With the new speed reload feature in INS:S, I believe that adding a revolver would be an interesting weapon type pair with the unique mechanic. Though unpractical in a warzone, this gun type would be interesting to use in INS:S.
I don't know if this is safe to do or possible to do in reality (I shot revolvers before, but never reloaded like this), but for regular reloads, the character manually takes out used cartridges out by hand and load the revolver fully. For example: I shoot 2 bullets, and when I reload regularly, I only take out 2 of the used cartridges and load 2 rounds.
The speed reload is obviously to push the ejector pin to empty the entire cylinder, regardless whether or not you used up all rounds in the cylinder.
A Speed Loader would be a gun mod like always, but this time, it's more expensive with supply points than before (in INS, the supply cost is 1, and maybe in INS:S, it would be more expensive and valuable, so maybe 3).
If the addition of revolvers is not planned, but considered, I do have some suggestions on what revolvers should INS:S have:
MR73 (One of the best revolvers in the world; Used by RAID and GIGN)
GP100 (I personally shot this revolver before, it's great!)
RHINO (Chiappa) (A uniquely designed revolver to break conventional revolver design)
LCR (Developed for conceal-carry, small but very lethal)
MODEL 10 (Back from INS)
M1917 (WW2 Standard Issue Revolver; uses .45 ACP instead of revolver cartridges; has both full or half speed loader, so loads 6 or 3 in a row)
NAGANT (Old Soviet Weapon; Not Possible to Speed Reload with its archaic reload system)