I believe he means a gun mounted to a rail. Pretty solid rest.
I have been thinking of putting a saboted .22 bullet in a .45 ACP for years. I don't understand why one would go through the trouble of designing a whole new caliber with attendant barrel, brass, and platform issues, if an existing system can be easily modified to fill the job well. Is there some intrinsic flaw with using a sabot? It cannot be high velocity, as the Armed Forces experimented with SLAP sabot rounds for years. Why not put a sub-caliber saboted projectile into a .44 Mag, for instance? Then, one can still use the 'ole "hawg leg" for the task any sane gun owner would ordinarily use a .44 for, with the option of shooting through an APC as close as the cartridge belt or butt cuff. I'm sure the saboted rounds would be hell on varmints, too!