no can do- it has the word "uninterrupted" in the description- that means complete barrel- believe me this has been around since the 70's and carbine fans north of the 49 have tried everything you can think of to "get around" putting a new barrel on- it's nothing new except for the plainsfield aspectIf it's acceptable, I'd add 1/4" to the end of the barrel, but not as a muzzle brake, but just something that makes the barrel longer and looks the same as the end of the barrel now. That would be fairly easy to do. Just turn down the end of the barrel, and add a piece that slips over the turned down section, and silver solder in place. Refinish to match.
IIRC the overall size and barrel length came about with the 1934 Gun Control Act (NFA - National Firearms Act). That predated the M1 Carbine.I read somewhere that minimum US rifle barrel length used to be 18", same as shotguns, but was specifically changed at some point to allow the popular (and often war trophy) M1 carbines. Dunno if it's true or not.
Maybe the G changed the spec because they wanted to sell carbines through DCM?