Muzzle velocity Differences between bolt release and cycled round??

[wrinklestar]

I am seeing on average a 35 fps increase on any rounds that have cycled into the chamber. The first round when I hit the bolt release is consistently around 2500 fps. the subsequent rounds are all 2535 ish. Is this consistent with what some of you are seeing?
The gun is a Vltor upper, shillen barrel, non adjustable gas block, std spring and buffer with an m16 (full mass) bolt carrier.
The reason for the question is I am ordering a quigley -ford scope I purchased at an auction a couple of years ago and the fps discrepency may prove to be significant at longer range.
Thanks