fwiw, most of the sniper/practical/field precision rifle matches are won with a 243 or wildcat that strongly resembles it, like the 6.5 Creedmoor necked down to 6 CM or what i shoot, the 6 Super Long Range, which starts with 243win brass with shoulder bumped to 30* and a longer neck with a grain less water capacity.
many people shoot these with 115g bullets at 3100 fps or so, or the hot new 105 berger hybrids over 3200 fps. the result is well under 7 mils to 1k. i.e. extremely flat shooting, and very low recoil compared to 6.5/260 and 7 and 308 cartridges all based on the same 308win case.
generally, the problem with running 243win in an AR is seating the bullets to magazine length is a long way from where you want them (the neck is too short and the cartridge too long). cartridges like 6.5grendel, 6.5 creedmoor, 6.5x47 lapua (and all their necked down to 6mm counterparts) were designed mostly to be able to seat the long pointy high BC bullets near the lands and still fit them in the magazine.
the GAP10 in 6mm cartridges is selling like hotcakes. very popular in matches right now.