Dear Guru, Both of my rifles have nonscrew- cut, plain- end barrels. Will the slipover type of moderator be suitable for them, or should I get a barrel adaptor that slips over the barrel with a ½” UNF thread on the end, and then use a screw- on moderator? Or would it be a better option for me to get my barrels screw- cut to the ½”20-UNF thread that seems to be the standard, from what I have been reading in your magazine and on the Internet. Many thanks, ALAN
Hi Alan, Many thanks for your enquiry and I will try to set out the facts as I see them. Barrels, the same as any mass-produced, engineered item, have a manufacturing tolerance. That is a maximum and a minimum diameter that they must be manufactured within so that they conform to the required specification. Any slipover type of moderator or barrel adaptor must be made to go over the maximum possible barrel diameter, therefore there would be some clearance when fitted to barrels that were not on the maximum. This extra clearance will give eccentricity errors.
When screw- cutting a barrel, the bore of the gun should have been trued up in an independent four-jaw chuck on the lathe, and then screw- cut, and not relied on the outside diameter of the barrel being concentric to the bore (they never are) and a three-jaw chuck used. Even so- called quality barrels can have horrendous eccentricity errors of the outside diameter relative to the bore. O.K., so the bores are gun reamed, but even they can run off centre when going through 16” plus of barrel length. Barrels that have been screw- cut correctly in a four-jaw independent chuck should be concentric to the bore of the gun within a very small tolerance.
Ergo, when slip-over moderators or barrel adaptors are used, they may not be exactly concentric to the gun bore. Obviously, I have stated the worst case scenarios here and 99 times out of 100, slip-over moderators and barrel adaptors would be perfectly O.K., and there would be no pellet clipping of the internal moderator baffles and no point of impact alteration. I would always have the barrel professionally screw-cut so that no concentricity errors are introduced into the gun.
Slip- on silencers can work well if the machining is accurate
Accurate screw- cutting offers the best fit of all