I'd be loading 180s (or heavier possibly) in a 300 mag, so that would likely reduce that some. That is a significant amount of freebore in my opinion. Next question; how long can you load the cartridges and have them still fit in the magazine? You should be able to load them significantly longer than factory because this was originally a 300 H&H so it should have a longer magazine. If you handload I'd try loading some long 180s in it, reducing the jump to the lands and see how they group.

I know years ago when I got into long range shooting a lot of guys were loading the long Bergers touching the lands for best accuracy, and then could often only fit one in the chamber and one in the magazine because they were too long. I found not being able to fit them in the magazine unacceptable and through testing found loads in my gun that got minimal accuracy loss loaded as long as would fit in the magazine. Others intentionally built rifles like yours, barreling them in a shorter cartridge than the action and magazine was designed for, so they could load them longer than published maximum OAL. While quite a few shooters did load touching the lands, the accepted norm for safety (removing the possibility of a bullet sticking in the lands when ejecting a loaded round, for one) is to load them no closer than.010 to.015 off the lands.

The real question is how much bullet u got in the neck of the case and as was previously stated does it fit in the magazine
In actuality the real question is how does it shoot. Ive had loads shoot half moa jumping over 0.100" . The answer is in the target.

That 150 gr bullet is halfway down the neck. Not enough for me. I may try a few handloads with the 150 gr with the bullet base seated to the bottom of the case neck.
I do have some 200 gr bullets coming that I want to start handloading with. I have some Reloader 19 and imr 4350 to work with.
Thanks for the replies, very helpful!

Give us an update when you work up some loads for it. I'll be interested to see how it shoots.