I've had my Tikka T3 for more than 3 years now, it has a Whiskey-3 KRG stock and Khales 624i scope on it. It probably has around 200 rounds through it. Every time I would go to my favorite range I would shoot the farthest target 6 or 8 inch steel plate at 600 yards cold bore and its always a hit more often than not (I've only used factory ammo Federal FGMM 308 SMK 168 grains). And if its a miss I would hit it on my 2nd shot upon adjustment. Last week, I went through my usual routine with the 600 yards target and missed the target by .5 mils or more. I double checked my ballistic calculator and and it did say I needed 4.5 mils adjustment for my intended target at 600 yards. I finally hit the target on my 8th shot with a 5.1 mils adjustment. Long story short it was a bad shooting day that I decided to go home and thinker about what could have gone wrong. I checked the scope and mounts and re-tighten it as spec. I went back to the range the next day and zeroed the gun and sure enough it was .5 mils low on my original zero at 100 yards. With all screws being tight and everything, do I need to check my zero every now and then? Do you guys think something is wrong with the scope (or maybe the guy behind the gun). The only only I haven't done is track the scope. How often do you guys check your zero?