Excessive headspace=lower muzzle velocity?

[Mikecr]

Originally Posted by PapaChunk

does excessive headspace cause a slower muzzle velocity?

No. Same powder amount in a smaller case reduces barrel time, and possibly increases velocity. It won't decrease velocity.

[MNbogboy]

Mikecr has a good point, that a decrease in case capacity will probably not decrease MV to the degree you are talking about....You didn't mention if you were able to confirm the 7 to 10" change in POI with chronograph results...It is entirely possible to have that much difference in POI without much of a change in velocity in either direction...Barrel harmonics will always affect POI....And a change in case dynamics will most likely affect the measurable barrel harmonics...Again due to all the variables that most often come into play such as ambient temperature as Bassin93 mentioned....It is hard to say what exactly happened without all the stats...
An example is; both of my 260s always are slower with new brass (smaller in capacity and contradictory to Mikecr's theory) but burn rates complicated with the reflectivity of the inner case walls of the "shiny" new brass may attribute to this(my theory).....Second neck sized firings are a little faster and three and on fired cases stabilize just a tad under the second fired cases (now confirming Mikecr's theory)....
We can speculate all we want but this really is rocket science...We can only do our best to produce the most repeatable ammunition and deliver it in a redundant action to our target...Any deviation from the process will deliver different results....
Randy