Got a n550 18 months ago and fitted it on my 243 tikka used it for 6 months no problems then fitted it on my cz 22wmr and it stopped working while i was zeroing it.sent it back they sent me a new one had it on the cz for a month then put it back on the 243 no problems for 11 months.now just put it on a 223 tikka and guess what it packed up while zeroing.just coincidence? or do they not like the recoil from smaller calibers.