[SilentScope]

How will barrel length affect fps in bolt action rifles?

Barrel length can increase or decrease FPS depending on your cylinder volume and air compression efficiency. Assuming very good air compression efficiency, you should focus on calculating your cylinder volume to barrel volume ratio which would involve math. I was actually involved in a thread not too long ago so I will refrain from re-posting the conversation here. This thread was originally posted no longer than 4-6 weeks ago. Search and you will find them.

But what is the optimum barrel length (regarding fps) for snipers (VSR10 and APS2)?

Your question about optimal barrel length can be better answered if you can give your personal definition of "optimal" in these contexts.

Also, does having a longer barrel and a larger cylinder help when shooting relatively heavy bbs

What do you mean by "help"?

 

[SilentScope]

I beleive that the lighter BB the .2g is out of the barrel before it uses the full acceleration of the piston and all the air, hence a lot of energy is wasted. Through using the heavier bb it is using more of the air from the piston and cylinder and is more efficient.

You are mostly correct. This is actually what most people believe and have been spreading as truth. Aside from that, most people leave out a very important component- mass.

E= 1/2m*v^2

A heavy BB carries more energy at the muzzle because the heavier weight embarks less air resistance and more momentum as soon as the BB starts to move.

Lighter BBs slow down quicker/more than heavier BBs so this correlates to lower chronograph readings than what is mathematically expected.

If it were possible to chrono a 0.2g and a 0.36g BB perhaps 1/10th of the way in the inner barrel as it's being shot, the energy readings would probably be much closer than the moment it exits the muzzle.