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And here's a tortured Colt [url]www.galleryofguns.com/shootingTimes/Articles/DisplayArticles.asp?ID=1205[/url] HBAR which after more than 8000 rounds in 2 days shot it's best group of the entire session, so as stated above ya might want to revise your figures. Most chrome bores (not tortured) will go well past what you listed.
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A couple observations: The Match HBAR is an unlined barrel, although it may have a chromed chamber.
While the best group was shot at 8000 rounds, that barrel was shot out and keyholing by 9000.
So the article shows us that an unlined barrel with heavy semi auto use can go 8000-9000 rounds with acceptable accuracy (none of their groups were under 1MOA except the accuracy check at 8000 rounds). Figure on a 1/3 to 1/2 extension to that with slower rates of fire and more occasional use to keep throat erosion down.
I've yet to shoot out a chrome lined barrel on a semi-auto, although I don't have more than 6k rounds on the most heavily used of those I own, so far.
Full auto fire is much more demanding. I've shot out a 16" HBAR unlined barrel in 5k rounds doing mostly bursts, and never firing until it's smoking hot. I've got over 10k into a 16" chrome-lined barrel that's still going, strong though.
The M4 heavy barrel PIP came about because the specops guys were shooting out their chrome-lined Colt M4 carbine barrels in 1500-2500 rounds with regular, sustained, full auto fire - so even chrome lining can only do so much against the heat and strain of truly sustained fire.