In my 15 inch 300 whisper I'm shooting 110gr vmax with 18.3 gr of h110 and sighted in 1 and a half inch high,I tried some 110gr GMAX and they shot best at 18.2 but group three inches lower.I don't have a chrony and know I should get one but cant figure out the big difference in poi at 100 yrds.

I shoot the 110 gn vmax and the 110gn tac-tx in 30-30 and the poi is about 4" different at 100 yards even though the load is the same and the velocities are very similar. The coppers shoot lower for me. I attribute it to the different bullets. The pressure behind the Barnes bullet appears to be b higher as the primers are a bit flatter.

120 gr. Vmaxes shoot 4" high in my 6.5/.284 XP and Encore. 180 gr. .325 Nosler ETips shoot 3" to the left in my .325 WSM XP.
Anytime you get a bunch of bullets shooting to the same spot consider yourself lucky.
In other caliber handguns these two bullets shoot the same as all the other I use.
Welcome to wonderful world of ballistics/firearms/unexplained results. Some things "just happen".

Always remember not use your standard (jacketed) loads when using all copper bullets. The increased friction can cause over pressure situations. Even different brands of copper bullets can differ. Stick to loads specifically designed and tested for the bullet in question.

I thought it was an appreciable difference in velocity but if the gmx are running around the same pressure by all standard measurements the 3 inches lower poi were a mystery to me.

I had a 22 rifle that would shout 1/2" groups at 50 yards with one particular load, A different load of same weight bullet and similar velocity would also shoot 1/2" groups, but 3 inches apart from each other. This at 50 yards.

When I ran a gun shop years ago, people would buy cheap ammo to sight their gun in and buy one box of premium ammo to hunt with, never checking for POI. As you have witnessed, this could be a miss or worse, wounded animal.

This is why it important to practice with what you will use in the field.

I think ive got it figured out in the new Hornady manual it states the solid gmx dos not flex like the cup and core bullets so different barrel harmonics.