Gunsmithing 30 Sherman chamber quandary

[cast1]

So I’m chambering a 30 Sherman to you with. Chamber is perfect per the supplied gauge, which is lasered “Manson-30 Sherman Magnum”. Bolt falls closed on it, can be closed with moderate effort with a .0015” piece of scotch tape on it. After torquing, should have about .0005 clearance on the Go gauge.

Thing is, it falls closed on a piece of either loaded factory 300 PRC ammo or Hornady PRC brass. It occurred to me that possibly I was supposed to chamber short on this gauge, but I have a SAAMI 300 PRC gauge and it is definitely longer than the one for the 30 Sherman. What would y’all advise to do? I can take material off the shoulder and tenon to problem, just want to do the right thing.

 

[357Max]

Factory brass is usually .006-.008 short of a go guage. I’d add scotch tape to the back of a pc of that brass and see how many it takes to get resistance. If 2-3 then it’s GTG.

 

[Steel head]

Yep
Tape trick on brass.

If your worried about excessive initial expansion just fire form with a mild load and a good bit of bullet jam.

 

[iceng]

+1 what they said.

 

[cast1]

So ended up setting it back to .003” short on my SAAMI 300 PRC reamer, like I would any other Ackley-esque build and all is well. Little crush on the factory Hornady brass. Should form up just fine.

 

[iceng]

Be aware that Ackley chambers call for SHORT chamberings. Your go gauge becomes the no-go.

Often people chamber with a 4thou shim between the recoil lug and barrel, and complete the headspace/machining that way. When it comes to final assembly, remove the shim and your headspace will be tighter.

 

[cast1]

Be aware that Ackley chambers call for SHORT chamberings. Your go gauge becomes the no-go.

Often people chamber with a 4thou shim between the recoil lug and barrel, and complete the headspace/machining that way. When it comes to final assembly, remove the shim and your headspace will be tighter.

Correct. Go gauge in chamber, bolt in the action. Tighten it down till it stops on the go gauge. There is a .003” gap between the recoil lug and the tenon. The chamber is .003” short of what the non-improved chamber would be, allowing a thousandth of an inch for torquing the barrel to settle at .004” short. Good to drive it home for any passers by.

 

[Wannashootit]

Whenever I want to know exactly what the clearance is between boltface and gauge, I stick a piece of Plastigage on the boltface.