How to headspace?

[gravityfan]

I have a semi-auto conversion MkIm, and it closes (easily) on a Field gauge. No wonder it was ripping the heads off!

So, the locking shoulder is too short, obviously, but by how much? How can I work out/measure what size of locking shoulder I need to bring it back into spec? I can't see a way to get anything in there to measure the remaining gap, as it were.

Also, surely changing barrels must mess with the headspacing, unless all Bren barrels are machined to exacting tolerances - which I'm sure they were not. Have I got that right? Is there an approach to the headspacing where one just tries different barrels until one happens to pair properly with the locking shoulder to produce an acceptable headspacing? Surely the design is better than that.

Can anyone give me guidance or point me to an informative article?

Thanks for any help.

Information

Warning: This is a relatively older thread This discussion is older than 360 days. Some information contained in it may no longer be current.