The best mod i have seen on a hmr is an over barrel DM80, however my .17 hornet is quieter with a DM80 mod. I think the problem lies with the bullet speed. At 2,400fps ish it is a relatively fast rimfire round but slow in comparison to c/f bullets of the same calibre ( 3,600fps upwards). As such i think the extra time the bullet takes to reach it's target is the reason it produces a prolonged sonic crack.