Assuming you are shooting only 55gr XM193 and engagement range is <100 yds with a 16" bbl, will using a slower twist barrel result in any likelihood of greater terminal effect (more tumbling, fragmentation)? I know the earliest bbls were 1:14" twist…
Or will a 1:9 or 1:7 twist be a better overall choice?
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I wouldn’t think that the difference would be significant. 1/12 was selected because it offered the optimal accuracy for M193 ammo.
A longer barrel will do more for helping bullet frag than twist rate. Velocity is much more important.
do you lose any velocity by spinning the bullet faster? 
Sky, you increase pressure so your optimal load is going to be a little different. Theoretically it is more effective to use the lowest twist possible, just loaded a little hotter.
That said, good luck trying to prove that scientifically.
I remember notionalizing this years ago…
I don’t know that it’s ever been tested.
It would seem to make sense (to me) that the more aggressive twist rate would increase the resistance of the bullet traveling through the barrel, at least maginally. But that leads to another question - would that also increase pressures as well? At least to a small degree?
I’m not a physicist, but I play one on gun forums. So, I’ll hypothesize that it requires more energy to spin the bullet faster. That energy has to come from somewhere. You’d see that with a slightly reduced velocity. Granted, it may only be a few FPS (and may be impossible to reliably measure) but it’s interesting nonetheless (to me.)
in my opinion. … 1:12 has lesser spin. when it hits flesh… it’ll easily tumble.