7.62x54R should be fine at 100yds. The maximum rifle caliber at rated by energy is 30-06, and less that 3150 fps. Its not a diameter restriction.
Looking at ballistics tables, even 8mm Mauser (7.92x57) would be ok. 30-06 on the high end has 3300 ft-lbs.
Unfortunately, those rules are out of date. handgun-caliber rifles are not allowed on the pistol range any more.
I do think is odd that 308 on the Barnes range is fine (at 100 yds), but 44 Mag is not.
You're 100% correct.....actually a copy of an older post from when those rules were in effect. I have edited/deleted to reflect, and thanks danb as I certainly don't wish to further confuse this issue!