Most 155s shoot well in 14-inch twists even if by current standards, some are too long for this twist rate and won't get near the now desired 1.5 stability coefficient value. The original shorter 155 Sierra MK (#2155) is a perfect match though for the rifling, and so any of the tens of recipes with IMR / H4895, IMR-4064, VarGet, Viht N140 etc, etc that are around for this bullet will let you work a good load up.
Unless you've fallen very lucky with a little used replacement stainless barrel from one of the match barrelsmiths, don't expect the sorts of results that are regarded as the norm these days. Anything that is 14 twist has to be an old barrel as GB TR moved from 142-146gn military spec 7.62mm ammo to a fatter bulleted 155gn a long time ago now, a change that saw the standard twist rate moved to 13. (Nevertheless, a few competitors stuck with the older 14 even when a new barrel was put on. Also, if the rifle has come to Texas via Canada, I think their TR shooters used IVI 7.62 144gn stuff a lot longer than we did in the UK with our RG equivalent.) Even when new, the original hammer forged chrome-moly P-H barrels were generally 'adequate' for an era when standards were a lot lower than today's with a minority being much better performers than they had any right to be, but another minority that were absolute 'dogs' and simply never performed well.
The other very important thing to be aware of is that GB and British Commonwealth TR rifles of the M84's era used very 'tight' barrels by today's standards to match undersize military bullets as the norm was to shoot 7.62mm military ammo back then - here in the UK it was the slightly improved (ie slightly less bad!) Radway Green 'Green Spot' 146gn FMJBT sniper round, some lots of which were frankly dreadful. This means that these older barrels generate much higher pressures than a modern 0.300/0.3080" dimension bore / groove job. I bought an old Paramount (predecessor of the RPA) TR rifle a few years ago, likely younger than any M84, which had been rebarrelled with a Lothar Walther tube in 14-twist, so was 20 or so years old. Despite a large amount of throat erosion which normally reduces pressures significantly in these rifles, I had a few leaky primers with 155gn SMK loads I wouldn't normally regard as at all 'hot' - until the penny dropped as to how much extra pressure the 'tight' internals generate. So take the usual 'top loadings' for 155s in 308 and drop them by a couple of grains to get a new maximum, then drop more below that to get starting loads to work up from.
