I had a bunch of Eley's misfire on me. Happend frequently. Eley is THE best rimfire ammo, period. Especially when it comes to quality control.

I rebuilt that Ruger 77/22 a few months ago, and while I did it, I replaced the firing pin. Have put over 1000 rounds of all sorts of different stuff through it since, and haven't had the first hiccup.

I suspect that the problem with some (not all, but some) misfires in a rimfire comes from the firing pin. Everytime you shoot it (but espeically when it is dry fired) you are doing just a bit of harm to your firing pin.