Amongst the FN literature on the FN303 is the option of an underbarrel mount for the M-16, M-4 and SCAR. After an extensive amount of Googling I have yet to find any evidence of use in this configuration by police, military or anybody else. The mounts are listed by various FN resellers but seem universally out of stock. Midwest Gun Works, which for several years have been the definitive source of FN parts, does have them in stock however they are $424 and even if you were inclined they won’t sell them to mere civilians. So what’s an amateur gunsmite to do?
From the few pictures I have found online the FN configuration of an undermounted FN303 simply utilizes an M16 with the lower half of the hand guard removed which, in my estimation, looks like crap. To make this a package more appealing to discriminating tactical operators such as myself I went with a 16″ Dissipator style upper which offered a compact length but could also utilize an M203 style hand guard which solved the aesthetic issue nicely.
Though FN also offers an M4 mount I have yet to find any pictures
of a FN303 in that configuration and I have my doubts about it as the barrel and tank would extend past the muzzle of a 14.5″ M4 barrel. Even with the 16″ barrel the aluminum tank full of 3k psi air would be exposed to muzzle blast from the right side of the flash hider. As spare tanks for the FN303 are both expensive and extremely difficult to find for civilians I chose to replace the flash hider with a Manticore Arms Afterburner Linear Comp.
Upon initial inspection of the FN303 the mounting rail that attaches to the standalone stock looks similar in dimensions to the 11mm rail system found on airguns. A simple 11mm to Picatinny adaptor could be attached to a barrel clamp mount which is all available for under $40. The reality is a bit more complicated.
As it turns out the 11mm rail is much too “shallow” for mounting to the FN303 lower. When choosing a 11mm to Picatinny adaptor it will be necessary to choose one with as much height as possible to allow for removal of material without compromise of the integrity. The best solution, in my estimation, is a HZW adaptor mounted to a UTG barrel mounted clamp linked to below. It’s all cheap Chinese stuff but it also seems to be solid when tightened down with Loctite.
Modification of the 11mm adaptor is relatively simple though, depending on the tools you have available, will take either minutes or hours. At the most basic level the required modifications could be performed with a triangular file in a few hours however I took the rich man’s route and used my Harbor Freight mini belt sander to remove material a bit more quickly. I cannot give exact measurements as I hand fitted these parts however I would estimate that it’s necessary to remove 3-4mm of material before the 11mm dovetail will fit into the FN303 rail. Gross material removal done with the sander the rail can be fine tuned with a triangular file to ensure a tight fit. Additionally it will be necessary to drill the rail to fit a roll pin to engage the catch to hold the launcher on the rail.
Once the adaptor can be fitted to the FN303 it is straightforward to connect it to the Picatinny barrel clamp. I used a prodigious amount of Loc-Tite an the various screws to secure the whole assembly would not come loose on firing. I suggest using a PMag to line up the center seam of the FN303 with that of a magazine in the well along with ensuring compatibility with all standard capacity and 40 round magazines.
Modification of the M203 hand guard was simply a matter of removing a very small amount of material on the right side to clear the tank mount and a 1/2″ x 3″ from the left side for the thumb screws of the mounting parts. It may be possible to further modify the mounts to minimize this modification to the hand guard however I chose not to bother at the time and now mildly regret it. In any case the M203 hand guard fits snugly on the FN303 as if it were designed for it.
Fully loaded the AR-FN303 combination pushes the scales at 13 1/2 lbs with loaded magazines, air tank and optic shown. An optic zeroed for the rifle is more than sufficient to achieve center of mass hits on targets with the FN303.