Is there any way to estimate the pressure change going from a full length sized case to a neck sized case? I was wondering if this could have a significant affect on muzzle velocity?

Shouldn't be significant for sure.

You'd have to have a pressure measuring setup to try to dothis, and even then the likelihood of getting an average pressure level with a statistically significant difference is nonexistent. I would be very surprised if you got much difference in MV either.

Try this: Take one of your fired cases and FL resize it. Then weigh it, fill it with water, and weigh it again. That'll tell you the case capacity in grains of water when FL sized. Next, dry it well, reload it and fire it. Nowweigh it again. Then size the neck only,fill it with water again, and weigh now. How much more water does it hold now vs when FL sized?? Any difference you can detect with your powder scale? I'll bet the difference will beless than 1 grain of water............ (the reason I suggested using the same case twice is that the weight difference between two apparently identical cases can be enough to invalidate the comparison. Additionally, the water capacity of the "identical" cases may be different as well.)

You really won't see much difference in velocities, but you will see your velocity spreads go down and more consistent velocitieswith neck sizing only. And even that might be slight.

I GENERALLY do a partial FL sizing with cartridges I have already fired in the gun they'll be going back into, and I have a number of such loads that have chronographed velocity variations between shots UNDER 10 FPS-some have even chronographed THE VERY SAME VEL for several shots......

I have yet to get such nice variations in velocity. I do not know if its my chrony or my loads. With my bows, I get spreads of 2fps, and with some pistol loads, I get +/-10fps, but with my overbore guns like 300RUM, I got spreads from 40fps to 100fps. And neck sizing seemed to help lower the high ones. Maybe its just the load. Best I get with rifle is my 7mm08, I get 20fps spreads.

I always contributed it to my F1 chrony.

I will confess that the cartridges that have shown those low shot-to-shot variations have been small capacity cases, like the 7.65X53 Arg., 7X57mm Mauser, etc. The larger capacity rounds have generally all had larger spreads. I believe that case capacity vs bore size had a lot to do with this. I too use a Chrony (Beta) these days, but originally used an Oehler. The Chrony is SOOO much easier and faster to set up!

The difference in powder capacity was mentioned. However, what about the room for case expansion in the chamber. A full resized cartridge should have a little room to expand in the chamber and a neck sized one will have none. Could that make a difference?

It certainly could-any change can make a difference. But the question is, would the difference be significant enough to cause a problem, and I don't think so!

Velocity is definitely proportional to pressure(obviously). IMO your question depends on each individual rifle. As stated above the difference is minimal. The major difference is obviously going to be case capacity when it is neck sized. Tom.