Have two of us in New Zealand with bushmaster rifles both done 6000rds and looks like the chrome lifting or is very ruff at the start of the rifling. have had plenty of chrome barrels before do lots more and some bushmater too. Have a shooter over here with a armalite with 15000 rds and looks the same and they are replacing the barrel under warrenty????? have contacted three people at bushmaster with no reply. the rest of the barrel is mint and I have only used factory ammo.
any thoughts, please.

cheers Kiwiaug.

MTBF on US military M16 barrels been reported at 9600 rounds.

Those are 1/7 chrome lined barrels firing full power military ammunition in a mixture of semi/3rb/automatic fire regimes.

Your 1/9 bushmasters firing civilian spec ammo in a semi-automatic fire regime for you kiwis should last at least that long, on average, if not 50% more for the slower 1/9 rifling, less rigorous firing regime, and possibly lower-pressure rounds.

After 6k rounds, you're nearing the halfway point in an average barrel's lifespan, so that roughness you're seeing is throat erosion - gas cutting and friction abrasion wearing down the chrome and steel under it, and eating into and extending the leade.

Eventually, in the fullness of time, this gas cutting and abrasion may mean your headspace gets too long, and the barrel must be replaced for safety reasons.  More likely, the leade erosion will result in the bullet tumbling or twisting ever so slightly as it makes the jump to the lands, and your accuracy will start to go to hell since the bullets are not engraving symmetrically.  If accuracy is not your forte', you can keep firing it as long as it doesn't become unsafe.

I've seen some "blaster" rental MG barrels that looked more like smoothbores than rifles, but still wouldn't close on a field headspace gauge - not accurate, but safe to fire, and easily more than 100k rounds on them.

Ive only shot one barrel out (gas port erosion) and it was around the 12k-14k mark.  Was a CMMG M4A1 SOCOM profile.

Mine is the other Bushmaster barrel that KiwiAug is on about. It's only a rough estimate of the round count of 6000-6500 based on what reloading components I've used. Rifle is a 20" 1/9 HBar purchased new in 2005. Accuracy is still all right at this stage. I've heard of another Bushmaster barrel doing the same from another club member so that three in our club alone. Thanks for the replies so far guys.

What? Both the .223 and 5.56, as do most all bottle necked cartridges, headspace on the shoulder, not the case mouth. Throat erosion will not in any way effect headspace. The barrel will be worn out, accuracy will suffer, but not because headspace has become excessive.

OP, you will know when your barrel is really worn out when your groups start to open up. As for when it needs replaced, thats up to you and what you expect your rifle to be able to shoot.

Beat me to it on both points