Originally Posted by DonP
Well, that's true - for AEGs you have a set spring and a set volume, etc. It always pushes the same amount of air at the same pressure when it fires.
Well, not always. The piston head o-ring takes time to expand, and the lube in the cylinder will play a huge part in how fast this happens. This is not constant over time (as effective lubrication is reduced over time) and susceptable to temperature as well. The faster that o-ring expands, the more air volume is available behind the BB.
Ever notice how a poorly lubed cylinder can have huge deviations in velocity? I've seen 30-50 fps in lots of cases, or how re-lubing (and changing nothing else) will bring velocities up and keep them consistant. This was an easy issue to see in the PTW, as you used to need to perform periodic, specific cylinder maintenance which included a cylinder re-lube.