I just picked up a mk v in 30-378. got to say I'm kind of disappointed in the performance. I've had nothing but problems with it. Granted I'm the third owner of this rifle (my cousin, my father, then me) no one could afford to shoot the thing rounds are 130$ for 20. I reload so not as much of a problem for me (haven't calculated per round price yet, don't want to know and don't tell my wife). When I first shot it I couldn't get on paper, found the action screws loose. Then I had a rail put on it for a 30mm scope and the screw on the back of the rail was too long to run the action. After some messing around I got the basic bugs worked out of the platform and got on paper. Best grouping I can get is 3-4 in at 200yrds. Cold bore shot to follow up shot is about 6"up and 6" left. From their they settle into the 3-4" range in the up left position. Could be that the rifle was beat on before I got to it but kind of doubt it. I'm giving the barrel 4-5min between shots to cool and am close to wits end with the thing. Don't get me wrong I love the challenge and feel this is a well built rifle, just expected more.
Loader, I have a stainless synthetic in 30/378 I bought new in 2002. The first thing to go was the Tupperware. Replaced it with a classic WBY Kevlar/Carbonfiber HS Precision. (Not offered by HS anymore). Bedded the HS and the top triggersmith around took it down to 2.5#. I have always loaded my own and this rifle has always shot sub-MOA out to 500yds. I also bought a .270Wby synthetic 4 years ago from a older gent who retired from hunting for $600 which he said didn't shoot very accurately. He wasn't kidding, it shot 2-4" groups at 100yds. Tossed the Tupperware and bought a B&C Medalist, bedded it and had the trigger done. It is shooting very well now .88" @ 200yds. If you have a plastic stock, perhaps upgrade, if it has the carbon fiber factory (B&C) get it bedded. If it helps, I'm loading 215Bergers with Retumbo and 215M primers. Here in Az we use a homemade air mattress pump barrel cooler to expedite cooling. Good luck