The MK12's rifle-length gas system and longer 18-inch barrel would tend to favor a heavier bullet, yet the faster 1:7 twist would allow a lighter bullet to properly stabilize and could be very handy in closer range. So to test the rifle's versatility, I grabbed a box of every match grade .223 ammo I had in my safe, including Black Hills 52-grain Match BTHP, Federal TRU 55-grain Sierra BTHP, HPR Hyper Clean 60-grain VMAX, and HPR Hyper Clean 75-grain Match BTHP.
I zeroed the MK12 at 50 yards with the Federal 55-grain BTHP. This is currently what my agency carries as a duty round, both for the SWAT and patrol rifles, and we're continuously impressed with its performance. The quality of the Leupold scope and ADM mount came in handy here, as I was only about 3 MOA low and left during my first group.
I adjusted the scope to zero and began my accuracy testing. All shots were taken from the prone position with the rifle resting on a pack, just as the MK12 would likely be hastily deployed in the field. I also shot most groups from the 100-yard line, as I felt this would likely be the maximum distance for engagements in most law enforcement scenarios. There have been longer distance shootings, but the vast majority of police engagements are much closer.
My first five-shot group was with the Black Hills 52-grain round. I was convinced from the start this round was simply too light to maintain accuracy at distance, even as close as 100 yards. I have always been a huge fan of Black Hills' match ammo (I run a ton of it through my bolt gun), so it wasn't the brand that concerned me, just the bullet weight. Three five-shot groups confirmed my suspicions, as I was only able to maintain about a 3-inch group with little consistency.
Moving up the weight class, I repeated the process with the Federal TRU, the HPR VMAX, and the HPR Match BTHP. The 60-grain VMAX is where the MK12 really became consistently accurate, holding groups of about 1 inch with about a half-inch of variance for the cold-bore shots. The 75-grain Match was the clear winner, and I was able to hold between half-inch and 1-inch groups with little noticeable difference in the cold-bore shot. To be honest, the real limitation was the clunky military-standard trigger. I'm certain a precision trigger would make a big difference in the results, even for a novice shooter, and I would hope BCM has considered offering one in the future.