The Gun Workshop by Peter Davie Hello, and welcome to a new series of articles intended to illustrate and de-mystify what happens to guns when they enter a Gunsmith’s Workshop. I hope to achieve this by taking you step-by-step through a variety of jobs carried out for our customers; from removing a broken screw or correctly mounting a scope, to more complex jobs such as chambering and fitting a new barrel or blue-printing a rifle action for ultimate accuracy, there will be something different here each time.
Threading a rifle barrel for a sound moderator
(Figure 1) A French Crapahute Bullpup .270 Win Stalking Rifle, fitted with a Wildcat “Predator 8” Over-Barrel Sound Moderator
Sound moderators are understandably popular on vermin, stalking and target rifles, but if your gun didn’t come with a suitable thread on its muzzle, you will need to have it threaded or “screw-cut” to be able to fit a moderator to it. Next to cleaning and servicing, this makes muzzle threading one of our most frequently requested gunsmithing jobs. The threads and other features we are going to make must be cut exactly concentric with the barrel’s bore and perfectly in-line with the bullet’s travel to properly align the moderator; doing this job correctly will optimise the gun’s accuracy, whilst preventing the dangerous situation that can occur if a bullet strikes the inside of a moderator. The main problem for the Gunsmith to overcome is the 38
fact that the bore of a rifle barrel rarely exits exactly in the centre of the muzzle and is almost never perfectly straight. Now, this doesn’t mean that most barrels are poorly manufactured or can’t be made to shoot accurately; these imperfections are just consequences of the difficulties involved in drilling a small hole through two feet of solid steel, whilst keeping the drill exactly in line with where you started. When you think about that, it’s actually pretty amazing that most rifle bores are within about 20 thousandths of an inch (0.5mm) of being concentric at the muzzle – however, amazing as this might be, it’s nowhere near close enough for moderator threads - we will be aiming for better than half a thou’ (0.01mm) maximum bore run-out; so, how do we address this difficulty?
One common method is to use a device like the one in Figure 2 to hold the barrel in the lathe when machining the threads. This fixture is called a “spider” and using eight brasstipped bolts it grips the barrel outside diameter, but allows it to be adjusted off-centre with the correct degree of up/down and left/right compensation to get the bore running perfectly on-centre at the muzzle. We also adjust the horizontal and vertical “cant” of the barrel to ensure that the part of the bore nearest the muzzle is rotating perfectly in-line with the axis of the lathe. All measurements at this stage are taken directly from the rifling inside the bore using bespoke long-reach instruments. It takes time and patience to adjust the bore alignment for minimum run-out before
we can start cutting metal; then the first task is to machine the end section of the barrel to the correct diameter, ready to receive the chosen moderator thread. A perfect ninety-degree shoulder must also be created for the moderator to stop against and sometimes an extended spigot, multiple diameters or other features are also formed on the barrel to further guide and support it. After all that preparation, the only practical way to machine the thread with sufficient accuracy is with a “single point” tool like the one in Figure 3 – the position of this cutting tool is synchronised by the lathe’s leadscrew and gearbox to trace the chosen thread pitch on the barrel as it turns. It will then cut a near perfectly concentric thread form onto the prepared diameter. A conventional threading die is