I have a RimX with a heavy stainless M24 Muller barrel. Muzzle is not threaded but has a Harrel Tuner tightly clamped on. Reciever is glassbedded to a heavy black walnut stock. I'm shooting Eley Match black box ammo. First shot will go high ³/8" and left ¹/4" with a higher velocity a out 50 fps faster than the following shots. With a delay of a couple minutes the cold bore shows up. Other wise the gun is fantastically accurate. It's just the first round is faster than the follow up shots. Not sure what to do with it, could it be the barrel? The rifle barrel and chamber is clean, still does it. Not sure what to check next. Anyone seen anything like this?