Whengunswentpew

I'll crack out the calibre tonight but from memory it should be an M3 nut.

the "gas" in a gbb magazine is a mix of liquid and gas.
 
when you fire the pressure drops, liquid portion starts boiling and the pressure raises back up, this is also why they tend to not like colder temperatures as the equilibrium pressure, and the rate of boiling will decrease. same reason why they don't like being fired quickly (where you're venting gas pressure faster than the boiling can top up)
 
it's not perfect, but it approximately keeps the pressure the same (close enough for our purposes) as long as there's liquid left in the magazine. if your familiar with cheap gas lighters it's exactly the same principle (and you can even see the liquid portion decrease over time in the transparent ones).
 
for hpa, this isn't the case, it's 100% gas, so to acheive a usable working volume it has to be compressed to much higher pressures then regulated down to the pressure you want to use. it's not really practical to build all this into a small enough package to fit inside a magazine hence using an external tank+regulator.
 
there are systems out there, notably the daytonagun kits, that operate like a gbb only with the air feed going through the gun rather than the magazine so you run it with a standard hpa setup. afaik the WE p90 can also be made to work like that due to the way they worked around the top mounted magazine.

The first time my dad's landlord came to his flat to collect the rent, he had to quickly explain why I was sitting on the floor with a bunch of disassembled guns.

The first time my dad's landlord came to his flat to collect the rent, he had to quickly explain why I was sitting on the floor with a bunch of disassembled guns.

The first time my dad's landlord came to his flat to collect the rent, he had to quickly explain why I was sitting on the floor with a bunch of disassembled guns.

The first time my dad's landlord came to his flat to collect the rent, he had to quickly explain why I was sitting on the floor with a bunch of disassembled guns.

I'll move this to the off topic section for you Tezza... 
 
Instead of "Gas Guns"...

Thanks, I appreciate the help and I'll let you all know how the project goes.

should be good then as long as your not under voluming a long barrel you'll be fine.

Ta ,sounds about right .Think it was about 380-70ish still at this length. Other advantage of a lighter hammer spring is an easier trigger pull where you still thinking of keeping it longish?

The carbine version is what i'm starting with as well. I'll have it back this weekend (left it at my dad's place before the lockdown hit) so i can check the barrel then and I'll see what I can do about that spring. 
The overall goal is to get the FPS below 350 if possible so I can use it at my local cqb site, last time i put it through a chrono it was around 470 on green gas so i'm hoping with some modifications and white gas it'll run below 350.
Also your mauser looks brilliant, nice work!

Being a nbb is more straight forward I think the barrel is proprietary , not sure if it has any similarities to any other available barrels. A softer hammer spring or tweak the existing one would be my first goto and the easiest method. Made this a good few years ago with the carbine version still fires over 350fps at this length. 

Try a wider diametre barrel too, reduces some of the pressure
You could cut the hammer spring, but I wouldn't do it unless I had easy access to spare parts. Best thing you can do, though, is to hand it to a decent tech

suppose i should open with the caveat that gas pistols aren't something i mess with too much, so these are at best pointers until someone with a bit more practice at this sort of thing comes along.
 
however you could look at a couple of areas.
 
firstly is shorten the inner barrel, pretty simple and more importantly temporary (ie you can reverse it again by swapping back)
 
the other is softening the spring on the rocket valve, this is what @Wo1f did to my kwc makarov when i bought it and i can confirm it's done wonders.

Finally got my G&G L85 carbine today, haven't had a chance to fire it yet but it feels damn perfect.

Been waiting ages to get this and it finally arrived today, can't wait to test it

Finally got my G&G L85 carbine today, haven't had a chance to fire it yet but it feels damn perfect.

Been waiting ages to get this and it finally arrived today, can't wait to test it

Props to Finius for this.