Since I want to expeiment with a lot of different barrel configurations, and cartridge designs (plus some other esoteric stuff), I am giving some thought to building a universal receiver.  I will want to try some short barrels and long barrels (even cutting a barrel down progressively).  The thing would be mounted to rails, and be bench mounted (though portable).  It would not have a conventional stock.

Would registering it as a DD allow me to use any barrel of any length and any caliber, or not?  I'm thinking that the DD classification would allow me to use calibers above .50, though I at present do not have such plans.

Or, are there special classifications for univeral receivers that allow for any caliber and length?  This, of course, isn't going to be something that one could carry concealed.  

Dave.

I'm bumping this for those who use the Active Topics button.  (Like me).

Dave.

Different barrel lengths AND calibers??!  A universal receiver indeed!

I'd do a DD, then.  A SBR or SBS would allow for different barrel lengths but not the >0.5" bore diameter.  DDs have no restrictions on barrel length and they cover the bore diameter, so that seems like your best bet.

OR, you might be best served by sending a letter to the ATF's Tech Branch detailing your plans and asking how they want you to proceed.

Mike