This on a gunbroker auction for a Winchester M100 .284 carbine:

Barrel rifling is great. While I’m by no means a Winchester expert, while cleaning the bore for inspection I noted the rifling is cut back about 3 3/8 inches back from the muzzle. Little I was able to find out about this was that with certain bullet weights it improved ballistics of the round for the obvious and the taking of larger game. When you look down the bore you can see the work was done by a professional. While I’ve sold other 100’s, I never noticed this. Thank You.

Here’s the link:

https://www.gunbroker.com/item/858360748

So, as near as I can decipher, the barrel has been counter-bored 3 3/8 inches (by a professional)?  And the reason for this is improved ballistics for the taking of larger game?  I tend to believe his final statement – that he has not noticed this on other M100’s he has sold.