Given how well the common fast twist barrels can shoot the 50-55gr varmint bullets, I have to wonder - why bother? Is this just based on an assumption that a faster twist barrel won't shoot well?
You aren't gaining any significant velocity by going to a slow twist; even the theoretical advantage is miniscule and so small it gets lost in the noise of differences between one barrel and the next.
My own personal experience - I've had fast twist (i.e. 1:7, 1:8) 5.56/223W/223 Rem barrels from 10" to 26", in bolt guns and ARs, and every single barrel that shot the heavy bullets well was also a tack driver with the 50gr varmint loads I sometimes use. I've also had slow twist 223 Rem barrels, one that only was happy with 40gr bullets, and a second that was fine with 50gr but not better than the faster twists. That's just me and my rifles of course, but IMO the versatility of those faster twist barrels is worth a lot more than any imperceptible advantages of the old school 1:12/1:14 stuff.
I'll also go out on a limb here and say the results with the faster twists seem a little more explosive on little rodents. But some guys will say that's bunk, and that's fine too; if there's a difference it's not night and day, just something I've noticed.
My 2 cents, worth what you paid for it.