I don't know about the M54J but I am sure it would have the same twist rate as the M60 which is stamped on the barrel as 1:12 for the 7x61 chambering. This twist rate stabilised the sleek 160gr boattail bullets and of course lighter projectiles but would not usually stabilise 175 gr bullets so well. The later model S&L rifles, the M65 and M68, had 1:10 twist barrels.

I did load some Norma 175gr round nose bullets in my M60 and used on Whitetail deer in the bush and accuracy was fine for this application but Speer 175gr SS Magnum projectiles showed greater instability at longer ranges, not key holing but showing enlarged bullet holes on paper. Accuracy was still minute of big animal, shot a few large wild cattle with the Speer bullets at longer range and they certainly knew what had hit them. Never chronographed any 175gr loads but was using Norma N205 powder at the maximum recommended charge so likely achieving close to their advertised 2900fps MV.

As often mentioned in various reloading books, the 7x61 usually achieved high velocities with smaller amounts of powder than any of the other 7mm Magnums and was superbly accurate doing so.
Lyman and Speer were always praiseworthy of the 7x61 cartridge while Hornady seemed to be more interested in protecting and boosting the reputation of the 7mm RM and WM cartridges above any others. Ironically Joyce Hornady shot a record Moose with a 7x61.