What kind of rifle does he need rechambered with a nato reamer? The only chamber I would ever cut with that is an ar barrel that will just shoot nato ammo. If it is an ar barrel I wouldn't rechamber it. If it is a bolt gun then why does he want to use that chamber?
I don't know all of the details but in short, he has a Savage bolt rifle he uses for varmint hunting and he bought a lot of boxes of 5.56 NATO rounds to shoot through it from his "gun guy." The rifle is chambered in 223. I told him, after he asked me, that the SAAMI and NATO specs are not the same and it is ok to shoot 223 ammo in a 5.56 chamber but not really a good idea the other way around due to pressures. His complaint has been stiff bolt lift and erratic groups of late.
So, he's now deciding on whether he wants to build an AR and chamber it in 5.56 and keep all of the ammo, or build an AR in 223 and get rid of the ammo. At the time of my post, he thought he wanted to rechamber his 223 to 5.56.