When I posted about the L119A1 D.I.S. last time, I forgot to mention that Begadi actually makes the L119A1 barrel extensions as well. This allows the player to turn, for example, a M733 rifle into a long barreled L119A1. Making the SD version of the said weapon is not, however, possible with this extension. The silencer needs to go all the way to the front sight to be realistic. Either way, this extension is a good way to make a L119A1 long barreled version. You can locate the barrel here:

Begadi L119A1 barrel extension