website design software

Guncrafts.co.uk

Threading a Rifle Barrel

As shooters are now more aware of the damage that can be caused to peoples hearing due to noise, particularly rifle shooters. The fitting of sound moderators is becoming more common on full bore rifles.
For those of you who want to have some idea of the steps involved in fitting a sound moderator on a rifle I will explain the stages taken. This is not intended to be a definitive guide or a “How to do it” but an outline of one of the ways it can be done and some of the processes involved. However it is done, it must be on a center lathe with a thread cutting ability and NOT with die and hand die holder.
What ever method is used, the barrel must be removed from the stock, the bolt, magazine, scope and rings taken off the barrel and the trigger mechanism removed.

(a) Firstly, set up the chamber end of the barrel in a 4-jaw chuck with protective padding and using a dial test indicator adjust the jaws until the barrel runs true.

(b) As a double check, look in the head stock end of the lathe and visually inspect that the chamber and barrel are running concentrically. You may notice that the muzzle end of the barrel in not inline with the centre by as much as ˝ an inch. It is important to ensure that your chuck jaws are tight and pull the barrel to line up with the centre. When you turn the lathe on the barrel may look as though it is running like a banana. If you do not feel confident to continue, recheck (a) and (b).

It important to remember you are turning to the bore and not the out side diameter of the barrel.The lower  right picture is a good you example of this. As you will see the first cut does not make a complete cut around the circumference of the muzzle end of the barrel. On this rifle it took .018 housands of an inch before a complete cut around the barrel was made.

Let us assume that we are going to cut a standard ˝ inch U.N.F. 20 threads per inch. The first stage is to machine down the outside diameter to .500 of an inch  photo left.

Next checking with micrometer and to a length of approximately .525 or to suit moderator as per manufactures instructions.You do not want a tight fit on the thread as you want the moderator to butt up on the shoulder behind the undercut you are now going to make. It is a simple

matter of grinding up a basic parting tool with the cutting face 1.5 times the pitch of the thread. Set it up so the tool will cut at the base of the thread and index in to the full thread depth as seen in photo below left.

The next stage is to commence cutting the the thread. photo right If you are using a single point tool make sure the tip is sharp and set to cut a 60 degree thread form. Make sure the gears are selected to give you 20 T.P.I. It is important to make sure the tool is going to cut the right T.P.I. You can rest a soft lead pencil on the  tool  post  and

on engaging the thread cutting lever with the pencil point on the barrel, you will then be able to double check the pencil line with a pitch gauge to check it is 20 T.P.I. before actually making a cut.

Ensure the tool is set up accurately at right angles to the barrel and the top of the tool is on the centre line of the work. The thread depth of 1/2 x 20 UNF is .031 Thou. once this is reached the thread is complete. I always do a second cut at full depth on the same setting to allow for a spring cut. This ensures that if there is any pressure on the tool tip preventing a full depth cut, the spring cut will clean it up.

The final stage is to re-cut the muzzle crown, This is important as it ensures a clean exit of the bullet from the barrel. Before trying the moderator use a 1/2 UNF thread gauge to just check the thread is within tolerance. If it is satisfactory try fitting the moderator. Once in place look from the muzzle end and ensure that it all line up concentically. 

When a reflex moderator is fitted it is normally necessary to bore out the barrel bushing that fits in to the rear of the moderator. The manufacturer will give the tolerances as to the clearances required. Reassemble the rifle and submit it with the moderator (if it is not already proofed) to the Birmingham or London Proof Houses.

It is important to remember that when a sound moderator is fitted the point of impact will be different to what the rifle  is normally zeroed for prior the thread cutting. If you want to use the rifle with the moderator fitted all the time, then it is a matter of simply zeroing the rifle as you would normally. As I use mine with the moderator on and off, I have some 1/2 inch strips of paper under my scope caps with the alternative clicks on them. E.g. With no moderator, Top turret 14 clicks up -  Bottom turret 6 clicks to the left.