How Many Rounds to Wear it Out?
No offense meant, but that's a loaded question. That's like asking how many miles will my tires last? That all depends on several factors.
1. What brand of rifle or barrel?
2. What caliber or Cartridge?
3. Handloads or Factory rounds?
4. What do you shoot? Target, varmints, hunting?
5. How many rounds do you shoot per range trip?
These are just a few questions that should be asked. For instance, take number one:
What brand of rifle or barrel is on the gun in question? If you buy an el cheapo for $150 it may not be made of high quality steel. This also applies to barrels. I have heard from a competant gun smith that the Adams Bennet barrels that Midway sells are not as hard as say a Shilen or Lilja and may not hold up as long.
Number two:
What caliber or cartridge is the barrel? For instance you can't hardly compare a 30-30 Win. with a 220 Swift. The Swift is an overbore beast that breaths fire and can burn the barrel out with hot loads in under 1,000 rounds if you're not careful. However, you could probably shoot a 30-30 20,000 rounds before you ever got close to eroding the throat. A 30-06 is not as hard on a barrel as a .270 Winchester, and a .270 Winchester is probably not as hard on a barrel as a .25-06. A 7mm-08 is probably easier on a barrel than a .243, while it's probably harder on a barrel than its parent cartridge, the .308 Win.
It all boils down to how overbore the cartridge is as to how long it will take to shoot it out.
Number three is what kind of ammo do you shoot in it mostly, handloads, factory, milsurp? Some factory loads have been known to be really hot and hard on barrels, such as the 220 Swift. It is very overbore and in factory form in the past it was loaded hot and there have been stories of barrels giving way in as few as 500-600 rounds of hard shooting. Of course this also pertains to hand loads too. Some people like to push the envelop on velocity. If they have a Swift or a .264 Winchester Mag. or something this hot, they want to see the Swift go through their Chrony at 4000+FPS. A steady diet of that can be abusive to a barrel and shorten its life rapidly. The .264 suffered in sales because of its reputation as a barrel burner, because factory rounds were hot. And some people liked to load their hunting loads hot.
On the other hand, some people have .220 Swifts that are 30 years old and still on the original barrel because they slack their hand loads off a bit and go for accuracy over velocity. Same with the .264 Win. Mag. and other overbore rifles.
Number four is what do you shoot? or what style do you shoot? Are you a deer hunter that goes to the range twice a year and zero's his rifle before opening day and maybe for a club shoot now and then? People like this may not shoot 30 rounds per year through a particular rifle. Then you may be a competitive shooter that shoots a couple hundred rounds a month during a particular season. Then there are varmint hunters. Some prarie dog shooters may fire 500-600 rounds over a dog town and may do it twice a week for several months while the weather is agreeable for the dogs and man to be out. So you can see some people burning a barrel out in as little as a year or so.
Number five is how many rounds do you shoot per range trip? Say you're not a competititive shooter and you don't hunt prarie dogs, and you regularly work up loads and shoot for your own satisfaction or maybe shoot in informal club shoots. How many rounds to you usually shoot at the range? I may go out one day and shoot 10 rounds checking out a load making a step change up in powder and checking accuracy. I may go out and shoot a rifle 30-50 rounds some days. So you can see this may accellerate barrel burn out time or maybe you don't shoot enough to hurt a barrel in many years.
This is why I say this question is hard to answer. There's no two people and no two guns owned by the same person going to last the same amount of rounds. It all depends on what the shooter does.