Some of the very best shooting AR barrels I've had (6.5 Grendels) were used tubes. I took shot out high-power barrels which were generally 6.5-284's with well over 1500 rounds down the tubes (mostly Kriegers). Some barrels had firecracking for 4-6" past the chamber. Take one of those barrels, cut 6-8" off the chamber end, contour for your AR and chamber. Some of the absolutely best shooting barrels I've used. Was it more work? Yes it was, but I pick up the barrels for $50-$75 ea and it's worth it. NOT the same thing, but those barrels are not lost if there's enough meat on them for another rifle, maybe a shorter barreled sporter or an AR. Setting back a barrel tenon and rechambering can freshen up things, but it won't give you double the life. I do think taking 4" off does change things closer to a new tube (that's already very broke in).
Oh, and no, I didn't sell em for new price, a few friends of mine got some very good deals on AR's in 6.5 that shot extremely well.