The 6 Dasher is a "improved" 6BR, so performance is a bit faster than the standard 6br. Dan Dowling is usually credited with its invention. It offers about 4gr more case capacity than the standard 6BR case. Its got a 40 degree shoulder, and the shoulder is moved forward .100. You can usually get for an extra 125 fps with a 105-115gr bullets. It works very well with the heavy 6mm bullets. On June 26-27 of last year, Richard Schatz used an 11-pound 6mm Dasher to shoot a new 6-target NBRSA 1000-yard Light Gun World Record. His 6.125" aggregate beat the old record by about .75" If I recall he was using the Clinch River 106gr bullet.
The 6mmXC is a new version of the 6mmX, both designed by David Tubb. The 6mmx uses a 22-250 case fired in a chamber that was cut by holding a 243Win reamer short .132". The origional 6mmx used the standard shoulder of the 243Win (20 degree) The 6mmxc is shorter than the 6mmx by .023, and has the shoulder increased to a 30 degree angle. Capacity is about 7gr less than the 243Win. Capacity wise, the 6mmxc is first followed by the Dasher, then the 6mmBR.
Did I miss anything?
Oh, welcome to LRH!
What part of Oklahoma are you from?