Perhaps headspace is not the correct term,I am looking for what the gap between the block and the barrel should be.I am going to rebarrel a No1 that is currently in 220 swift to 6.5-06 but cannot find any info on what the gap should be.

regards

Griff

When I rebarrel #1s I strive for .0005

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A great many people claim that the best accuracy is achieved if the there is no contact between the breach block and the barrel AFTER, a case is loaded into the chamber. A certain amount of back force is needed to load up the locking block. So simply closing the block on an unloaded chamber does not really tell you to much. I have always sort of stuck to this rule of thumb myself but others will do things differently and no one has really convinced me that one way is better than the other. There is a point of course where to much space becomes dangerous. I should think that anything over .010 inch of clearance with a case in the chamber is possibly not unsafe, but is pushing it a lot on what I consider to be good and logical machining practices. I should think that if you can run a .001 feeler gauge between the barrel and block when the block is closed, but not a .004 inch gauge you should be in the ball park.

There are also a number of people who believe that the block should pre-load against the barrel. Again, I can't say if this is the best way to go about it or not. I have always avoided doing this myself for the simple reason that I don't want a guy coming back on me when he can't chamber a round because a cleaning rod bristle or a bit of grit got between the block and barrel and he missed an opportunity on a new, world record, Zimbabwe Death Skunk. Number 1s are not varmint or target guns. They are hunting rifles for shooters who can't be trusted with more than one round of ammunition at a time.

I just measured two factories; one is .017 and one is .007. I would not make it below .001, and for good machining practice, not much over .005. No, the block should not bear against the barrel.
On the M256 Tank Cannon, the block is about .375 inches away from the breech face.

Thanks thats great info.I am probably to to strive for 3thou

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I have never considered that to be a critical measurement and obviously from dpcd's findings of .007 to .017 inch, Ruger doesn't either. I am however positive that enough of a gap should be left to allow for the off chance that some sort of foreign particle can't get in and jam up the works when you need the gun the most. .003 should be fine. There are some that feel that the smallest gap possible should be used to prevent gas escaping in the event of a case failure. I have personally never experienced that problem with any Number 1's and in the half dozen catastrophic failures I have seen, they all preformed the same. In all cases the top end of the breach bock set back from .015 to .050 inch but the web of the cases did not fail and there was no gas leakage what-so-ever. Two of these failures were with 378 Weatherby case wildcats and one on the Ultra Magnum family of cartridges. The one failure was on a gun that I built in 30x378 Wolff and was loaded with a 22 caliber Sabot bullet. The gun was shot over a chronograph when things went south and the velocity was just over 5000 FPS. The lettering on the case was almost wiped out completely and the primer was the size of a pea so I would estimate that that the pressures were in excess of 110,000 CUP. Far to much fast burning dragon dung was the obvious cause of the confumblement.

I always aim for .005 clearance between the block and barrel breech. I add a couple of pegs to the lever, bearing on the bottom of the block, to positively locate the brechblock to center the firing pin. Some factory set-ups show a lot of clearance and I doubt that even .025 would hurt anything but it just looks sloppy. Regards, Bill

One hundred thousandths wouldn't hurt anything. That way, no need to cut that pesky extractor slot.

quote:

Originally posted by dpcd:
One hundred thousandths wouldn't hurt anything. That way, no need to cut that pesky extractor slot.

And it would probably work just fine too! LOL

I often think I should splash on a few ounces of deer musk or skunk scent before I head down to the DMV to get in the 30 minute lineup to pay a ticket or do my license renewal. I know I would be at the head of the line in less than two minutes. But I still don't do it. Though I'm not sure why.

LMAO ROFF