Would the 300gr. XTP from Hornady be too much for deer out of the 44 mag.?
I don't think so. Hornady's load is pretty modest from a velocity standpoint and there is a good chance the bullet doesn't open, but it'll kill deer.
300 gr. XTP in .45 Colt at 1100 fps worked perfectly on mountain lion for me. End to end penetration. One of three bullets recovered, other two somewhere in intestines. Full mushroom expansion, little weight loss. Can't say about the .44 Mag but 12 gauge at 2000 fps with 300 grain isn't "too much" if Hornady is to be believed. Expansion from 850 fps to 1800 fps is reported by Hornady for 300 grain XTP. Probably in ballistic gelatin so real work experience may vary. Black bear fall to the .44 Magnum 240 grain Hornady XTP in 24 Hour Campfire blogs are to be believed.
