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.

Worked up a load with Reloader 19 powder and a 200 gr bullet seated just off the lands of the rifling. I was getting 1” groups at 100 yds. Total cartridge length exceeded the factory specs, but since this rifle was built on a 300 H&H action I could still put the cartridge down in the magazine.

I may do some more tweaking on the load combo to get a little better groups.

Good deal, glad ya have it shooting better.

I built a 7mm saum that I was ready to sling down over a bank until I got hold of some H4831 and now its becoming one of my better shooters.

I would guess almost all my handloads are longer than the Sammi cartridge legnth. Its like Erick Cortina said " who's Sammi, ive never met her"

Part of the joy(or not sometimes) is working up loads for a rifle. I love the experimentation!

Same powder and weight(and seating depth as the 200 gr) with 180 gr Speer grand slam got me 1.25”. Not shabby but could definitely be better.

I’ll probably try some I-4350 in the future

When you think you have a good shooting load at 100yds, better shoot it at 200.

Sometimes they open up more than one would think.

So true! I did try it at 155 yds and the elevation stayed the same and so did the groups. I do want to try and get my groups at 100 to be closer to 3/4”.