AIRGUN GURU

Air Gunner - - Your Letters -

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

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