I just matched the Win factory ammo for 270 WSM 130gr Nosler Ballistic Silvertips using IMR 4350 and R19. I hit 3250fps, which is what the rifle shot them, no problem and stayed under max load in Lyman data. IMR 4350 did the best. With R19, the velocity went way up with .5gr over max load. But the IMR 4350 load was consistent 30fps with .5gr above or below max.
I did work up from way below max, as different load data has different max/min etc. E.g. Nosler had 65gr as max load for R22, Alliant 66gr and Lyman having 68gr as max. Nosler also had the highest velocities in their data. Lyman data had 64.5gr as min. R22 wasn't getting it, and R17 was popular for WSM, so I thought 4350 and R19 would do better than R22 and I had them in stock.
You can take a caliper up to a friendly store and measure the factory ones. However, I bought a box, at $48.50, and shot them up.
I've read that Winchester uses WLR primers for all their WSM ammo. I used CCI 250 primers. Most of the load data I have uses LRM-type primers.
Reloading WSM required different powder funnel.