Wow, this is some incredibly valuable information about barrel life, thx for sharing this info man, this stuff goes in my notebook. Cheers manSpecifically, at the last measurement, the throat had advanced 0.1" with 574 jacketed rounds fired. A mix of fmj 147gr ammo and 165-172gr match and hunting bullets. Divide 1" by .1" and multiply times rounds fired so far to get estimate of rounds at 1" of throat wear.
Since it'd been a while since I measured the wear in the XCR-M, I just now cleaned the bbl and took another measurement. It was between marks on the gage, so I gave my best eyeball estimate of the reading. I've now fired a total of 653 jacketed (147-172gr) bullets and 379 163gr AP bullets. Calculated rounds to 1" of throat wear is now 8,256. The more rounds fired, the more accurate the calculation becomes.
Back when the US Military measured throat wear (I do not know what they currently do in that regard, as I retired in 1988), 5.56 and 30 caliber bbls were deemed due for rebarrel when the throat wear read 1" on the gage. I know that when the gage read 1" on the 3 M1919A4 30/06 bbls that I wore out (on the unlined bbls it took 5000 rounds each at which point the gage read 1"), the 100yd groups would suddenly go from inches to feet within 250-500 more rounds. I do know on 1 M16 bbl a friend had that ball ammo key holed when the gage exceeded the 1" measurement.
I'm not surprised in the calculated differences, as it's been my experience that the throat wear slows down the more the bbl is fired, thus the use of the words "continues to wear at the same rate".
I track throat wear (aka the wearing away of the full height of the lands) in all my 5.56 and 30 caliber bbls. It just adds to my enjoyment of the shooting hobby. Except for the 2 5.56 bbls I use on the M16, all bbls are let cool after 5 shots so I can keep the comparison even. Using a gage is an easy way to monitor. Other ways would be seeing a reduction in velocity and seeing a reduction in accuracy. A "match" shooter would probably rebarrel long before throat wear had advanced 1".
As long as I'm typing, I'll add this, as I have the most data (about 19,000 rounds now) in 300 Blackout using every bbl steel and bore treatment I can find. From shortest life to the longest life with the Blackout:
Plain 4140 steel non cold hammer forged (CHF) and no bore treatment
Plain 4140 CHF and no bore treatment
416R stainless and no bore treatment
4140 steel nitrided bore
4150 steel CHF nitrided bore
4140 steel chrome lined bore (Per Alex that is used on all the RA barrels)
4150 steel CHF chrome lined (some makers use the term 41V50 or "machine-gun steel")
I do not mean to imply that plain 4140 gives a short Blackout bbl life (calculated at 11,820 rounds). The longest Blackout bbl life is calculated at 22,667 rounds (FN CHF 4150 "double" chrome lined). My next Blackout bbl will be 4150, button rifled, regular chrome lined. Copper plated bullets in the Blackout wear the same as jacketed bullets. Cast bullets wear at about 1/3 the rate of jacketed bullets. Using .310-.311" jacketed 7.62x39 bullets, I get no more Blackout bbl wear than when using .308 bullets.