Can someone explain Barrel Nuts and their numbers?

This is in reference to my experience with Semi-auto remanufactured Bren guns here in the U.S., but the question should apply and actually originate from the original full auto Bren gun as there is no modification in that area of the U.S. conversion.

I see that barrel nuts (locking levers) are available from those with no numbers stamped at all ("originals" perhaps when a gun is new?), and then some having "0" stamped" and versions with numbers "1" thru "5".

I had thought these barrel nuts pushed the barrel slightly towards the rear (into final locked position) and allowed for another way to tighten headspace. I am now thinking that is not the case. So can someone explain to me how these barrel nuts worked in the process of locking a barrel? I mean I understand the overall method of interrupted threads and how the basic locking works. But what happens in the nuisance with these numbered locking nuts?

I have barrels that I have tried in my Mk I body, but the original barrel nut won't completely close on some of these extra barrels. I tried a "3" marked nut and it wasn't any better, in fact may have actually been worse, in the fact it did not get any closer to locking closed. The original barrel nut has no number stamped on it at all.

I looked up "barrel nut" in the search, but found nothing that explains what the alternative numbers mean or how they work.

I think you can ignore the numbers, I think Peter L said they actually made little difference. If you have some that are tight, try placing sand paper on a surface plate or glass surface and then rubbing the back face of the nut in one direction and then at 90’. There’s also a sleeve that can be pressed over the breech end of the barrel to help bring back the head space and tighten things up, Kev Groom used to have these.

Thank you Brit plumber for that. Here is what I observed with the barrel shims (wafers), as I purchased a couple from a company called BRP here in Georgia. They carry quite a few Bren parts.

Those very thin barrel wafers get snugged up against the shoulder of the barrel, forward of the locking threads. This also means they are on the forward or "muzzle side" of the barrel nut, not between the "rearward side" of the barrel nut and receiver body. So that seems to me that while they may be another way to tighten the interface between the barrel nut and the barrel, they in essence, push the barrel ever so slightly FORWARD, right? How would this tighten the head space?

I can get easily confused by all this forward and backward motion. But it would appear that supposedly the numbers mean the barrel nut attempts to push the barrel back into the receiver body against the bolt face, to achieve a slightly tighter interface? I can see that this might all be theoretical and as you say, may have no bearing at all on how a barrel seats up agains the bolt face.

OR maybe it is more the instance of cutting various barrel nuts in an attempt to simply match a barrel nut to fitted barrel, as the barrel can be determined to headspace correctly without a barrel nut, but then you must have a nut that holds the barrel in place firmly, but not SO firmly as to take a lot of force to lock or release the barrel. So the numbers may only reference that you can loosen your barrel (by the numbers?) to have less force in lock up?

Anybody else confused?

I am trying to wrap my head around this as we say...

The locking nuts are selectively fitted only to ensure a snug fit (and eliminate wobble) of the barrel to the receiver. This can be roughly gauged by "feel"; the amount of force necessary to lock and unlock the lever should be enough to firmly secure the barrel but not so much that the lever has to be pounded open or closed.

This has nothing directly to do with setting headspace, which is adjusted by changing out the locking insert in the receiver, using a gauge.

M

Thank you Mike.