I looked at the 338 RCM and went 338 Fed. I rebarreled a Kimber Montana with a heavier contour Lilja at 23". It balances well and with the heavier contour doesn't have all the muzzle rise the 22" factory barrel had. I get an honest 2700 with the 185 TTSX, 2600 with the 210 Part. The RCM seems to get about 150 ft/sec more than those loads - but who cares about 150 ft/sec at 50-100 yards, which is the primary reason I built it. Shortish range elk, Moose, bear. Plus it weighs 6.75 lbs and doesn't beat the crap out of you. I dig it alot. And I may have 6-7-800 308 brass.
