Ah, yes...., glad you asked that. I'd call a shim anything less that .060" or so. Others will have other ideas. But a shim is really not supposed to be subject to a rotary loading (this is being very simple now..........). An example is if you were rebuilding your MG gearbox and needed say, .008" endfloat and had, say .024" end float, then you'd back the endfloat thrust washer with a .016 shim. Perfect. But you wouldn't put the .016" shim where it's subject to the rotary forces of the gear train.

In much the same as breeching up a rifle, the shim is not made for this purpose. Better to machine back and insert a proper medium carbon steel (but good mild steel will do) spacer/breeching up washer....., call it what you will and then breech up on this, properly.

I know that someone out there is going to mention Bren guns but that's exactly a point in question. We do use .003 and .005 shims there but the barrel step is being pulled directly down on the shim in this case. They're not subject to rotary forces.

It's bad practice. Even worse are those butchers that 'gently swage' the breeching up face of the barrel to get it to breech up tight. Ok, I realise that it was easy for us to simply go to the machine shop and get a spacer made or even jusy scrap a rifle if need be BUT, shims. NO

This brings me to a point that was raised on the CSPicon forum many moons ago when the little D within a circle often seen on a barrel was mentioned. The indexing of the barrel thread was a very wasteful process and eventually barrels were finished AFTER the thread was cut. Alas, the indexing was also critical to the rifle body too. During the run-up to the L39/42 conversion programme, several hundred rifles were found to be slightly under or over cut in the body and THIS is why the L39 and 42 use breeching up washers (as opposed to shims...ugh or the old breeching face to breeching face method)

Steel helmets on here. I bet this will bring in some flak from the shimmers and barrel swagers!