I have and shoot a 6mm in the ackley version. The barrel has a 1&8 twist so that heavier bullets can be used, ie... 105 &107 grains.
The ackley version hols about 2 grains less than 257 Weatherby mag, mine aslo likes to shoot light bullets, esp, 58 grain V-max. With 47.5 grain of Varget - CCI benchrest primers - Reminton brass - i get a loading that shoots right at @ 4000fps - with arounf 21 standard deviation. It is an excellent varmint loading out to 400 yards.
The 6mm popularity was diminished by the introdution of 243 Winchester round. Still it has its fans, with the wide variety of powdrers, bullet weights, it is a good overall cartridge for varmints thru deer sized animals.
12-34hom.