Overpressure problems
Need a little help here. Building a couple of Turks in 308 Sporters. I've done two rifle projects so far with no problem but I'm in a quandry now. Using a Clymer reamer I reamed my barrels to a point that I would just "feel" the go guage when I closed the bolt.Right at the bottom of the bolt throw. When I tried to chamber some Lake City Mil.ammo, bolt wouldn't close without considerable pressure. Rem. and Win ammo chambered better but still had a tight bolt. Reamed them deeper. Now the bolt handle drops on the go guage but won't close on the NO-go. Now Lake City rounds chamber "slightly snug" and the Rem and Win. rounds chamber easy. Fired three test rounds. Cratered primers and bolt very stiff on opening. Bullet is not into the lands and I'm not using hot handloads. Anybody know whats happening here....
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My guess is surface roughness in the chamber from tool marks. If you can get a rubber wheel in there and polish it bright, it should help.
It sounds like you have the headspace right.0 -
v35. That gives me a place to start. Last bad chamber I cleaned up I used fine grade lapping compound and fine steel wool. Rusted chamber polished up great. I'll hit it yet tonite. Anyone else have any ideas?????????Thanks v 0 -
sealyon:
You are mixing 7.62x51 NATO brass and .308 Win brass. They are not the same, and the tolerances you are using are close enough that the differences are apparent. 7.62 NATO brass has a longer headspace length, the brass is thicker resulting in lesser case volume (and higher pressure from the same bullet/charge combo than you would get in a .308Win case).
Which guages are you using? 7.62 NATO or .308 Winchester? Judging by what you say, you are using .308 Win, and setting your chamber up pretty tight. The 7.62x51 NATO brass is bigger than the .308 Win brass.
.308Win Headspace guages: 1.630" go 1.634 no go 1.638 field reject
7.62x51NATO 1.635" go 1.6405 no go 1.6455 field reject
7.62 average case headspace is about 1.632, or longer than your match tight .308Win chamber. When you lengthen the chamber, the 7.62 brass fits.
You are not saying which brass you were using when you had the sticking bolt and cratered primer.
A sticking bolt can be overpressure, or it can be headspace so excessive that the elastic limitations of the brass are reached and it doesn't return to nominal size after firing. The cratered primers are more troubling.
There is also the issue of barrel leade. Some experiments show that barrels with excessive leade can produce erratic pressures, and can sometimes cause drastic over pressure.
Remember that the .308Win cartridge, even at medium loads, is still producing pressures significantly greater than the cartridges these rifles were designed to fire, so you are pushing the envelope, and little things may therefore have big consequences.
Which brass were you shooting?
redcedars0 -
Sometimes, the locking lug recesses in the action get pounded to the rear. The bolt closes tight as it rides over the edge of the recess and then has slack when completely closed. This can cause the situation you described as the case stretches to fit the longer headspace and then is "sized" as you open the bolt. The case may not show a typical excess headspace measurement as it has the shoulder reset when the bolt opens. If this is the problem, there is no recourse other than "recutting" the recesses which isn't a recommended practice.
Mobuck0 -
sealyon,
Primer cratering can be two things.
- Excessive headspace and pressure from the same.
- The firing pin hole needs to be bushed and re-drilled to the
correct size for the firing pin. This should be done on all
military Mausers all the time when you move up to a higher
pressure cartridge. Too many avoid or ignore this step when it
should be done as a matter of course.
The other posts answered your headspace question.
Regards.
redcedars,
I would like to see the reports on the experiments with "long leades and high pressure". Can you point me to the right place or is this something that you have formulated on your own and can pass on to me?
Best.0 -
redcedars...Thank you. I learned somthing new and I always appreciate that. I knew brass thickness was different but not headspace. It was the Lake City rounds that were cratering. I'll polish the chamber and fire some Rem's. and check it out.
Mobuck.......I checked for bolt lug setback as soon as I disassembled the Turk. Looked good but I've been wrong before. If I continue too have problems I'll take it down and recheck. I wouldn't put a 308 together on a normal small ring but the Turk has a thicker receiver ring than a standard 98 and a lot better metal than the Spanish 98's you see in 308.
Thanks to both of you for the imput. This is why I like "Expert" forums.0 -
Nonosense....I replaced the bolt body first thing because I thought the firing pin hole looked excessive when I started. Very stiff bolt on opening led me to beleive it was an overpressure problem....Thanks for the reply. I'll keep everyone posted on what I find....... 0 -
Mobuck:
Another reason for a sticky bolt, and a new one to me. Very interesting.
Nononsense:
Mystery Solved, Charles E. Petty, Handloader #187, June 1997. Lots of speculation, not all of which is justified, but very interesting nevertheless.
redcedars0 -
redcedars,
Thanks, I'll read it.0
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