DealHunter,
I thought you may be interested to know that a gentleman on RFC tested his 18" American .22 WMR against a 22" Marlin XT .22 WMR.
The results are in post #5 of this thread:
Final Chapter.
Summary:
He lost 13 fps with Dynapoints, lost 23 fps with CCI 40 gr TMJs, and 'gained' 1 fps with CCI 40 gr JHPs.
As far as I'm concerned, those are all well within the standard deviation of those loads. So, you can look at it as
no change in the velocity.