1-10 failing to feed is a lot check mags for damage.- the 455 is basicly a .17 reciever ie it is designed for the longer round, to enable the shorter .22 round (and mag) there is a plastic plug inserted in the action , this usually works very well, but check it is fitted correctly (nearly impossible for it not to be), ive seen one or two with the odd (very odd FTL as you describe). also some of the actions can be a little rough- tight when new, but they smooth out fairly quickly so until this happens try thumbing the rear of the bolt ie start the forward motion of the bolt with a STRAIGHT push from your thumb this stops any stuttering or squeing of the bolt resulting in a smooth feed. als ive noticed that some of the larger capacity mags when loaded fully the top round is under pressure and nearly jumps out of the mag causing the problem you described. do you have the 5 or 10 shots polymer or steel, is there any particular round FTF first last ect, will it FTF with 1 round loaded.