What is the life span of a AR15 barrel/upper reciever?  

Depends.

Depends on what material it is, chrome moly, stainless, chrome lined, etc.

Depends on what type of ammo and how fast you shoot it. Shoot it real fast one 30 round mag. after another and life will be shortened compared to slow fire or being able to let it cool off between shots.

In short, there are many variables that contribute to barrel life. You can only give generalizations at best and there is always the exception to the rule.

Generally speaking a chrome lined barrel will outlast the other types of barrel steels.

I bought the AR about 20 years ago the barrel is a chorme lined heavy barrel.  I have fired it slow fire most of the time very seldom fire a mag as fast as i can.  But, have put a lot of rounds down range and not always let cool.  I use 69gr. HPBT Sierra.

US AR barrels are rated to 4,000 rounds 1/7 twist non-chrome lined to 9,000 rounds 1/9 chrome lined before developing significant throat erosion. AK47 barrels are rated to 35,000+ because they are hammer forged chrome lined. The hammer forging makes the metal much denser than standard US barrels. Also most US barrels such as on the Springfield M1A Match are something like less than 30RC for hardness because they are not hammer forged or heat treated for that matter.
Truthfully US manufacturers skip out on the necessary measures to produce a lasting barrel because of costs such as heat treating, hammer forging, and sometimes chrome lining. However despite the US myth hammer forged barrels are capable of being extremely accurate such as you find with Styer hammer forged twist chrome lined barrels they use on the SSG series or Gewher 69 rifle. The Styer hammer forged twist chrome lined barrel (same as FN produced barrels) is a 30,000+ barrel and it is mounted on a bolt action sniper platform capable of sub MOA groups at 800 yards well past 15,000 rounds- you would be lucky to get the same results after 2500 rounds with a Remington 700, Springfield Match, or other US produced barrel. Yet Springfield, Remington, Badger etc. all contend that hammer forging, chrome lining, and heat-treating is not the way to go but this is simply because these manufacturers are cheap and want to save money. Luckily the AR is designed for the cheap barrel produced in the US- you can change them out easily unlike with an AK or FAL for instance. The only fault in the AK system is the bolt locking design- otherwise the AR market would have a run for it's money. The same applies to FAL design versus the M1A, once again poor locking bolt design on the FAL. Some idiots would have you believe it was the barrel making processes fault such as the twist hammer forging, chrome lining, heat treating Styer process but this is not the case intead it is the poor engineering of the bolts that causes the lesser accuracy when compaired to US designs.
Compaire bolt actions for instance because they are free of the complications of the Auto type systems as with US barrels versus the traditional foriegn Styer barrels and you will see what I am talking about- the Styer barrels perform just as well and for ten times longer.
So buy a second barrel for your AR- it was designed as such for easy barrel changes to accomidate the US cheap barrel production. Styer technology barrels last nearly as long as the reciever they were designed to be housed in- this is not the case in the US.

Can you recommend a source for a Styer barrel?

You cannot buy a true Styer twist hammer forged chrome lined barrel for an AR that I am aware of. Styer is a manufacturing process named after a conglomerate of companies in Germany where this process originated. However companies like FN, SAKO, and just about every AK manufacturing facility adopted the process to make barrels as a staple. Any barrel from Europe just about that is chrome lined is made in this fashion.

They litterly take the hot steel and twist it till it looks like a big log spiraled all the way through, then they use giant presses to hammer it rolling it several times to make the metal much denser, after that they heat treat it so it becomes even more more dense, and then that is followed by chrome lining the bore for even more longevity. The process litteraly makes the steel so dense that it cannot be duplicated otherwise- why they bother chrome lining is beyond me but they do anyway. The barrels up close have a spiral ring pattern on the surface, not machine marks as you can see those as well but the steel itself is striped all the way up, each ring is about 1/8th of an inch thick from the twist process. You could buy one perhaps but most likely a take off from a $1,500+ bolt action rifle. FN only does this process on .30 caliber barrels that I am aware of because this is the predominant barrel used for sniper rifles. However you can buy an el-cheapo Bulgarian AK47 barrel and it was built using the same process with the exception of the twist part- this is sometimes negated as it is to costly to produce in signifcant quantity. However all SSG barrels are produced this way and replacement barrel blanks cost nearly as much as the rifles. You can buy MAG58 barrel blanks from FN but they run about $1,000. I just wish they would import Bulgarian AK barrel blanks that were not turned down- these would be ideal because AK barrels come in just about every major caliber and they are cheap thanks to nearly third world conditions of Bulgaria.

