I don't know if anybody besides me pays that close attention to the guns used, but there are several episodes where the Sigs are out in a "Non-firing" scene and they are actually using the Sig Sauer brand P229 (to stand in for the real P228) with the little orange cone taken off.
The most obvious scene is with Tony and Kate outside the nightclub when they are talking to the sister of a dead sailor (guest star was/is Mia the singer). Tony pulls his sig on one of the bouncers and you can see that the barrel is nowhere near the proper diameter.
So technically, the airsoft would be screen accurate.
That actually is a common problem when REAL guns, also, are used. We "blank" real guns in one of three common ways (generally).
1. Special barrel with a tapered bore. Allows the right amount of heat, gas & flash to escape but occludes the exit end enough to build up the required amount of pressure to cycle the gun (semi-auto).
2. A plug is fitted into the regular barrel. The plug is removeable (usually) and has a precision hole cut in the middle of it. The hole size is carefully determined in order to allow the semi-auto cycling to function.
3. If it is a common gun with lots of barrels available, a used (often shot out) barrel is simply welded closed, and the appropriate diameter hole is machined into it.
ALl 3 methods result in a smallish looking bore when filming a gun where the end of the barrel is seen, especially if light hits it.
Now, my comments are largely "old school" and YMMV, but my experience has been that crews will generally use a rubber duck (casting of gun, made of soft to hard rubber, sometimes cast aluminum or resin, but usually rubber). This eliminates the need for a certified gun wrangler since blanks, squibs, etc. nor any real guns will be used, a prop guy can handle static props. They will, sometimes, simply use the real that have been rigged for blanks but that requires an armorer to be on set.
You're probably right on the NCIS example, though. Clearly there is a surge in the use of airsofts and replica guns designed from the beginning to shoot blanks but there are still lots of real guns out there that have been converted by entertainment industry armorers as described. Good eyes on your part! :ninja