You mean transport as opposed to "mount" I assume.
Folks have adapted various scabbards and hard cases to motorcycles. Generally a hard case is mounted to a luggage rack on one side. Others have tried various take-down rifles or shotguns either in a pannier or hard case. Even a 48" Garand case isn't a big deal to carry sideways on your luggage rack.
If you are talking about something that would allow you to easily deploy a rifle, well, that's one reason why military motorcycle use is pretty limited: it's not easy to provide bounding overwatch from a hard case or scabbard.
Scabbards may not be usable depending on your state laws about transport of firearms: for example, if it was easily reached, some states require you to have a hunting license, and others prohibit or restrict such transport.
2005 DL650 (sold) | 2009 DL650A (sold) | 2008 Tiger 1050 ABS | 2012 WR250R | 2008 CRF100F