There are many factors that affect usable barrel service life, including how you want to use that barrel.
A Designated Marksman or the spotter for a small reconnaissance or sniper-recce team will have different accuracy requirements than a foot Soldier or vehicle trunk monkey looking for close rapid fire and multiple-round training sessions.
If we had one barrel maker who knows his business (let’s say he apprenticed under a master like Wisconsin barrel-maker Boots Obermyer) and he makes us a dozen barrels of identical profile (say a SOCOM Heavy Barrel M4 profile with 1-8 twist) those barrels will all have different service life.
4150 Chrome-Vanadium will have a slightly longer service life than 4140 Chro-Moly. They are harder than stainless so are slightly tougher on gunsmith tooling. They hold accuracy well, declining slightly but constantly over their entire service life.
Chrome-lining helps extend their life by putting a hard coating over steel. As the chrome starts to wear or chip off the throat and high spots (the lands at the leade), the steel gets attacked by the high heat. Chrome-lining the bore does nothing for gas port wear. Chrome lining also tends to “Round off” sharp rifling lands, which makes it tougher to consistently imprint rifling and impart spin on a bullet – groups from a chrome-lined barrel tend to be bigger than from an un-chromed barrel. If chrome is not properly treated you’ll get hydrogen embrittlement of the steel and cracking.
A hammer-forged barrel will be harder due to the steel’s forged crystalline structure and helps contribute to longer barrel life, however cold hammer-forging may induce molecular stress (which may affect the barrel as it heats up). Stress relief is critical for hammer-forged barrels for optimum accuracy. Makes no difference if the rifle is used for mostly short-range or full-automatic use.
Gunsmiths like 416 or 416R stainless because it is easier to machine and rifle well. Stainless barrels give excellent accuracy, but as the throat and gas port wear away by cracking (as high temperature gases burn out the impurities of stainless particularly at the barrel’s throat) when it’s time to die the barrels’ accuracy falls off quickly (as bits of the “Cracked mud” surface at the throat break off and scratch the bullet’s jacket as it makes jump from case mouth to first rifling, and as gas blow-by during the bullet’s free-bore jump makes velocity inconsistent).
17-4 Precipitate-hardened stainless is used in aircraft landing gear and surgical tools and lasts a long time due to fewer impurities (and ratio) in the alloy. It’s also hell on gunsmith tooling (rifling buttons may last a quarter of the life of those used for 416 or chro-moly barrels, so you can do 5 barrels vice 16-20).
Button rifling surface hardens the bore of a barrel every time it’s pulled through a barrel, which will help it wear a long time but doesn’t help when the throat starts to go out. Cut rifling will give you the best accuracy.
For long-term precision shooting your best choice might be a 17-4 PH hammer-forged, stress-relieved, cut-rifled stainless barrel.
For a line doggie or door-kicker you’d probably be happy for years with a 4150 button-rifled Chrome-lined barrel.
Note that high-heat range sessions (any single range event that exceeds say five 30-round magazines in double-tap training without cooling) changes the steel in the barrel at the molecular level (especially when the internal steel’s temperatures exceed 350 Fahrenheit). The Army has also measured barrel bursting failures once the barrels get REALLY ignorant-hot without letting rifles and carbines stand and cool.
