Barrel, No.4 will not index properly

Trying to fit a No.4 Mk2 barrel to a No.4 mk 1 action (used to keyhole at 50yards). However the newer barrel turns too far and the extractor cut out is too far round to use even when turned on by hand without any torque. Is this simply as tolerance thing and I need to find another new barrel or is it possibly due to the fact that the old barrel had to be cut to remove it and maybe the receiver got stretched or something in the prior efforts??? The smith has always been solid and good with LE's before so he know's his stuff

Its a normal variation in tolerance.

The ideal way to fit a barrel is to try several until one hand-tightens about 15-20o before the index - which can then be torqued into position. Unfortunately, most people don't have access to several No4 barrels these days....

Yes the lack of several barrels is a problem.I have on barrel and one action. Grrrr. Might go down the .223 conversion rout at this point.

Barrel shims are used to correct index. Determine thickness you need by positioning barrel on receiver then measuring the gap. Correct shim size will give you the top dead centre index you need. Ron

Additional tap-dancing may be required.

If you can get the barrel to breech up correct aligment and reasonable torque, that is a good start. Breeching washers may be helpful in this regard.

However, in the likely event that the previous barrel-fitting involved a bit of thread crush, headspace is your next issue. If it all goes together and you get away with it by using a No2 bolt head, that's great.

If you cannot get within spec even using a RARE No3 bolthead, you are in trouble.

Swapping bolt bodies MAY pull things up, but you need to ensure correct bearing on BOTH locking surfaces of the body and bolt.

Here is the relevant part of a 4 page article written by Peter Laidler :

"...........Someone suggested that his gunsmith will insert a .028” steel shim and that’s the answer. Others have suggested that it’d be a good idea to gently swage the shoulder of the breeching-up face of the barrel, sufficient to take up the slack. DO NOT USE THESE METHODS. THEY ARE XXXX POOR ENGINEERING PRACTICE and verging on the best bubba practice you will ever have the privilege of seeing. Have you digested that?

This is what you do. Knowing that your barrel needs .028” underturn, get yourself a proper breeching up washer made. I’m not going to teach you or your machinist pal how to suck eggs but if you need .028”, then get the breeching washer made .128” THEN machine .100” off the breeching up face of the barrel (no, the breeching UP face, not the BREECH face silly…..). But I’ll let you into a secret. At our large Base workshop in Singapore, we were running major overhaul programmes of everything including L1A1 rifles. Then, someone noticed that the tough, hard, readily available and exact diameter required L1A1 breeching up washers were between about .055” and .070” thick. Now, we’d just take the barrel to the little Chinese fitter/turner (he had a big pile of breeching up washers in his tray anyway) and say .”028” please Lim” and he’d mount the barrel and machine away .032” from the breeching up face. You’d walk back to the Armourers shop, past Steve’s Magnolia ice-cream van where you’d spend the next half an hour discussing politics or the Viet-nam situation or the new flower arrangement in the church with the rest of the blokes……..Oh, I’ve gone off at a tangent again…… Anyway, armed with the new barrel with .032” machined off the breeching up face PLUS a new .060” L1A1 breeching up washer you’d know that .060” - .032” was .028” which is JUST the underturn we need to tighten the barrel to make it PERFECT on the flat-plate we used to ensure that it was perfectly tight, upright and square.

Is that simple enough? It might be a tad more thoughtful that a steel shim or a good battering around the barrel flange that won’t last twenty minutes but it’s how the pro’s do it............"

How does one measure the underturn, and are the dimensions of the washer please (excluding thickness of course). Does it need to have additional work for extractor slot, gas port etc? And of course the next question is where you get a couple of 'washers, barrel. Rifle L1A1, breeching up'?

Q1- You look up 18 degrees on a protractor, see what it looks like and guestimate it. 2) You can work the inner and outer diameters out for yourself by looking at the inner and outer shoulder of the barrel. 3) No, the extractor slot is already there and will remain in its relative position. There is no gas port!!!!!!! 4)From any company doing L1A1 rifle rebuilds

Got it. After I posted the question I got home and looked. It was a stupid question a bit. NOW who has one or two of these thing lurking????