Well when it comes to cylinder volume you have two at least two variables at play at least two important ones. One is the length of the barrel the other is the weight of the ammo your going to be using. The longer the barrel the more volume you need, the heavier the ammo the more volume you need. If you have excess volume, increasing the BB weight will cause joule creep because the BB is staying the barrel longer and imparting more energy onto the BB. Basically joule creep is sort of unavoidable, if you've got enough volume to shoot the heavier ammo it's going to chrono lower with lower weight IE .2g BB's. If you don't have that setup then your undervolume which is also possibly a problem.
So all of that done I'm not honestly sure the best way to test that, there are allot of theories on how much volume is required, and how to calculate it. But from what I've seen and heard over volume really doesn't effect accuracy all that much if your fairly close to where you should be. It will be louder at the muzzle if you have too much volume, you'll hear more of a pop when the excess air exits the muzzle. I'm running a VSR10 G-Spec and it's hard to get more overvolume even with the .43g BB's I'm running I've only got a 303mm barrel and a cylinder that with that weight would be about spot on at 400mm. So it's very over volume but it's also pretty dang accurate and ranges past 100 yards.
For your build depending on brand of m130 and air seal that shouldn't be a problem at all, actually I'd start with an m120 for your FPS goal since I think an m130 is likely to overshoot it if you've got a good setup behind it. My M14 is running an m120 spring with a 550mm or so barrel and it's currently shooting around 440 or so. I expect it will tame down a little over time but it's pretty hot shooting right now for an m120. I've also got an M16 DMR project I've been building with an m130 in it and now that I've got the air seal coming together and working the bugs out it's shooting a pretty consistent 460-470fps with .2's and that's with an M16 length barrel. I'm actually likely going to drop an m140 in that gun to bring it up a touch since my goal with that rifle is 500fps.
On both of those guns they are full cylinder guns, I did put a ported m4 cylinder in the m16 at one point but the FPS dropped indicating it's under volume. AEG's don't have nearly the volume to begin with of a bolt action so I run the theory especially for a DMR that you almost always want as much volume as you can get IE full cylinder on any longish barrel accurate platform. Since you're going to be running heavier BB's, you'll need as much volume as you can get to really fling them out there. For that matter my M4 with 363mm barrel is also running a full cylinder from another longer barrel I tested in it, and I honestly can't see a difference in accuracy or FPS in there. It does a little better with heavier ammo because of the full cylinder but otherwise it's not a big deal.
So short version toss in a full cylinder it will give you more ammo weight options without going under volume, and should work better in just about any DMR platform. Maybe a short barreled say A&K SR25 with the extended gearbox you might need to port, but really most of those are longer platforms as well so likely full cylinder there.
Take care!
Luke
