Been working with a new Rem 700 in 300 Blackout and the 1st shot out of a cold barrel registers about 100 fps faster than the following shots, which in turn prints it 3" higher at 100 yds.
All loadings are at subsonic levels and I have another rifle in the same chambering that shoots respectably and does not suffer from this problem.
We have tried with and without the suppressor and it doesn't change at all.
Has anyone run into this before and is there anything to do other than get another barrel put on ?

It is common; there is no remedy except if it happens after cleaning, then don't clean. Normally 100 fps won't make a 3 inch difference in POI, but at that low velocity, it might.

Unfortunately it's not after cleaning and it is about 3" at subsonic velocities.

Not necessarily anything to do with the barrel temperature as such although we do use the term "cold barrel" figuratively. Likely a bedding issue where there are pressure changes over time when the rifle is not in use and then the first shot 'sets' the bedding back for all the consecutive shots.

If it were a problem caused by actual barrel temperature then you would expect this to vary depending on the ambient temperature of the day. What happens if the rifle is left in the hot sun or pour some warm water over the barrel before firing the first shot.

Usually barrel temperature only starts to effect shot placement once it gets quite warm or hot from shooting and this is often a bedding issue with thin sporter barrels where free floating the barrel normally helps to some extent.

What happens if the rifle is left in the hot sun or pour some warm water over the barrel before firing the first shot.
Sorry, but we have not had hot sun here for 6 months or so. I have been messing with rifles and reloading for 30 yrs or so and have never come across this before.
Then again this is my 1st experience with subsonic, don't know if that's relative ?