Geoff,
Wow! Your rifle's bore/groove dimensions are larger than normal. I have a Marlin 336A made in '78 with micro-groove rifling and it has .301" bore and a .308" groove diameters.
Based on the shallow grooves in your rifle, I am wondering if it is an older marlin.
When micro-groove rifling was introduced in 1955, the factory specs called for .304" bore and .308"groove diameters. Yours is larger but the .004" difference is close to being the same.
Several years ago I had some experience with 2 different marlins of early vintage. Both had barrels close to the Marlin 1955 specifications. The bullet I chose to test was Lyman's 311041. Bullets were made from Linotype and lubed with NRA 50/50 lube. Bore riding diameter was .301" which was .003" smaller than the bore. Driving band diameter was .310".
The initial test load was 36 grs. of H414 giving 2,000 f.p.s. Both rifles shot this load well and averaged slightly over 2" @ 100 yards for 5 shot groups (Both rifles had receiver sights.):-D
However, when I switched the RCBS 150 gr. bullet (Linotype .302"/.310") and pushed it to 2,300 f.p.s., groups expanded to 5+". The "small for the bore" nose diameter was not supporting the forward portion of the bullet causing groups to really open up at the higher velocity. By comparison, the RCBS bullet shoots just fine in my 336A (.301"/.308") at 2,300 f.p.s.
What diameters are the bore riding / groove diameter portions of your bullet?
Perhaps a .311" diameter jacketed bullet (.303 British) would give you an accuracy benchmark to compare to and to try and exceed(?).
Sincerely,
w30wcf