The throat on my xm16 upper's barrel looks wierd like it has been recut. the beginning of the lands is a lot deeper than any other m16 barrel I've seen. it dosn't look worn it is just deeper. that thing shoots fine but I'm wondering what gives? was this at one point worn out and remachined?

Any markings on the barrel?  Numbers or letters that might help us with identification?

Typical NATO chambers are 2.550, what does yours measure?

That sounds like it would be rough on both the chrome and the chamber reamer. Chrome is much harder than the HSS chamber cutters unless I am missing something here.....

well it is a 1964 production colt barrel. the only markings on it are the "C" on the barrel extention, and the "VP" in a triangle (verified proof) on the barrel below the sight base. there is NO other marks so it INS'T magnetic particle checked and it dose NOT have the 1-12 mark eventhought it IS a 1/12" twist. Also it is not at all chromed

What I'm describing can be best summed up as it looks as if some one bored out and refinished the lead on the throat of the rifling to possibly restore a barrel that was "shot out" the rifleing dosn't start untill about 1/10 of an inch past the end of the chamber. the head space is correct so thats not the issue

I can't figure out why this oulw have been done I thought if it wasn't made this way then what was done would do to the throat basically the same thing that counterboreing a muzzle would do.

The rifle shoots great so what ever it is it works fine but it just seemed wierd. I first noticed it when comparing the barrel to a 1970's one I had and confirmed it with a military throught erosion gage that there was definatly a big difference. more than the variation between one being plated and the other not would have.

hope this clarifies this.

I know this was done between the 601 and 602 barrels.

"Model 601 production ended in 1963 and was replaced by the Model 602. The Model 601 was chambered differently then later Colt rifles. These barrels were chambered per ArmaLite specs with significantly shorter leade/throat then was used from the Colt Model 602 through current. With the exception of examples with heavily worn throats 601 barrels gauge a 0 with my commercial T/E gauge. 602 and later barrels gauge a 1.25 when new with this same gauge."

Not sure still about what you are describing...

this one basically has a much deeper throat/lead than any other 5.56mm or .223 barrel I've ever seen (I do small arms repair for the army. so I've seen a lot of wierd stuff.) the lead is about 0.10 inches deeper than the later 1970's A1 barrel I have.

here's and illustration. not to scale the top drawing represents what I'm talking about the bottom a normal chamber more or less.

http://i46.photobucket.com/albums/f113/m1garand_man/ar15/chamberdrawing.jpg