6.5 x 37 Custom Load Data & Reloading Guide

The 6.5 × 37 Custom is a wildcat cartridge in active development by James Anderson, built on .223 Remington / 5.56 × 45 NATO parent brass (Lake City) necked to 6.5 mm (.264"). It is intended for the AR-15 platform as a 500-yard semi-automatic carbine delivering substantially greater retained energy at 300–500 yards than a 77-grain .223/5.56 OTM load. The chamber drawing is attached. Maximum working pressure is self-imposed at 60,000 psi, deliberately below the 5.56 NATO MAP of 62,366 psi, with the goal of maximizing velocity under that ceiling.

The current Generation 1 case, as designed by Anderson, mirrors 300 AAC Blackout up to a 23° shoulder before necking to 6.5 mm and runs slightly longer, with a chamber case length of 1.455" and a working trim length of 1.445–1.448". Maximum cartridge overall length is magazine-limited at 2.260". Mean fired-case capacity measured over ten Lake City cases is 25.0 grains H₂O; useable capacity with the reference bullet seated is approximately 19.1 grains H₂O measured. Generation 2, with reamers currently being cut, opens the shoulder to 30° and increases case capacity by roughly ten percent; all other dimensions are per the attached print.

The reference bullet for load development is the Hornady 123-grain ELD-Match, with a measured length of 1.240" (lot range 1.238–1.243"). Field testing by Anderson ranks A2200 and CFE BLK as the two most consistent performers, both producing single-digit SDs and ultimately limited by case volume and the 60,000 psi pressure ceiling. A1680 has proved erratic and pressure-limited; W296, H335, and A2460 are all volume-limited, with H335 and A2460 additionally too slow in burn rate for the case. Database defaults imported from .223 Remington will need to be overridden for case capacity, COL, chamber and trim length, and bullet length as described above.