sownare:

A thought on your shooting problem - the clip is meant to attach to the Sherman 105 barrel, and its an even diameter ay that point, right?

Perhaps there is a slight taper to your longer one, and its causing the emitter to be partially blocked by the muzzle brake - the stock barrel had no brake, right? You might try putting a shim on the front edge of the clip, like a scrap of vinyl tape, to counter-act the angle of the barrel.

Other option is moving the emitter into the mantlet, but thats not an easy task after the kit has been assembled.