One data point for longevity of ar15 barrels was a test done on a 20" Colt HBAR shooting 10,000 rnds in two or three days.  Don't have the link but as far as I know, the barrel was not chrome lined and being a colt probably had a 1/7 twist.  If I remember correctly, they started seeing the bullets keyholeing around the 9,000 rnd mark.  Should keep in mind that all this ammo was shot off at a rate of about a round a second with no concern over letting the barrel cool and the rifle being cleaned every 1,000 rnds.  Pretty extreme conditions for a semi so I would assume that 10,000 rnds would be a minimum semi barrel life if barrel temp was not considered.

Barrel heat is the biggest factor in barrel longevity.  Each time a round it fired there will be a bit of throat erosion, however, as barrel temp increases, the amount of erosion per each round will increase as well.  So if you don't do mag dumps and keep an eye on barrel temp, you could see a chrome lined barrel last up to 30,000 to 50,000 rnds.  My rule of thumb for shooting rifles where I want the barrel to last is to not let the barrel temp get to the point I can not hold it with my hand for more than a second.

"One data point for longevity of ar15 barrels was a test done on a 20" Colt HBAR shooting 10,000 rnds in two or three days. Don't have the link but as far as I know, the barrel was not chrome lined and being a colt probably had a 1/7 twist. If I remember correctly, they started seeing the bullets keyholeing around the 9,000 rnd mark. Should keep in mind that all this ammo was shot off at a rate of about a round a second with no concern over letting the barrel cool and the rifle being cleaned every 1,000 rnds. Pretty extreme conditions for a semi so I would assume that 10,000 rnds would be a minimum semi barrel life if barrel temp was not considered."

That is 4 times the mil-spec maximum sustained rate of fire for the M-16A2 for a whole weekend.  No wonder it got screwed up.

I think that it all depends ([b]WHAT DO YOU CALL WORN OUT? 1.5MOA? 2MOA? 4.5MOA? too dangerous to continue to use?

You can roast a rifle length chrome bore as much as you want and I doubt accuracy will ever degrade much past 5MOA.  (At the same time you could dump 600rds through an A2 gov't profile as fast as it would cycle, and you could turn a 3/4MOA barrel into a 5MOA barrel in about 2 minutes.)

If you keep it within the mil-spec maximums you will have a serviceable barrel for 60,000rds or more.

[b]There really is no way to compare SS barrels to chrome lined as the standards are different, Krieger puts accurate life at 4700rds (but what is accurate to Krieger? probably 1MOA.  it might take many times that to get accuracy to errode to 5MOA.[/b]


And comparing life to an AK is BS, The Ak fires a slower velocity round and that alone will scew results. Besides how many AK's will do even 2 MOA BRAND NEW? not many.  Mine was a Mak-90 and it would do 3MOA new (roast that and if accuracy degrades at all it will be shooting worse than an ar-15).

Just some clarification.  What erodes a barrel is the heat from the burning propellant.  The erosion of the barrel is also helped along if rounds are shot in full auto or very rapidly in semi auto because you are adding more and more heat to the barrel.  But again this heat is coming from the burning propellant.  The heat basically makes the atoms of the metal vaporize.  Increasing the temperature of the barrel thru rapid fire just speeds up the process of vaporization.  Friction from the twist of the barrel or from the bullet going down the barrel is insignificant because bullets have a copper jacket which is very soft.  Also, some barrels are eroded because the residue left behind from the burned propellant is corrosive.  The reason heavy barrels or HBARS last longer is because they can absorb more heat because the barrel has more metal and they cool down a little bit faster because the heavy barrels have a larger surface area.  The reason chrome lining a barrel makes them last longer is because chrome is a very hard metal that has a very high melting point and so can resist vaporization.  Also, chrome is unreactive to most chemicals that can react with metals.  Chrome also can form a very smooth surface.  This is why anything coated in chrome is very shiny.