Remington Sling tension?

[Minesweeper3433]

So I was keeping my Matchmaster in the back of my car so I could shoot after work on the way home. I was learning how to use my sling by shooting at 50 yards and groups have been getting better and better. All of a sudden groups shoot back to around 4 inches at 50 yards with any ammo I used. If I stop using my sling it goes back to shooting 1 hole groups. Why is it changing all of a sudden?

 

[jpretle]

Might want to check contact between bbl and stock. Sounds like any sling tension you apply makes a change in the bbl vibrations.