Groups sometimes in a triangle pattern and a semi-circle,
THIS.

Check to be certain your barrel is indeed free-floated. My Mk II was NOT - the stock was cut wrong.

As a barrel with pressure points heats up, it will begin to move against those pressure points, usually in a circular pattern, but often in other patterns as well.

[img width=600 height=450]http://i19.photobucket.com/albums/b191/WabashShootist/Guns/M44/Shootinupammo.jpg[/img]

These were shot from a Mosin-Nagant M44 carbine at either 50 or 100 yards (forget which; probably says on the target and will be visible when I post). You can see the distinct semi-circle made by the ammo as the barrel heated.

The .17 is going to be less pronounced as it's not a .30 caliber round running at 2500fps, but the principle is the same.

If it's a skinny (sporter) barrel, bed the barrel.

If it's a heavy bull barrel, make sure it's floating.

Should cure your problem.

Josh