Ream or New Locking Shoulder

[sundance8]

Hello, I'm new to the forum but have been building for awhile now. I guess I've been lucky since I'm building my 4th Semi-Bren and have not had this problem before.
Too tight of a space between the locking shoulder and barrel will not allow the bolt to lock up. Is it easier or more recommended to try a bunch of locking shoulders or just ream and adjust the barrel? I don't know what kind of variation there is in the locking shoulders in order to give me enough clearance. Trial and error I guess.
Any advice from those that have had the problem would be welcome.
Thanks, Steve

Information

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[WallyG.]

Easiest starting point will be adusting the gap with a shorter locking shoulder... a source for a large range of sizes in the US is: 30: Bren Locking Shoulder - BRP CORP Store

[sundance8]

Thanks WallyG. Ordered them today. Now to get that pesky screw loose.

[Peter Laidler]

You are describing insufficient CHS. If I'd answered first I'd have asked you a few Q's to start with. For example. WHEN does the breech block fail to lock up? Is it when you are using the the .064" GO gauge? Then I'd ask, by how MUCH does it fail to lock up? What happens when you try a different barrel? Does a different barrel make it tighter or looser.

Answer those and I'll tell you the rest.

[Brit plumber]

If this is a reweld, are all the dims correct?

[Peter Laidler]

Then there's the breech block stops and running clearances and all sorts of things that are associated with it.

Maybe Tankie and Skippy could come in here too. They're Bren era Armourers too...........

[ActionYobbo]

when I got a spare barrel for mine the HS was too tight
because I was set up with 1 barrel I made the second barrel fit by reaming and shortening the breach end so the bolt would close

[WallyG.]

I'm no expert and I have not stayed at a Hotel lately... My reply presumed that this was a US reweld and a large pool of barrels/bolts to try was not an option. By default the other adjustment point that can be changed out is the locking shoulder... and at a relatively reasonable expense. This is a crude adjustment but might be appropriate for a receiver that has been cut and rewelded at multiple points affecting this critical interplay of the components. The rest of the geometry issues and a proper set-up proceedure are best left to the pro's like Peter, Tankie and Skippy.

[sundance8]

It fails to lock up with or without a .064 gauge. It is my 4th reweld but the only one with a problem. Heat shrinkage finally got me.
It's difficult to get a measurement that makes since, but here's what I get.
With a .064 Go gauge in place I get 4.355" from the face of the gauge to the locking shoulder. The area I measured to on the locking shoulder was just where it breaks over the edge. Hope you can visualize what I mean.
I have two barrels that fit and both give the same dimensions(within a .001).

On one of my Semi's I had a similar problem but by milling a few thousands off the bolt, where it mates to the locking shoulder, I was able to get proper lock up and it shoots really nice. Didn't want to do that this time since I fear I need more than a few thousands and I don't want to mill thru the case hardening.

Thanks for all the suggestions so far.

[Peter Laidler]

It just sounds to me like locking shoulders won't fix your problem because the variation between the selection is insufficient to rectify the shrinkage you have. I don't think that the shoulders are case hardened. They are simply tough material and I'd start by surface grinding, say, .010" at a time off the angled locking surface until the breech block JUST closes over the .064" gauge. That way, you have retained the working geometry of the breech block in relation to the breech, piston post and therefore mechanical safety and so on and on. Not that I have ever welded one up again. But cut a few up! and repaired about a zillion of 'em