Accuracy deterioration generally results from throat erosion. Accurate life depends on a number of factors. Acceptable standard of accuracy must be defined. In a precision rifle, accuate barrel life can range from several hundred to several thousand rounds, depending on the calibre and cartridge. Throat erosion can be easily observed by noting how far a bullet must be seated out to make contact with the leade. For long range target shooting, a barrel is often considered to be failing when vertical dispersion in the group appears. This results from velocity variations. High intensity, overbore cartridges will have shorter barrel life than more moderate ones, all else being equal.
To the best of my knowledge, the first .50 calibre rifle sniping was done by William Brophy in Korea. He started with a .50M2 Browning fitted with a 'scope, then continued with a PTRD rebarrelled with a Browning barrel, also scoped.
Chrome plating, Stellite throat inserts, special steel, etc, all contribute to longer barrel service life for military firearms. Broach, button and hammer forging are all faster methods of rifling a barrel than traditional cut rifling. Doesn't matter if the barrels are for rifles, pistols or machine guns, production time is reduced. Hammer forged barrels can be very accurate: the Steyr SSG is an example. Many rifle manufacturers use forged barrels, and accuracy is excellent.
Early machineguns were water cooled for a reason. With the steels available, barrels could be quickly burned out. The original barrels used in .50 aircooled guns had rather short service lives. The .50s intended for sustained fire were water cooled. Quick change barrels were an attempt to create a lighter gun that could still produce an acceptable volume of fire. The MG42 was issued with three barrels, and these were routinely rotated. One spare for a Bren. Even the MG08 water cooled gun carried a spare barrel on its mount.
Sleeving can be a way of restoring accuracy to a barrel. The large naval rifles are tubed. Generally speaking though, only low intensity barrels are routinely sleeved. One reason is the bottlenecked cartridge is much larger in diameter than the bore. It is a safety issue. It is easy to sleeve a .22, and if the liner is of high quality, accuracy can be just as good as with the original bore. A piloted drill is used, the tube is bonded or soldered it. A successful job can be done with hand tools.
