I didn't know that the carbine barrel will fit the bullpub model so I asked to Charles and he said they drop right in you could even use the factory shroud..the length of the carbine barrel is almost 22" the same length in the taipan long..so if you have a standard or short you could upgrade to the long barrel .. the standard with one plenum..the short with 2 it should look like a factory setup...I order one in 177 n one .22 will post the pics ones they are installed..