Note that a 1:10 twist in a 300 Winny Maggie will spin bullets lighter than 200 grains much too fast for best accuracy. As 97% of all bullets are unbalanced to some tiny amount, it's enough that spinning them too fast creates enough centrifugal force to make 'em jump off the bore axis when they exit.
Which is why 1:12 twists were (and still are) popular for 30 caliber cartridges of that size with heavy bullets for long range matches. I've shot 180 HPMK's from my .300 Win Mag's 1:13 twist with excellent accuracy through 1000 yards.
So, for best accuracy with a 1:10 twist in one, I suggest a 220-gr. Sierra HPMK. But it'll only work well accuracy wise if the barrel's groove diameter is smaller than those bullets; they're typically about .3082". Lots of 30 caliber factory sporter barrels have larger groove diameters (Winchester was nororious for that and may still be). If the groove diameter's bigger than that, then I'd try to find some fatter bullets to use.
The 1:10 twist for so many 30 caliber magnum barrels came about when Winchester built their first Model 70's in .300 H&H Mag. They used the same twist as the .30-06 had, but even Harry Pope (famous barrel maker a century ago) said that was way too fast for best accuracy. Roy Weatherby used that twist for his .300 Wby. Mag. 'cause both he and his customers felt if the H&H version's twist was good enough, then so would his be. But Winchester and arsenal engineers were smart enough to make the 7.62 NATO's and .308 Win's twist 1:12 for 150-gr. to 200-gr. bullets leaving slower than the .30-06 shot them. They were more accurate with that slower twist.
Whatever bullet you choose to test, shoot enough shots per test group to be meaningful. At least 15 and 20 is better. Judge accuracy by the largest group for a given load as that's what you can count on all the time. Rarely does anybody shoot a group the size of the smallest one shot with a given load.