I know just bringing up the topic of Barnes X is enough to get one banned from this group, but I have a gun that shoots them and a question. A mentor of mine with vast experience loves the Barnes X for moose, but he will not use them on deer because (for him) they do not open up unless they hit bone. I know the people at Barnes say different but does anybody have any thoughts. I like to use them because of the higher velocity with their coated bullet.

I have used Barnes .284 XBT bullets on whitetails and have not had a problem with them openinmg up. It is unlikely that you will recover an expanded bullet from a broadside shot. They zip right through.
Frankly, I don't think the Barnes X bullets are all that great for whitetails. They are stronger then necessary, lack the shocking ability of other conventional bullets, foul barrels, are usually the least accurate bullet in a given rifle and cost more than you need to spend for shooting whitetails. Sure, they work but they aren't necessarily the best for thin skinned game.
Deer that I have shot broadside with other bullets (Sierra Gameking, Hornady Interlock or Nosler Ballistic Tip) have usually dropped faster then the deer hit with the X bullet. However, if total penetration is the bottom line, than the X bullet is hard to beat.
As to the higher velocity of X coated bullets; I really haven't seen that nor would it make any difference to the dead whitetail. I did notice that the blue coating gets stuck up in my seater dies. Another X pain in the butt. I'd rather buy the bullets naked and moly coat them. Works fine.
Just my opinion. I'm sure the die-hard X bullet men may disagree.

VH

I've shot a lot of animals with X bullets, and I like them. I have never lost a deer, elk, caribou, bear, eland, wildebeest, cape buff, nilgai and many other animals shot with an X bullet. I prefer Hornady for deer sized animals, cheaper and usually very accurate, but the X will definitely do the job..

I've had too many failures with Barnes X, but I do like the GS HV monolithics so far and I like the failsafes also, so far....but I am not sold on monolithics as yet, but I'm trying.