Gunsmithing AR10 chambering ?

[Rthur]

I guess it is not so much a chambering question as a chamfer question. I would like to hear from the hide what angle and how deep a chamfer/angle you cut at the back of the chamber to achieve reliable feeding. I am using a Bat Machine extension. The caliber is 6 creedmoor. Any help would be greatly appreciated.


Rthur

 

[Turd in the Pool]

The bbl extension is technically the feed ramp. The chamfer is really just a formality. I've never seen one that could make it past the extension and miss the hole regardless of the angle or lack of. Just don't go too deep!

 

[ugsly]

I chamfer the chamber edge and counter bore edge just enough to break the sharp corner. I touch off on the edge and go in .005 and cut with the compound backing out with a very sharp boring bar at a 29-30 degree angle.

Kc

 

[Rthur]

Thanks guys, I was using a JP 308 barrel as a comparison on the cut at the back of the chamber. RJB I want to leave as much support as possible at the head of the case. This may be a case of the 6mm neck not riding up the extension as well as the 308.

R

 

[Delta4-3]

Cut it shallow like stated above, then round the outside edge with emery cloth, then polish the barrel face where the bolt contacts. I have used a deberring tool lightly on 50 or so barrels then polished and had great results.
Also, lap your lugs like you would on a bolt gun. I have found it to be more beneficial on an ar than I have bolt guns. I'm sure most here will argue me on that, but it's my personal experience.

 

[Rthur]

I would also call this the meplat clearance. With berger 105 hybrids it will contact the tenon face before finding the angle cut at the back of the chamber. I would say that the smaller bullet on the 6cm is going forward further in the extension.

R

 

[Turd in the Pool]

RJB I want to leave as much support as possible at the head of the case. This may be a case of the 6mm neck not riding up the extension as well as the 308.

R

No need to reinvent the wheel. As long as you have no more than .150" of the case head exposed, you are good. I'd be surprised if the 6mm bullet fitting in the mag could reach the bbl before the shoulder would kick it up.

 

[Rthur]

Not reinventing the wheel, FYI the berger 105 hybrid will reach the barrel before the shoulder will kick it up. This is with a col of 2.800ish.

R

 

[Turd in the Pool]

Not reinventing the wheel, FYI the berger 105 hybrid will reach the barrel before the shoulder will kick it up. This is with a col of 2.800ish.

R

Could be but I'm inclined to be skeptical. Wish I had one handy to look at. If it were that critical, I'm sure we would see more 308s that wouldn't run.

What is the length from the shoulder/neck juncture to the bullet nose?

 

[Rthur]

Per your request. Meplat to base of shoulder at neck 1.160ish, Meplat to major dia at shoulder 1.350. The loaded neck dia .268.

R

 

[Turd in the Pool]

Shouldn't he a prob. Base of neck to tip is about the same as my Palma 308 load. Once the shoulder hits the extension it will kick it up. I'd be more worried about that skinny bullet hanging on the extension lugs but you can work around that.

 

[Rthur]

RJB I think the problem is the neck diameter not riding as high as the 308 on it's way up the extension. Thus going deeper towards the tenon on its way to the chamber before the shoulder rides up the extension.

R

 

[Turd in the Pool]

R,

I've been wrong more than once (though it pains me to admit it) but the neck dia. is irrelevant. If the bullet nose isn't hitting the bbl before the shoulder hits the extension, there is no issue. The meplat is a point just like a fmj. The mag, shoulder and extension dictate it's path to the chamber. My semi is an HK clone but I have a bud that runs the same load through both of his AR10's and they run fine. Worst case is you'd have to seat the bullet a little deeper.

Not that this relevant to your case but I've installed a ramp on at least half a dozen ak's that had the problem. Kicking up the shoulder is the key.