Abstract: A tubular shaped collar (22) of predetermined mass having internal threads (23) and an internal tapered area (21) which is threaded onto a tubular shaped insert (26) of flexible composition that has threads (44) that match the threads (23) of the collar and is tapered in a manner to match the taper (21) of the collar. The insert has a plural of longitudinal slits (25) in the tapered area. The insert is threaded into the collar and the collar and insert are slid over the muzzle end of a firearm barrel where they are locked in place by tightening the collar onto the insert whereby the predetermined angle of the taper (21) of the collar compresses the insert tightly against a firearm barrel. The insert and collar are adjustably positioned on the firearm barrel and held in place while being tightened by a tubular shaped tool (32) which has means of mating with the insert and has a means of calibration to determine the relative position of the collar and insert on a firearm barrel.