PDA

View Full Version : Is this pressure indicator?



Jim Kobe
04-10-2011, 10:55 AM
I recently assembled a 308 for a friend, headspaced minimum on new barrel, trued action, etc. He called the other day saying his brass has stetched and his primers have backed out, even had one blown primer. He claims he is using new ammo and new brass and it still is happening. Reloads, according to him are 4-5 grains under max. I have not had a chance to check it but I am under the impression he is using brass and ammo that is sized too short. He says bolt lift after firing is very gentle. To my way of thinking, it ain't pressure problems. What say you?

Jim

Herzo
04-10-2011, 11:13 AM
Headspace???? If it is occurring with new factory ammo.

He could be pushing the shoulder back on his reloads.

jackie schmidt
04-10-2011, 12:38 PM
The one major thing that will cause a primer to back out is too much case to bolt face clearance. The firing pin pushes the case forward, the case grips the chamber walls upon ignition, and the primer, not being against the bolt face, backs out.

The big question is how is he arriving at too much bolt face to case head clearance.

If he shot brand new Factory Ammo, then the first inclination is that you messed up, the chamber is simply too deep.

The second is he stated he used brand new ammo, but was this brand new cases that he ran into a die? Is he arriving at way too much shoulder bump by what ever he is doing?

What ever the answer, tell him to CEASE what he is doing, before he blows the head off a case. Then find out what is going on......jackie

Butch Lambert
04-10-2011, 02:39 PM
Jackie,
Jim Kobe has been doing this for a long time. If he says the headspace is right, it is right.
Butch

jackie schmidt
04-10-2011, 03:33 PM
Well then Butch, he has a client that is quite a few coins shy of a full bank account.......jackie

Boyd Allen
04-10-2011, 03:36 PM
If loads are too light, a case may be knocked forward when the primer is struck by the firing pin, and pushed farther by the primer exploding and pushing against the bottom of the primer pocket. This is more of an issue with smaller shoulder angles. At that point, the pressure generated by the powder may cause the case to stick to the chamber walls and if there is enough pressure, the case will stretch near the head and come back to the bolt face. With a light to medium load, sometimes this stretching does not take place, or does not fully take place, leaving the primer protruding. If the case is then reloaded with the same load. The primer will stick out more than the first time. With rimless cases, with flatter shoulder angles, it is a good idea not to go too light on your loads, or this will happen. BTW this is not a guess. I have done the experiment with an '06 in a Springfield, loading mid book loads, and neck sizing.

Butch Lambert
04-10-2011, 04:30 PM
Jackie,
I don't know about his friends dollars and cents[sense], but I think Jim was probably correct on his guess. We will hear when he gets a chance to inspect it.
Butch

Rustystud
04-12-2011, 03:27 PM
Jim:

You might check and see if your customer bought a "tight bore .297 or .298 barrel. They are designed for Palma shooters who are limited to 155 grain bullets that have a shorter bearing surface. The standard bore for a 308 is .300.
I have had customers buy tight bore barrels thinking they are doing themself a favor and it in a combination with longer bearing surface bullets i.e. 185-210 bullets have all kinds of pressure issues especially on warm days.

This may or may not be a cause for his issues.
Good Luck

Nat Lambeth

zippy06
04-12-2011, 11:17 PM
5 grains below max. might be tooo light.....
Tell your buddy to buy some New Federal Ammo.......

Jim Kobe
04-13-2011, 09:33 AM
Mystery solved. I had trued up the action and bolt face and after the rebarrel, I checked the headspace with the bolt stripped, no ejector pin in place. I discovered that the ejector pin was protruding into the cartridge head when chambering a round; it did not recess fully into tyhe bolt. How it did that when I normally remove only about .003"- .004" from the face of the bolt is beyond me. You live and learn.

zippy06
04-14-2011, 12:29 AM
Cool Jim.
This is a new one to me. But, I am not a gunsmith.
Good job.

JerrySharrett
04-14-2011, 07:09 AM
I recently assembled a 308 for a friend, headspaced minimum on new barrel, trued action, etc. He called the other day saying his brass has stetched and his primers have backed out, even had one blown primer. He claims he is using new ammo and new brass and it still is happening. Reloads, according to him are 4-5 grains under max. I have not had a chance to check it but I am under the impression he is using brass and ammo that is sized too short. He says bolt lift after firing is very gentle. To my way of thinking, it ain't pressure problems. What say you?

Jim
Jim, I wonder if the "new" ammo is US commercial or imported. There is a bunch of imported here now that never heard of SAMMI !!