6mm ARC Gas Gun Barrel Lengths and Muzzle Velocities

I'd like to know what muzzle velocities are actually being achieved by "Hide" members for their pet handloads that are tipped with Hornady 103-grain ELD-X bullets and fired from 6mm ARC gas guns having either an 18" or 20" barrel length. I'd also to know what the preferred choice of powder is for those handloads.

Thanks in advance for your input in response to my inquiry.