. I am going to assume it is some early pressure signs and slowly and carefully work up until I get some other signs of pressure.

 

[boomtube]

"... I set the shoulder back 3 thousandths from my fired brass, "

Well, that's 3 thou more that they need to be, considering the shoulders have already shrunk back some 1-2 thou from chamber size.

 

[renegadelzard]

"... I set the shoulder back 3 thousandths from my fired brass, "

Well, that's 3 thou more that they need to be, considering the shoulders have already shrunk back some 1-2 thou from chamber size.

really?...not being a smartass here, i just really didnt know that. I just ordered some possum hollow trimmers, and they index off the shoulder...should i account for this when im setting up the trimmers?

 

[MSLRHunter]

"... I set the shoulder back 3 thousandths from my fired brass, "

Well, that's 3 thou more that they need to be, considering the shoulders have already shrunk back some 1-2 thou from chamber size.

Thanks for that insight, I didn't consider that. So do you recommend not moving the shoulder when you full length resize? Doesn't this make your cases more difficult to chamber? One of the reasons I full length resize rather than neck size is so that my cases are sized the same every time and I don't have to bump shoulders back every 2-3 times when they become difficult to chamber.

 

[Loner]

A belted cartridge head spaces off the belt until it is fire formed in your chamber.
Setting the shoulder back will not change the headspace from what it was as a
factory round or piece of new brass. However as stated you are stretching your
brass and shortening it's life by setting the shoulder back each time. Neck size only
until the bolt lifts a little hard. Check a few rounds as they come out of your neck sizer
die and if they chamber, neck size them all and load them up. If not bump the shoulders.
I bump the absolute minimum it takes to chamber. .003 is enough for any semi auto,
too much for a bolt gun.
.2 Grains of powder in a case that size is not going to noticeably change pressures.
I wouldn't be working in less than .4 grain increments in a case that size. (about 5%)
So you are, for most practical purposes, at max with the 77 grains.

 

[Kevin Thomas]

MSLRH,

Primers are one of the most commonly used indicators for pressure, but it's impportant to understand that they'll lie to you like an incumbent politician. As Boomtube's already pointed out, the primers can flatten due to sizing issues just as readily as they can to high pressures. You cited the lack of cratering, but this too can be caused by other factors unrelated to the pressure per se. Firing pin fit (or more accurately, a poor fit) can yield very pronounced cratering even with perfectly safe loads. Bolt lift is another that can be useful, but must be used with some discretion. Basically, some cartridges will give sticky bolt lift when pressures start approaching the upper limits, while others adamantly refuse to to stick even when sane pressures have been left far behind. This has to do with case geometry and tapers more than pressure alone. Cases with more built in body taper (22-250, 6mm Rem, 6.5x55, etc.) will stick far more readily than those with minimal taper (think Ackley Improveds here). On cases like the Gibbs family, it's **** near impossible to get sticky bolt lift even when the loads have gone well into the proof-load level.

Bottom line here is, there's a range of pressure indicators that we need to pay attention to, but don't get too focused on any one by itself. Take them as a whole, and get the big picture. Sticky bolt lift, deep ejector marks on the case head, high velocity, flattened primers with pronounced craters, yeah, you probably have a pressure problem. Flattened primers with no cratering, mild velocities and no other pressure indicators . . . probably isn't a pressure problem. Look at all the signs, and base your decisions on the combination of them.

Kevin Thomas
Lapua USA

 

[300weatherby]

I had a budy that shoots a 300RUM, he was asking me if I thought flattend primers were a sign of over preasure... I hadnt even thought about that, when I work up loads I look for cratering of the primer, stamped brass or hard lifts. So I started looking at fired brass in the big magnums and found that all most all of them even factory ammo had flattened primers.