I have a Tikka T3 Lite in 7mmRM. It has a Mk4 6.5-20x50 Leupold, PRW rings, and an EGW 20MOA rail fitted. It is stock other than a rear ported muzzle brake
that I had fitted about twelve months ago. Prior to fitting the brake this was a consistent 0.5 MOA shooter.
Shortly after the brake was fitted things started to go wrong. Constant slight shifts in point of impact and groups opening up to around 1.25 MOA. I wasted a lot of time, powder, and projectiles trying to figure out what had gone wrong before surrendering.
I decided to ditch the rifle and therefore reattached the much maligned 1" rings that came with it, and fitted a Leupold 3-9x40 VXI. I couldn't bring myself to sell it to someone without testing it with some factory ammo, so off to the range agin! This time with some Hornady Superformance 154 SST's. It shot three <0.8 MOA 3 shot groups!!!
Change of plans. I refitted the rail (expoxied), mounts, and Mk4, and then resighted the rifle. All good. Groups back to 0.5 MOA. Then after about twenty rounds the same wandering and opening up of groups reoccured.
This time I paid a lot more attention when stripping down the scope and mounts. I detected a very slight looseness in the rail screws. Plan B was then activated. Talley rings secured with Loctite thread lock, screws torqued to 25" lbs. Off to the range for an extended session. All good for about thirty rounds, then the screws worked loose again.
A couple of hours on google that night and a few articles were found with similar issues occurring with rear ported brakes. The pervading theory was that the dual direction recoil pulse (similar to spring piston air rifles) was the culprit. It made sense to me.
Any suggestions on securing the rail or mounts to the T3 so that this issue can be dealt with? One of the down sides to the T3's is the limited bearing surface on the receiver for the rail or mounts. It was suggested to me by a gun smith that I drill the rail and receiver and pin it with a roll pin or pins. I would rather not do that.