Feenix: I just posted this in response to a similar question by NavyChief. Hope it helps. Bottom line, I like it so far.
I started work on 4451 for my sons 270 win and 7977 for my 300 RUM yesterday. As a matter of fact, I just returned from Cabelas after buying two more pounds of the 4451 with the same lot number. After what I saw yesterday, IMR 4451 looks like it will be my 270 powder.
Barnes 129LRX, Federal cases, CCI 200:
900' elevation, 82 degrees
55.5 gr IMR 4451: 3062, 3059, 3048. < 3/4" for 3 at 100 yds
60.0 gr IMR 4451: 3082, 3080, 3075. > 3/4" for 3 at 100 yds.
I seat the 129 LRX to mag length in my sons Tikka T3. Looking through my load journal, I have three separate 1 hole (for 3) groups at 100 yds, but the 129LRX at mag length is a solid 3/4-1" combination. 48 gr of Varget is the only more consistent powder in this combination (followed closely by H4831SC), but nothing can touch this speed. Every other powder I have tried is in the 2800-2900 range.
I will be doing a little more load workup, but I think I'll be sticking with IMR 4451 for this gun and bullet.
The IMR7977 showed promise in the 300 RUM (200 gr Berger Hybrid), but I think it is tit for tat with H1000. Yesterday my chronograph showed 7977 to be slightly faster, but my load journal shows H1000 to be faster on another day (same powder lot). My guess is the chronograph (and lighting conditions) are to blame.
Hope this helps.
Erik