No surprise really, but I vote ballisticboy.

I think another factor will be the pressure build up ahead of the pellet.

And how much pressure there actually still is in the barrel when the pellet is at the muzzle.

However - with a moderator the effect of blast is measurable. Again no surprise. And again the factor above is critical. A small but measurable and consistent increase in MV was noted on an FAC PCP when a moderator was fitted.

Possibly with a springer there may be none at all - at best the blast maybe just reduces the drag, thus reducing the deceleration, rather than causing acceleration. This would seem to be the case not far beyond the point where any extra barrel length would cease to accelerate the pellet. It *will* be slightly beyond - as not having a barrel from that point would mean that there is no friction, thus the pressure there is still sufficient to accelerate a bit more, as some of it is currently just overcoming friction.