I've had FTFs with my NAA Minirevolver. Out of 500-600 rounds fired I've prob had 5 to 10 FTFs. Rimfire ammo is known to be less reliable because of the way that the primer is pored into the case - sometimes there are places where the bottom of the case does not get primer mixture all around. So I expect that this is "normal" since my other rimfire guns also have had FTFs with about this ratio. BTW, all of my FTFs had a good hammer strike, so I'm pretty sure it was the ammo.
However, I just bought a new Minimaster ( 4" barrel minirevolver) that had 2 or 3 FTFs per 5 round cylinder. This was with both 22 mag and 22 LR. I just sent it back to NAA to be fixed.
FTF is Falure To Fire and not Failure To Feed, right? I've had very few Failure To Feed problems with my revolvers!