I wonder about that as well. My custom Mauser has the original twist rate use for the cartridge. Something like 1 in 8.68" The M70 FWT is, I heardh came with the original twist then some one said a 1 in 9" twist and some say a 1 in 10" twist. I've tried measuring it myself with varied results. The Ruger #1 has a 1 in 10" twist so I can understand it's not doing all that well with the 150 gr. ABLR.I agree with the twist rate maybe being a problem , these are long bullets .
I've run over 100 rounds of test loads at various seating depths with no luck at all. FWIW, I've had the same problem with the 120 and140 gr. Barnes TSX. Good speed with any of the loads but no accuracy.
PJ



