Unlike many other cartridges, the 300WM case has a very short throat. Much less than even a bullet diameter.
As stated above, trim 0.010 less than SAAMI min. I.e. 2.620 - 0.010 = 2.61".
If you've already run then through the FL resized and bumped the shoulder correctly, and only ended up at 2.60", then you must have trimmed them too short last time or your calipers are off. Nothing you can do except reload and shoot them. After 2-3 more firings they should grow to > 2.61".
I wouldn't worry about 2.60", as generally SAAMI min is 0.020" under max, and trim to length is 0.010" under max. Nothing bad happens at < SAAMI min, you just have less bullet /case neck bearing surface. As I mentioned above, 300WM is already much less than others (I.e. 30-06).