I think sometimes it's done for legal reasons (rifles below a certain barrel length in the US are special and need some extra permits, but permanently attaching some kind of muzzle device counts as increasing the barrel length, and using a dummy suppressor specifically can get you the aesthetic of suppressed compact carbines like the AKS-74U or CQBR without the legal hurdles), but in this case it might not be that, since the M1 Carbine already has a sufficiently long barrel. The page I got it from states it might be a movie prop or display item.
https://www.wargm.org/armoury/index.php?mode=2&cat=Fully+Automatic+and+Self+Loading+Rifles+and+Carbines&item=125
My favorite gun to use on the NLF side playing Rising Storm 2. In early war they only get access to WW2 surplus weapons compared to the yanks M16s, I always found the SKS to be a bit too much gun for that game, and preferred switching to the 30 round M1 since it didn't really matter how big the bullet was, getting hits in with a faster fire rate was really effective
It's currently on sale on steam atm for like, 1.50 CAD, btw, so I imagine that's like 1$ USD or something. It's a good time playing as the Viet Minh, less so as the imperialists but at least you get some fun options to play as there, such as the Australians.
