I think more military barrels were boogered by negligent cleaning under field conditions(which permitted corrosion by primer salts) and by improper use of cleaning rods and pull thrus which created undue muzzle wear. Over the course of firing many rounds, hot propellant gases will progressively erode the rifling from the barrel throat forward to a point where the rifling is eroded to the point of unservicability and rejection.
There are established reject standards for military barrels. These are well documented for the Lee-Enfield, M1903 Springfield, and M1 Garand to quote a few well known rifles.
For the '03 Springfield the bore reject limit is .308 (for rifles on issue to troops,not arsenal standards) on the proper gauge when measured from the breech end. I have one rifle with an unpitted,but worn, bore which reads .307 on this gauge-almost the reject limit. The muzzle is concentric on this rifle and free of cuts and puckers, and measures .303 on a plug gauge. In spite of this degree of wear this rifle will still group to 3" with quality handloads,so it would appear that the reject standard is appropriate. I have installed several new '03 barrels with known round counts on them following installation. One began with .300 in the throat,and after just plus of 800 rds the throat now measures .301. I would not necessarily extrapolate this to mean that the bore will erode a further .001 for every 800-1000 rds fired.
The reject limit for the M1 Garand in the hands of troops is .310 on a GI throat erosion gauge. That's a swack of wear from a starting point of .300 or .301 on a new barrel. I'd like to see one of these as the rifling would have worn to a shadow. I've never owned a Garand which measured more than .304 in the throat, but have had several that were a bit plus of .303 at the muzzles. They still shot acceptably well in spite of this after the crowns were cleaned up. For really worn muzzles, some barrels can be salvaged and restored to better accuracy by counter-boring back to the useful rifling. This technique has been used on many military barrels.
When the US re-worked the P14 rifle to produce the M1917 in .30-06 they retained the 5 groove Enfield form of rifling and concluded that this rifling was more resistant to erosion than was the standard 4 groove M1903 Springfield barrels. Having said that, the .30-06 and .303 are fairly benign on the bore with cooler burning modern propellants, so I would hazard that life expectancy might be in the order of 8000-10000 rds. Gilt edge target accuracy would begin to fall off after perhaps 3-4000 rds, but who among us shoots that well with irons to notice this.
By comparison some ctgs, like the .264 Win Mag and .257 Weatherby, are badly over bore capacity,even with slow burning propellants,and bores will begin to wash out in fairly short order.
As an aside, I have a No4 .303 barrel, which I got from the late Tom Richardson many moons ago. This barrel has had approx 150 rds thru it since new and measures .3015 at the muzzle vs the nominal .303. I've been tempted to try some .308 bullets in it vs the standard .311.
