In the barrel in order for the bullet to stabilize?

I have a colt M4 barrel that looks to be thin on its rifling.

This rifle barrel was never used with a scope\optic so i never tested it for accuracy nor have a point of reference.

And you are waiting to shoot it because...?

What do you expect anyone here to tell you other than to go shoot it and see how it does?  There are a lot of folks here with tons of experience, but not a damn one of them is good enough to tell you how your rifle will shoot.

Hey, maybe call Sister Cleo, the Physic.  She might be able to tell you, but then if she asks "who is calling", probably not! Sorry, I could't STFU.

Short answer: "Not Much"

The Marlin Micro-Groove rifling has very shallow grooves and can stabilize a wide range of bullets.  Now, if your rifling is visibly worn, chances are you will not get particularly good accuracy.  

How does the rifling look in the throat area and near the crown?  Can you post pics?

last 2 inches

Quoted: last first 2 inches

...found a typo.