I can’t figure this out

[kmilner65]

It's not really that weird. Muscle tension and anticipation or apprehension of the upcoming shot could explain the first few shots. After those were over you relaxed and your group came in to play, your breathing relaxed, all the elements of long range shooting fell into place. I wouldn't change a thing, take another 20 rounds and hit the range and see how you do before you do too much thinking

 

[RobKen]

Have a read of Nathan Foster's article, "Hold that Forend" on his website "Terminal ballistics research", ( www.ballistic studies.com). He has a lot to say about technique and bipods.

 

[Slick8]

Shoot the gun over a regular rest. Bipods often give vertical if you load them with different pressure. Confirms not the gun.
If it is the gun, clean it!!

This is my thought as well.

My opinion, change one thing at a time. Start with a good solid front and rear rest as you've already confirmed you and load will shoot a good group at 300.

It's now about consistency.

 

[6MM06AI]

Sounds like neck tension could be an issue, check your fired brass by trying to put a bullet in the fired cases. And like others have said clean then shoot off sand bags.

 

[RockZ]

It could be those particular pieces of brass. I've have some that just didn't shoot with the group. I use those as foulers. See if those pieces group the same next time.
I picked up this idea from the book
Handloading for Competition.