First - this is clearly something that can be on the backburner.
Second - I have a hardcopy of the official guide - I like to actually flip open something in my hands ...
Third - in the manual the stock P416 compared the Bushman makes no sense, to me. The stock P416 has a somewhat long barrel as presented - similar to what you see on the screen in-game when in the shop file - but not carried. The Bushman OTH has the suppressor, I assume, replacing the original barrel, with that 1-2" inch gap as already noted. The other Signature ARs which now have suppressors, Warrior and Trooper, clearly have the suppressor attached to the front of the existing barrel.
So I guess we're back to what happens in the real world as it relates to the P416 barrel and how a suppressor would actually be attached - as compared to how that is represented with the other models utilized in FC4.
There's no sight on the P416 barrel for example, basically a straight tube, unlike the others. You could slip, I suppose, a suppressor over that barrel and it ends up with that small gap (but how is it locked down?), or do you detach the original barrel and then fit a suppressor unit which comes already set with a few inches of barrel, to be attached to the P4216 body?
Clearly the last two paragraphs have nothing to do with the game ...
"The NUKE, are you crazy?" "Oh relax, it's got a very small yield. Just enough ..."