Stubblejumper is correct. Have you compared the capacities of both brands of cases? There will almost certainly be a difference between the two brands, and this could very easily account for the difference in accuracy.

Generally, Remington brass hasless capacity than Winchester, but not always! So you should do a capacity comparison. Then redo your load workup with the Win brass.

Hehehehe.. Some consider the .300 mags the ultimate all around chamberings and some consider them neither fish nor fowl...If you want a real NOTICEABLE increase in power over a 30-06, go to a .338...

Just for conversation's sake, regarding using a relatively fast powder in a big belted case... I saw a young fellow set up the bolt on a .300 Weatherby one time...Gas rolled out of the action and he had to hammer the bolt open with a block of wood... He was using a manual published load of 3031, which would be a real fast powder for that case....

Has nothing to do with fast powder. Same thing has happened with me with RL25 and 300RUM. Its the amount of powder you use and did a person work up the load.

I changed from some Lake City to Rem. in an '06 the cases weighed+- .5 grains and the same water cap. I don't have a chrony but the report and recoil was like I'd put a 308 load in the '06 . In the mil brass I was at a min. load that was well about as good as it gets in the field . In the Remthe shots were about 8" low at 100 . That was 3031 150 Horn. cci 500 .