Agree, the 6 Dasher, 6 GT and, 6 XC ARE All, SO, Close in Velocity to, the 6 SLR and .243 Win,.. BUT use, LESS Powder for, much BETTER, "Barrel Life"Just a footnote - I wouldn't build another 6mm SLR. Whilst it seemed a good idea at the time, it's not one of the most efficient cartridges out there - achieving very little over the smaller cased 6mm's and using a lot of powder to do it. Sure, it's "better" than a 243 - longer neck, nicer shoulder angle -ticks the boxes and barrel life should exceed the 6mm Creedmoor- and it's fast -3000+fps fast. But only a little faster than a Dasher for 25% (ish) more powder. Proof of of the Law of Diminishing Returns.
It was built in the heady days before COVID, when powder was plentiful and cheap and a 6mm barrel could be delivered to your door before you had chance to change your mind and build another 308 ...it does shoot rather well and I have the dies and 4 Kilos of H4831.