RE: changing case brands - ever happen to you
 

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.

RE: changing case brands - ever happen to you
 

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....

RE: changing case brands - ever happen to you
 

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.

RE: changing case brands - ever happen to you
 

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 .