Accomplishment aside, I proceeded to the second phase of my project: compiling the necessary specs and materials to build the carbine variant. About this time lady luck smiled on me: my friend Brad began taking orders for replica carbine kits. Getting one would provide not only the opportunity to obtain a carbine stock; the included barrel extension would also give me the necessary measurements needed to produce a functional barrel. (Yeah, when all was said and done I'd have enough replica components for an attractive non-functional display for my office along with a shooter (I doubt my co-workers at NRA would object)—so, what's not to like? When ordering my kit, I asked Brad if he knew the overall length (OAL) of the carbine barrel. He responded by telling me the extension piece that screwed into the pistol measured 6 ½ inches—minus its threaded stem, which suggested both prop pieces (pistol barrel and carbine extension screwed together) totaled 9 ½ inches OAL. Eureka! But that raised another concern. Any firearm containing a barrel less than 16 inches and a stock requires registering it with the Bureau of Alcohol, Tobacco, Firearms and Explosives (BATFE) as a short-barreled rifle (SBR) along with paying a $200 stamp tax (what fellow co-worker Ed Friedman here at Shooting Illustrated refers to as "The ATF compliance fund").
With an overall length of 9.5 inches for it's carbine barrel, the U.N.C.L.E. Special must be registered as an SBR.
Further challenges involved seeking a method to securely anchor the handguard to the carbine barrel without adding reliability-hampering mass.
The carbine barrel could be made couple ways. While the sectional attachment looked cool for television, overall practicality and machining costs ruled it out. The remaining options included making one out of a section of rifled 9mm barrel blank (but replicating the portion that sets inside the frame—otherwise known as the barrel block—would prove to be another machinist's cash cow). Another (also involving use of 9 mm barrel blank,) was to cut an original barrel at end-most portion of the chamber, just before the rifling begins and to thread them together and permanently bond the pieces with Red Loc-tite. (But again, that left the high labor cost of replicating the barrel block). The most economical option involved cutting and threading an original pistol barrel—similar to how the pistol barrel was prepped for the flash hider, and then
permanently attaching an unrifled extension to it. After purchasing a spare P38 barrel from Earl Sheehan (of Earl's Repair Service) I sent it to my gunsmith (along with a 9 mm Olympic Arms AR upper receiver—part of another SBR project) and asked him to use the excess from the Olympic barrel to produce a 9 ½-inch P38 barrel (with the end threaded for either the flash hider, or my suppressor). Later, I bought a plastic carbine hand guard for Larry Green and prayed the added weight wouldn't cause cycling problem. The new barrel functioned fine with the flash hider, but the added inertial mass of my suppressor caused problems. In addition, the handguard would also slide during recoil despite the use of its built-in setscrews.