Site Notices
10/24/2014 2:42:22 PM
Author
Message
kiwiaug
Offline
Posts: 43
Feedback: 100% (44)
Posted: 9/23/2011 2:57:43 AM
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.
Circuits
Member
Offline
Posts: 5129
Feedback: 100% (21)
Link To This Post
Posted: 9/23/2011 3:32:08 AM
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.
"The only real difference between the men and the boys, is the number and size, and cost of their toys."
NRA Life, GOA Life, CSSA Life, NRA Certified Instructor, FFL/SOT 07/C2
nf9648
Frustrated Cargo
Military
Offline
Posts: 8540
Feedback: 100% (26)
Link To This Post
Posted: 9/23/2011 10:42:33 AM
Ive only shot one barrel out (gas port erosion) and it was around the 12k-14k mark. Was a CMMG M4A1 SOCOM profile.
"The essence of Government is power; and power, lodged as it must be in human hands, will ever be liable to abuse." --James Madison

NZJeeper
Offline
Posts: 7
Feedback: 0% (0)
Link To This Post
Posted: 9/24/2011 1:35:10 AM
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.
colklink
Member
Online
Posts: 21553
Feedback: 100% (4)
Link To This Post
Posted: 9/25/2011 10:31:07 PM
Originally Posted By Circuits:
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.


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.
If obama is the answer, the question must be really stupid to begin with.

Taft
colklink
Member
Online
Posts: 21554
Feedback: 100% (4)
Link To This Post
Posted: 9/25/2011 10:33:03 PM
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.
If obama is the answer, the question must be really stupid to begin with.

Taft
458winmag
Member
Offline
Posts: 3811
Feedback: 100% (83)
Link To This Post
Posted: 9/26/2011 12:26:17 AM
Beat me to it on both points