1899..........you don't need a 1-8 twist in 243/6mm unless you intend to shoot 115 gn bullets. 1-9 will handle anything up to 105 grnrs. For primarily a varmint rig I wouldn't go faster than 1-9 as you give up velocity with sharper twists.
I get that the theory of velocity loss due to fast twists sounds viable, but my observations don't support that the theory extends to reality. My 1:7 .222 with 52 gr match shot to a similar velocity with the same loads as they did with the 1:14 the rifle originally came with. My 28" 1:8 .308 drives a 210 Matrix VLD to just under 2700. My 1:7 5.56 makes good velocity despite a 14.5" barrel. I've ordered 3 other fast twist barrels that will be part of a switch barrel system I'm having built, although with a gain twist, a 24" 1:14-1:7 .243, a 24" 1:16-1:8 .280 AI, and a 22" 1:20-1:10 .375 Scovile. I don't expect the velocities from these barrels to be disappointing, but time will tell.
At extended ranges, faster spinning bullets handle trans-sonic velocities better. At near ranges, faster spinning bullets penetrate better, since their time in precession at impact is shorter. I've proven to my own satisfaction that there is no practical loss of accuracy that can be attributed to a fast twist barrel, in fact my Gaillard 1:7 .222 and my Krieger 1:8 .308 both showed exceptional accuracy. I've been warned that gain twists can come with accuracy issues, but again, time will tell.
Fast twist barrels do require tough bullets that will stay together despite the increased centrifugal force, but an important consideration is that bullet technology seems weighted towards longer and heavier bullets, and the fast twist barrels will shoot them, while the standard twist barrels will not. I have .308/240s that won't shoot in my wife's 1:12, but will shoot in my 1:10. I had a 1:10 .243 that won't shoot 107 gr MKs, the 1:9 probably would, on a warm day, at 4000'. My 1:12 .223 won't shoot Barnes 55 gr MPG, a bit of a surprise until I measured it's length. I haven't found a .224 bullet the 1:7 won't stabilize. Add to this the fact that as air density increases through a drop in temperature, or altitude, the less able a standard twist is able to stabilize a bullet that has a marginal stability factor under "normal" conditions.
