Have a word with your local players, See if you can get a mentor to show you how to strip and rebuild the gearbox. Offer beer or pay for the next skirmish ect but ask that they show you rather than just fixing it. Failing that there are many you tube videos, success is not guaranteed and you may make things worse or not be able to rebuild it. Once you have done it a few times it's very easy to do, and will save you a fortune in having to pay a tech to fix minor faults.
The spring might be weak. M100 is a safe bet for 330ish fps. Because you have the quick change system you don't need to open the gearbox. Just make sure you have the correct tools for that skirmish you are paying your mentor to go to, and get him to show you and chrono on site. It'll take 5 mins, and you will know instantly if you need to do any more tweaking.
It could also very feasibly be a poor airseal. Piston 0-ring, Cylinder head 0-rings, air seal nozzle are all contenders here. Pull the gearbox out, Check the piston is forwards by looking at the spring compression. Then blow reasonably hard down the airseal nozzle. The gearbox should resist and stop you been able to blow into it like this. Even a new gun can have airseal problems, and it's worth checking before you spend any money on springs ect.
If you hear air coming out at a frighting pace at the back of the airseal nozzle you have suspect number 1 (a slow leaking airseal nozzle is common if not an o-ring type, generally this isn't a problem).
If the pressure is slowly backing off, or you feel like no pressure at all is building then it's probably the piston o-ring. You should be able to feel this o-ring pop and lock. If the cylinder is ported you may find the piston is further back than the port. You can use something long and thin down the airseal nozzle to depth gauge the piston position if you have to. You can always cover a port with a thumb if you have to. Or cycle to gearbox with a dry fire to try get a better piston position.
If it feels like air is been blown back at you, or escaping round the front of the cylinder area, it'll be the cylinder head o-rings.
If it feels like it pops and resists even your hardest efforts to blow into it the airseal is perfect. (There is no need to strip a box to check the o-rings).
Spit bucking, hop wound on too much, poor hop bucking seal. A new rubber is a few £. You should always have a spare so get on the net and order a set. Watching a few videos, and rebuilding it with a new bucking correctly is not a hard task. If you mess it up, ask your mentor! Chances are they will fix it in 5 mins in the time before briefing in that game you are paying him into
A split bucking can significantly alter FPS.
Tight bore barrels are also a bit of a money sink. You don't need one at all. It's going to add a very small fps boost. Provided the stock barrel is in good order then there is no need to upgrade. At most it's going to have a minor FPS increase. You would be better served with an o-ring airseal nozzle, A shim set, and some decent bushings for the gears if yours are nylon (make sure to measure them before you buy). If you decide on this upgrade then get the parts before you get your mentor over, That way they can install them and shim the gears as well.
