What is the typical barrel life a 16" barrel?  I have a Colt 16" M-4 barrel (1/7) with roughly 4-5k rounds through it and I am having problems with accuracy.  I thoroughly cleaned it, including copper remover, but gun is not grouping well.  I'm shooting XM-193 and am getting groups measured in inches at 50 yards from prone and from sand bag rest using both iron and an Aimpoint.  Visual inspection of the barrel after the thorough cleaning showed a line dark spots interrupted by the rifling along the top and one side of the barrel.

I'm generally a decent rifle shot with multipe professional courses under my belt.  I plan to return to the range tomorrow and shoot some premium ammo to see if that tightens things up.  I'll also take a different rifle to make sure I haven't developed some unrealized marksmanship problem.  If I can track someone down, I may have them fire groups as well.

Any other ideas?

Barrel life, measured in terms of accuracy, is a finite value. There is a window of optimum accuracy for every quality barrel. Once the throat is shot out accuracy will degrade rather quickly. The throat gets burned out by heat. The faster a barrel is heated up, by such activities as 'bump firing' or 'mad minutes' or 30 second 30 round mag dumps, the throat will burn out VERY quickly. Barrels that have been fired at the rate of 1 round every 30 to 60 seconds will last much longer than barrels that have been shot very rapidly. It is entirely possible that thethroat on your barrel is eroded and the bullets will not properly stabilize before they exit the bore. I hope this helps and invite you to ask any other questions you may have. Charles the Gunsmith.  

I went to the range with a variety of rounds to see if I could get better groups.  I tried Winchester  55gr JSP and 64 gr JSP as well as 55gr Federal Tactical rounds.  The premium ammo grouped better than the XM-193 but still nothing impressive.  (All shooting was from a prone rest position with iron sights or an Aimpoint).  The one standout were the early Federal Tactical rounds.  These are moly coated bullets and I could shoot groups that were 1/4 to 1/5 the size of the other ammo.  I tried the more recent incarnation of this bullet (non-moly coat) and could not replicate the tight groups.

Does anyone know why the moly coated bullets would shoot so tight when nothing else will?

If you are areloader I would suggest you buy some moly coated projectiles and attempt to dupliacte the early Federal Tactical ammo the barrel seems to prefer. TWL may be right about the barrel condition, but, I would not give up just yet. Take care. Charles.

You might try a standard magnified scope for your testing as well. The Aimpoint's dot is 2" in diameter at 50 yards, which isn't very conducive to extreme precision shooting.

The above posts are right on.  

Gas corrosion of the barrel's leade at the throat is fastest with sustained rapid-fire groups.  Although chrome-plated, the chrome will start to peel off the highest parts of the lands closest to the throat.  If you have a gunsmith close by with a borescope you can clearly see this.  You will also see (when borescoped from the muzzle) the muzzle side of the gas port will show erosion, while the chamber side will remain nice and round (this is also caused by gas erosion).

Theory on moly-coated bullets is they have a very small "Lubrication" benefit as they pass over the rougher eroded parts of the throat, however you may also see a small increase in copper fouling.

The average GI barrel can last 10,000-odd rounds and remain within MIL-SPEC, but remember that MIL-SPEC is up to 6 MOA to still be serviceable (ten rounds grouping inside an 18-inch circle at 300 Meters).  That means (using a maximum mean radius spread) at 300 Meters all shots should land within 9 inches of point-of-aim and be "Good enough for government work," but probably not good enough to keep you happy.

3,500 to 5,000 rounds should keep you chugging along and happy to about 200 yards at about 2 MOA, then I'd recommend you buy another barrel.  If you're shooting that much you'll notice the difference if you're shooting at least to 200 yards/meters.  If you're just rapid-plinking at 100 you may not notice.

A new quality 16" barrel should go between $200 and $400, installed.

Funny this has come up. I have been experiencing similar problems recently.
My 14.5" barrel has recently started throwing 62gn bullets sideways at 50 yds. It's still giving decent results with lighter bullets.
Anyway, yeaterday, curiosity got the better of me and after rebarreling it, I decided to "section" the old one and have a look


The pics are the best I can come up with.
This is at approx 5500rds
As you can see, the throat has completely disappeared just forward of the chamber and there is no rifling present for the first 1 1/2".
What appears to be rifling is actually only a trace of what used to be there.
If you run a pin across the bore you can't feel the lands.
There is deep erosion and pitting at the juncture of the land/groove.

All in, the bore was in a very sorry state.
Hope the pic’s are clear.

Mark

Mark, your barrel looks exactly like one we sectioned at work, except the guys blued it after.  The chrome areas are intact, while the areas that have been eroded are blued.  You can see how far the throat has advanced by seeing how far up the blue area goes up the lands and bore (which should be chromed).

You'll be surprised how the chamber shoulder has had much of the chroming worn off.

