Can you tell us your type of rifle?
Also you need to match the barrel to the cylinder. If the barrel is to long you will not have enough volume. So if you can tell us your inner barrel length, then that would help.
Also, could you tell us the current spring rating that you have in the gun?

 

I have a j-bar 10 and waiting to here the exact parts from a tech that helped rebuild it. Pretty sure the cylinder and barrel is stock. Current spring is worn and about to be replaced, but it shoots currently 374 (.2) and 345-349 (.3)
Spring is most likely about to be swapped for this
Angel Custom PSS10 Mega-Up VSR-10 Power Up Spring - SP150 (420~520 FPS), Accessories & Parts, Internal Parts, Springs, Sniper Rifle Springs - Evike.com Airsoft Superstore

 

Current spring is worn and about to be replaced, but it shoots currently 374 (.2) and 345-349 (.3)
Spring is most likely about to be swapped for this
Angel Custom PSS10 Mega-Up VSR-10 Power Up Spring - SP150 (420~520 FPS), Accessories & Parts, Internal Parts, Springs, Sniper Rifle Springs - Evike.com Airsoft Superstore

I believe I read that those springs from angel customs take a set very quickly and lose about 25% of their power in about 2-3 months

 

This may help. It is from my VSR guide that is in my signature. You should read it. http://www.airsoftsniperforum.com/41-vsr-rifles/6075-advanced-vsr-sniper-building-guide.html

"TUNING THE CYLINDER TO BARREL TO BB WEIGHT.
Note: This will give the bb the maximum energy that your rifle can produce and it will make your gun very quiet.
(This will work better than any other mod to make your gun quiet. It is even better than filling the stock with foam which will make your gun heavy.) If the cylinder to barrel ratio is not matched right (Barrel is to short) then the excess air after the bb leaves the muzzle pushes the cracking sound from the piston slamming into the cylinder head, out the end of the barrel. If the barrel is the perfect length for the cylinder (depending on your bb weight), then most of that noise will be trapped in your barrel.
The way this works is that a lighter bb does not take much pressure to push it through the barrel. So it needs a longer barrel otherwise it will be pushed out to early and therefore making the slamming sound. A heavy bb needs more pressure to push it out and so if the barrel is to long there is not enough air in the cylinder push it out properly and this will make the bb fly less accurately. So you need to get the perfect match between cylinder to barrel to bb weight.
In effect your bb weight can alter the barrel to cylinder ratio. The heavier a bb the shorter the barrel it needs or the lighter the bb the longer the barrel should be.

Here is a rough guideline as to which bb and barrel to use to get the most efficiency from your cylinder.

So chrono your gun and work out the FPS. Then look at the chart and work out the weight bb you need for your FPS. (You can always use a slightly heavier bb and the energy/backspin of the gun should still be able to carry it)
330-370fps = 0.23-0.28gm
370-420fps = 0.28gm-0.30gm
420-450fps = 0.30gm-0.32gm
450-500fps = 0.32gm-0.36gm
500-540fps = 0.36gm-0.4gm
540fps upwards = 0.40gm-0.45gm
(These FPS/BB weights can be used with any gun setup. So if your pistol is firing at a ridiculous 540fps then use a 0.40gm as long as there is enough air in the cylinder to push it out the barrel)
(Also note that often you can increase the weight of your bb if you use a longer nub on you hopup rubber. The longer nub will help to create more backspin and this will help carry the bb further. This is why R-hop is so effective)

Now to get the best efficiency from your VSR10 cylinder with this bb weight you need to get the right length barrel
Note: I use 6.03mm barrels and information in purple is for the tech head type people and so if you are not into tech stuff and are just starting out please ignore the purple text

Action Army Piston Dimensions
Piston Travel Length: 90.2mm
Cylinder diameter: 22.5mm
Compression Volume: 35864 mm^3

So see the chart below:
0.3gm bb use a 554mm barrel (15821 mm^3) = 2.26:1
0.32gm bb use a 530mm barrel (15136 mm^3) = 2.36:1
0.36gm bb use a 500mm barrel (14279 mm^3) = 2.51:1
0.4gm bb use a 470mm barrel (13422 mm^3) = 2.67:1
0.43gm bb use a 430mm barrel (12279 mm^3) = 2.92:1
0.45gm bb use a 400mm Barrel (11423 mm^3) = 3.13:1

Note: These Barrel lengths may change as I try more barrel lengths. Also when testing for the perfect barrel length for each bb weight I had no air break on my piston. If you have an air break then the barrel lengths will be shorter than I have stated here."

 

I wonder how the Cylinder-Barrel-Ratio (CBR) is calculated with TNT barrels. They have an inner diameter of 6.03mm at the height of the hopup unit and at the end they are 6.35mm.