I'm new to airsoft shotguns. When I was looking for a skirmishable shotgun to use as a primary weapon, I came to the conclusion that the best options were the 6mm ACM M500 or a tri-shot springer (TM, UTG, TSD etc.) I settled on the M500 because I loved the feel, look, lack of shells and 5bb spread. However, I was a little disappointed after fielding it for the first time. The gun shot very consistent with good range and a nice spread, but only for the first 5-8 shots. The gun holds enough ammo for 24 full shots, but will only do ~16 shots with usable range. I wanted to use this gun as a primary, and having to carry gas around and refilling under fire would not do. Not mention the the problem of finding parts, especially fill valves. So I jumped in and decided to go for broke. I was either going to modify the gun to meet its potential or completely destroy it in the process.
Air Rig Modification:
The most irritating feature of the gun is its inadequate gas reservoir. The gun performs well at higher pressure and only the first several shots get to take advantage of that. I had my CO2 tanks for paintball around, so I decided to build an air rig and adpt the M500 to constant air.
Here is the fill valve:
The valve screws into a metal nub the sticks out of the bottom of the gas reservoir. I had two options to adapt the gun to take air fittings: fill valve external CO2 adapter or drill/tap the gun directly for common 1/8 npt fittings.
KWC style fill valve CO2 adapter:
I chose to drill and tap for 1/8 fittings. I didn't like the idea of the adapter because it is difficult/expensive to get parts that are compatible with the quick connect adapters. Also, no one could tell me with any certainty that the adapter would fit. Here is the finished rig (no pressure gauge installed in the pic):
I was planning to do a step-by-step build report, but I was on a roll last night and neglected to take photos while working. Maybe if I convert an 8mm version, I'll do a build report. Feel free to ask questions if you're thinking of doing the mod though. We need more shotgun players
I used a paintball remote hose to get the hose and quick connect fittings for this project. It is a quick/cheap way to get the parts needed, but they put some sort of adhesive sealant on the threads that take A LOT of force to unscrew. I damaged the on/off valve and ASA adapter on the ends getting the hose and fittings off.
Paintball remote hose:
Here is the fitting into the bottom of the gun:
I looked up the vapor pressure of propane. It reaches ~200psi when it is at 90F. This is the max I'll run the gun at. Currently I'm running 150 psi into the gun and It's firing better than a full fill of gas. The modification is working fine for now, but still need to test it out in a game. I don't think I'll have any problems with the system though, it's straightforward and simple. I am now the proud owner of the coolest shotgun ever Not really, but it's perfect for me
Next project might be an 8mm version or add a tightbore to the current gun. I also want to shorten the external barrel.
The ACM M500 is a beast. It's heavy, intimidating and has power that few airsoft shotguns can match (400+ fps with one bb). Unfortunately, it is underpowered and finicky in its stock form. If you want a super reliable shotgun with good performance, get a tri-shot springer. If you are willing to put in time and money (not a lot of $$$ compared to AEGs), the ACM M500 will be one of the coolest airsoft guns you'll ever own.
Edited by filipinofreak, 01 November 2008 - 07:49 PM.