My Ruger 77 mk II .223 will shoot the cheapwinchester varmint factory loads (45gr hollow point, 3600 fps, 40 rounds per box) into a group less than 1/2" at 100 yards. I want to duplicate these loads with my handloads and I was wondering if they use the Win 748 powder for these factoryloads? Does Winchester only use their own powder in their factory loads?

Thanks
Tom