KJW M9 problems
 

So I recently purchased a used KJW m9 and I'm having a couple of issues with both the pistols and the mags.

Firstly, at least 4 of the 5 mags I got with the gun are leaking. I know three of them are leaking from where the baseplate seals with the body of the mag, and the last one seems to be leaking from the fill valve. What would be the best fix for the leaks without having to replace the fill valve? They're all green gas mags by the way.

Also the gun seems to be shooting to hard for it running off of green gas. I dont have a chrono but I did the pop can test and from about 1-2" away the BB's are cracking the bottom of the can. The gun will also through and through a pop can from about 15 feet away as well. So I'm not sure if this is proper for the green gas mags or not.

Lastly, with the one mag that doesnt seem to be leaking, I cant seem to shoot more than a full mags worth of rounds before I run out of gas. The slide wont even lock back after the last round. Im wondering if this has to do with the second problem that I was having, where maybe the gun is expelling too much gas at once which is raising the velocity and killing the efficiency, but im no expert with gas guns.

Any insight into any of the three problems is greatly appreciated

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?

They should sit around 330 with .20s on GG/ Propane (if that), a bit more when the gas is very warm, less when the gas is cold.

The pop can method is highly unreliable. It'll give you a very rough ballpark, but it can't be trusted as results are inherently unrepeatable. I'd actually be very surprised if your gun is shooting 360-370 unless your house is over 30C.
Under those conditions they tend to run into malfunctions where the valve knocker won't be able to hit the valve hard enough to open it.

Fair enough. Im currently trying to track down a chrono so once I get one of those hopefully shed some light on how hard the gun is actually shooting.

If you're going as far as trying to use the 'pop can method', it shows that you're a conscientious player, and that's commendable.
Buying a chrono will simplify your life a great deal. :)

I definitely dont wanna go out and hurt anyone, and I go crazy if any of my gear isnt performing perfectly haha