It sounds like you have a very accurate rifle, if the individual barrels put their shots within an inch of each other.

The current regulation is a result of whatever lot of ammunition the rifle was regulated with on that particular day. Changes in ammunition specs, powder lots and such will change the velocity and other characteristics over the years.

To get the results you desire, a little handloading might be in order. You're very close now, and with a little work, you should have a rifle that'll put them all in 1 1/2".

I would not want to accept average with this rifle.