I have a 300 BLK pistol I just put together.  Free float rail.

My first shot is 3-4" high then the rest settle into nice groups.  Wait for 15 minutes to shoot another group and 1st shot is high again.

This is an 8" WASP barrel.  It was on the very low end of the torque scale  so I went to the next hole on the barrel nut-same issue.

I don't ease the first round in- I use the carrier release.

I know about cold bore but this is a stubby little barrel and 3-4" seems like a lot. Scope is tight and using a Larue SPR mount.

Any ideas?

Thanks for all the thoughts.  

I bought a lapping tool and worked on the front of the receiver.  I was nervous but noticed it was not actually flat/ true.  As the compound was working, I could hear the "high" spot getting worked more until it evened out.  

I also added some Loctite to the barrel extension after seeing that done in several other forums.

A side affect of the lapping was a reduced torque value on the barrel nut.  It was mid range compared to my previous jobs doing this so the lapping slightly reduced what I thought was a bit heavy torque on the nut.

I had just enough time to stop by the range and run 4 rounds between appointments.  4 shots within 1.25 inches at 100 yards.  No flyers or first shot high.  I really need to get back out when I can not rush through.  I feel better but taking a little more time will make me feel better.  All in all, I don't know if it was the "truing" or the barrel torque or the Loctite.  If I had more time I would have forgone the Loctite to see if it mattered.

I did have this in another section and will copy there.

I will update when I can slow down and try it again.  Hope this helps someone else!

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That's bizarre. I hope you get some answers.

Sounds heat related.  Is the front of the receiver true?

They're called fliers.  I generally shoot 1-2 rounds offhand before going for groups...

Yeah I'm afraid you probably don't want to hear that it's likely the shooter and not the weapon for things like this.  1 shot doesn't heat anything enough to matter.  The only way that is possible otherwise is if the barrel can actually move.  If you think that's possible and not you then pull the barrel, lap the upper, and put it all back together.

My 308 does the same thing.   One of my ARs is an inch high in the first round every time but the rest are good

I always look at the nut behind the trigger 1st.  I went back and took another AR just for the heck of it.  All shots in a neat little group.  

The 300 is doing the one shot high thing.

I may pull the barrel and so a re-install again

It is called CBZ (Cold Barrel Zero)

Shooters will see the same thing in M21's and M24's at range if they shoot on a KD range for proper feed back on targets. Usually not 3 or 4" on a stout barreled M 24 (rem 700). But you can see a different POI with the first bullet out of a cold barrel even on stiff barrel "tactical" Mil-Spec rifles  out beyond 300 meters.

Shoot an record the deviation of your CBZ and apply that dope to your first round.

It's an 8 inch barrel.  It is too short and therefore too stiff to have a single round make that much of a heating difference and POI shift.  

Until the shooter is taken out of the equation they are still part of it.  Strap it in a sled and check it that way.  

If this only happened on a freshly cleaned bore then I buy the first-round theory as the fouling round is often necessary, but first round of every string?  Nope..

It was answered in your other thread. You're really going to need to pull it apart and take a look at everything.

http://www.ar15.com/forums/t_3_66/652610_First_shot_high_then_nice_groups___HALP_.html

I ordered a lapping tool. That will square it up for sure and change the torque value.  If that doesn't work, I may try another barrel.

What lapping tool did you order?

Wheeler Engineering Lapping Tool

Update in op!