Can some one please tell me which model will fit perfect on a M203? Im trying to buy one of these but dont know the background/tech about them.
Thanks for looking,

ARMS S.I.R. systems.#45 #46C, #50M, #58M, #59

If you mean the conventional barrel hanging, the #45M and #45C, will allow them to hang the old way.
Jack

So even if its a military model (#45M) which I thought they said the delta ring must be removed, how or to what will it mount and what is so different from the rest? such as size,the bulk,..etc. Please correct me if Im wrong,
Thanks Jack

The M203 never attaches to the delta ring, only into the barrel nut and front provision on the barrel, so #45M or C makes no dif., the M203 goes up inside of the #45 SIR's and attaches to the weapon just like the SIR wasn't there.
The #45's are about 1/8" wider ea side to allow the launcher to mount in the conventional way.
The #45 hand guards channel are cut the same way as the slim lines, and the M203 mounts in free float fashion on all the models to include the #45, via alum. panels that look and act something like the hand guards except the bottom is clear to allow the bottom of the panel to attach to the M203.
Only the #45 allow dual mounting of free float and or conventional off the barrel. Right now ARMS only sell the panels to military customers.
Jack

Jack,
Great breakdown!! awsome.Things make since now. I guess I will go for the #45M, hopefully my M203 will fit perfect without the bottom of the panel.
Thanks again,
Apeman8