For information purposes here are some velocities I have obtained from some of mine and twin brothers 6mm caliber cartridges;
2-6XC's with Rock 1/8 26" and Win brass 3000fps with 107SMK but with pressure. Realistic about 2950 tops. This with nekkid bullets. Tried some moly and did get just over 3000fps with less pressure and using 39.5gr of H4350 Just got some Norma brass but haven't had time to chrono anything yet. Tubb guaranteed 3100 using his Norma brass so will see.
243AI Lilja 1/9 at 24". Nosler 80BT's at 3610fps
6BR- Krieger 14tw @ 26" 65Vmax at 3400 and about tops in this barrel.
6x47 Lapua- Krieger 8 tw, 26", 107SMK @ 3080fps but haven't had chance to test it alot yet. May get 3150fps but think that about tops.
6mm Rem- Shilen 10 tw @ 26", I cannot get over 3600 without pressure using any 70 grainers nor accuracy. Best has been about 3550 with a 75Sie and RL19
6x47 Lapua- Broughton 5C 8 tw @ 26" but haven't put it on paper yet or chrono
The 6 Dasher is extremely popular with the 600-1000yd boys. Holds alot of records but needs fire forming and expensive dies.
If your goal is around 3000fps with the 105-107 class of bullets with alot of barrel life choices for me would be the XC or Lapua because of brass issues and magazine feeding. Dasher would be interesting if fireforming and die cost not an issue. Long range and highest velocity possible I would take the 243AI or 6AI mainly to have no concern over feeding issues from a mag. However Tubb and others are getting 2500-3000rds from the XC and I think top accuracy in the 243AI will be about 1500 with variances in how you use it.
All this probably will make things just more confusing