Ok, while everyone says it could be done, i haven't actually seen anyone do it, so here's some pics.
For parts we have:
Dummy 12G from Palmer's Pursuit
90* macro fitting
Custom Product rail
Custom Products on/off ASA
HPA tank...more than likely a 13/3000 or 22/3000
First thing i noticed with the dummy 12G is that it was very well made. It's solid, not hollow and just has the 1/8thNPT fitting and a hole from the fitting to the nozzle drilled into it. Has some feel to it so it's not cheap eventhough it only cost $15.
The problem however is that it is slightly shorter than a normal 12G due to the top being flattened
Not a big deal. Here;s what we need for install. The 12G, a penny, and macro and a fitting
Simply slide the 12g into the gun and postion the fitting hole on whatever side you would prefer
Next, put the penny in on top of the 12G. You'll notice it fits perfectly in the bore and is thick enough to make up for the small smount shaved off. You could prob make something but pennies are cheap and easily found. Make sure you use a copper penny (pre-1982) as they are softer and won't corrode like the new zinc crap. Put the cap on and tighten it up
Next, screw in the macrofitting. You can see it's a tight fit to the side. If you have access to a mill, you could prob enlarge the window slightly by maybe 1/8" for a little more room
Finally bolt on the rail/asa or whatever combo you will be using. I had to get longer screws for the rail to reach the threaded inserts on the ACP. Unsure of the side though. I beleive they were 10-32's but not sure. This setup is off my other paintball gun. Since i can only use one at a time with the HPA, i decided to just use my existing setup. If i ever want to play with the ACP as a secondary, i would convert back to use the 12G cartridges anyway.
Final shot. I used the 68/4500 just for testing. It's way too heavy to play with flipped this way. If you flip it with the tank as a stock, it does feel nice. A grip attached to the weaver rail would help and you would then have a sort of machine pistol. I'll be ordered a 13/3000 soon.
I've taken shots with it already. It's leak free, and just by the sound and by viewing it in action, i could tell you the velocity is definitly higher than with the cartridges. DO NOT shoot anyone like this until you chrono the gun. SInce work will keep me away from playing this month, i don't know if i will be able to get it chronoed anytime soon, but definitely be aware of it.
Now, i just need a hopper adapter