Originally Posted by Cliffradical
All of those baseplates will develop leaks over time. It's a common (inevitable) problem with the pinned baseplate style mags. To correct it, you need to remove the baseplates, clean the absolute hell out of the parts, and get some blue gasket sealer onto the mating points. This should be considered a permanent fix, so be sure to take your time and do a careful job.
Let them cure for *at least* 24 hours before you charge them and they should be good to go.
The fill valve may just need to be tightened, or it's teeny tiny o-ring may be dry/ warped/ damaged, or need to be replaced. Fiddle with it a bit, but prepare to shell out for a new one if you have to.
If you don't have a proper valve key or can't find one to buy, you'll want to cut one for yourself out of another tool using a dremel or a grinding wheel. Using a flathead on those things is not recommended.
Gas efficiency is a pretty complex issue dependent on all sorts of factors including temperature, time between charges, and how rapidly you're firing the thing. Without seeing what it's doing I couldn't tell you whether there's a problem or not.
KJ M9s kick pretty hard and aren't terribly efficient as-is, so if you're getting an entire mag out of one propane charge I'm inclined to believe that your gun is running just fine.
Failing to hold open on an empty mag is probably related to worn springs or general fouling. Clean your gun real real good and see if the problem persists once your mags are fixed. If it does, you can find a spring kit pretty cheap online or through a retailer, or you can just live with the lack of hold-open and keep track of your rounds.
Hope that helps, welcome to the Beretta club!
Thanks for the helpful answers!
I ran another couple of mags through the gun today with the good mag and this time I was able to get a full 26 shots and then some on a single fill of gas, and the slide locked back so thats no longer an issue. I think i just need to master the process of getting a proper fill.
As for the other mags, I'll try to find myself some gasket sealer and also fashion myself a valve key of sorts if I can, so hopefully I can get those fixed.
I'll also see if I can track down a chrono so I can get a more accurate sense of what the gun is doing.
Now if the gun is shooting around 360-370 like I though it was, is there anything I can do to lower the velocity down to CQB levels?