Encore frames are registered, in a sense, that is why you need an FFL to ship or recieve a frame. Here in NY, if you plan to install a barrel less than 16" you will also need to register it as a pistol. But that will not stop you from also using it as a long gun.

Encore frames accept all Encore barrels; pistol, pro-hunter, regular Encore.

glockman, you will only see a marginal increase on muzzle velocity in a .243 by increasing barrel length from 24" to 28".