No experience at all with the M-455 but I recently acquired a CZ-452 in .17HMR and I'm quite impressed with both the rifle and the cartridge. Mine is the Military trainer version with beechwood hogback stock and 25" barrel. I'm still testing loads but so far I've tried six different loads, three 17 grain loads and three 20 grain. At 100 yards by best group was 0.93" and my worst was 1.6". But then I learned something. After about 100 rounds I decided I should give the bore a good cleaning with Hoppe's #9 and Shooter's Choice Copper Remover. Next time out my group size doubled! After about 25-30 shots it settled down again to group like it had before cleaning. I was surprised by that and posted my experience on "Rimfire Central" and ".17HMR net". All replies agreed that cleaning the bore of a .17HMR will adversely affect accuracy for some time. Some said only for the first few shots while others said it took as much as a full box of ammo to restore best accuracy after a thorough cleaning. Most seemed to favor no more cleaning than a pass or two with a bore snake.
The .17HMR is a fine little rimfire. I found that with a 100 yard zero I was no more than 1/2" high or low from 25-125 yards and accuracy, with a fouled bore, is amazing. I'd never before seen true MOA groups at 100 yards from any rimfire. Now I'm thinking of repalcing the Simmons 3-9X32 rimfire scope with something a bit better just to see what it really can do.
