Here's my update on IMR-7977 in the 300 RUM:
I previously said I had not used it in the RUM but now I have and I'm pleased with the results.
Is it better than Retumbo? I don't know how it could be but I worked up a load at 92.2 gr of 7977 and it pushed the 200 AB at exactly 3200 fps without destroying the primer pockets of my precious prep'd brass too much.
I've shot the 200 AB's a bunch at 100-600 and 900 yards along with the 212 ELD-X (note: different powder charge, of course!) and I've settled back to the 200 Nosler Accubonds.
The 212 ELD-X did account for an elk already this year and the bullet's terminal result was about perfect at a scant 147 yards (I practice far and always try for close).
Remember: start lower than my loads and work up! Even a .5gr change will throw things out of acceptable pressure limits, in my experience. If you go too high you'll only get one pop of the primer before the pocket is loose.
PS: I've noticed that primer pockets suffer with either powder if the fps edges over 3200 in my rifle.