I am planning to put together a .250 Savage Improved on a Zastava Mini Mauser action with a magazine length of 2.37". The case length is 1.91". Neck length is ~0.275". The bullet of choice is the Barnes-X (or XLC) 100gr. boatail with an OAL of 1.09". The math says this bullet will extend into the case about ~0.355". Is this too much?
Thanks!

No, but you really don't need a boat tail either. Flat base bullets are a bit shorter. Sounds like a neat project. Luck to you.

OK, how about a theoretical question for the ballistic physicists... Assume a load has 75% of the case capacity taken up by powder and the bullet seated at the base of the neck. Another load, same charge, used the same bullet seated deeper for magazine clearance. The bullet takes up airspace (but not powder space), effectively reducing case capacity, but only until the bullet clears the neck, then the case capacity is equal, right?. Would the deep-seated load generate higher pressure or does the bullet clear the case enough before peak pressure is reached?

Waterfowler, Barnes recommends seating bullets a minimum of .050" off the lands. I also agree with using a flat base bullet in this weight. What were you planning on using it for? Whichever bullet you decide on it would probably be a good idea to have the chamber throated for the bullet and magazine combination. Good luck. Paul.