Mark,

How much rapid-fire or FA use did the sectioned barrel see?

Thanks,

Sam  

Quoted: Mark, How much rapid-fire or FA use did the sectioned barrel see? Thanks, Sam

This is a barrel from the UK.
Over here, we can't have semi's, let alone FA
Anyway, the barrel was a non chromed 1/9 and the rate of fire at most has ever been 20-30 rpm.
Most ammo shot through it has been Radway Green 62gn

Mark

You can see how with an advanced throat bullets leaving the case mouth have a "Free jump" before actually grabbing rifling.  That freebore jump does nothing for accuracy, as no two bullets will meet the rifling the same way.  Groups grow.

Wow!  That's interesting.  After years of listening to people talk about barrel wear, it's enlightening to actually see it for the first time.  Thanks!

Anyone able to cross-reference this thread for people asking about how barrels burn out?

No sweat, Steve.

We haven't seen any appreciable difference in barrel life between polygonal rifling and button rifling.  Corrosion of the barrel at the throat has a lot to do with bullet weight (the heavier the bullet, a la 80 and 90 grainers for slow fire) and barrel alloy.

The throats usually start eroding as the temperature of the gases closest to the case mouth start burning out the impurities in the alloy (for example the sulphur in 416R, or re-sulphured 416 stainless barrel stock which makes the stainless bar easier to machine.  As the sulphur is gas corroded out you will see the throat wear advance as in the photos).  While button rifling and hammer-forging can make the bore surface slightly harder than a cut-rifled barrel the sulphur or other alloy impurities are no more resistant to gas heat corrosion.

Chrome lining in chro-moly barrels are a little more resistant to the corrosion and helps barrels last a little longer (especially with slightly harder GI 4150 alloy).

Long line ammo tends to be harder on barrels than rapids (short-throated cut-rifle barrels used for Rattle-Battle will last longer than across-the-course guns).  

Some Schneider polygonals shoot well, some don't.

Probably the longest lasting button-rifled barrel is one custom made by John Benjamin in Portland, Oregon.  He has a special line of barrels made from VERY hard 17-4 Precipitate Hardened stainless alloy (same as used in aircraft landing gear).  He turns, bores, rifles, and heat-treats his own barrels in his shop.

I would have to pull the report on this, but the military testing of MTBF for barrels sets the *expected* service life of a 1:7 14.5 M4 barrels at 9600 rounds iirc.

A few things to keep in mind: Heat is the number one killer, the different classifications of use fall into slow fire, rapid fire, substained fire and full auto fire -- the above figure is based on primarily rapid, substained and full auto fire.

There was no noted diference between poly and regular rifling... three groove rifling seems to have the most hope for barrel life (more land surface).

"exoctic" platings, like NiB and nitrites had mixed results, NiB is great for lubricity and wear resistance, but offers poor corrosion resistance. Melonite is just too hard and caused early failure of the rifling by checking - it would be interesting to see a melonite lined poly barrel though.

Faster rate of twist was determined to promote premature throat errosion, gain twist barrels have historically shown excellent resistance to the same.

Stainless offers excellent resistance to both errossion and corrossion, although some are still hinky about it in very cold weather -- 17-4 PH, has proven to be an excellent choice.

Remember, though, that MILSPEC is 6 minute of angle accuracy.  That's ten shots within an 18" circle at 300 meters.

An average M16A4 or M4 should do 2 MOA (approximately 6.5 inches at 300 meters) all day long with GI M855 Green Tip.

Quoted: I would have to pull the report on this, but the military testing of MTBF for barrels sets the *expected* service life of a 1:7 14.5 M4 barrels at 9600 rounds iirc.

What did you go and say that for?
Now every keyboard commando on here is going to start saying that his barrel is done at 9600rds

Seriously though, barrel wear is a very subjective thing.
I recently rebarrelled my AR XTC rifle. The original Douglas 1/7 had seen 4000rds of 77's and 80's and the throat had gone .140" and 80's were just barely in the case.
I'd also just got hold of a batch of Border 5R blanks for £76 each ($138 ) and decided that it was time to rebarrel.

Does it shoot any better?? Maybe at 600yds, but I have some sight pic problems and I shot 755's before and am still shooting 755's with the new one.

It probably is shooting better than the old worn barrel, but having the new one on there makes me feel better.

Mark

Quoted: Absolutely, and green tip ammo sucks - always has shots flying out the top. I dont think people understand how accurate a basic AR/M4 is to begin with, or what all comes into the picture for accurate shooting. If you really look at it you can think about what it takes to shoot 2 MOA at 300yds.   The 10 ring in High Power shooting is 2 MOA for the 300yd rapid string.   These guys have 20" SS, free floated, match barrel, match trigger,  shooting coat, sling, glove, mat, hand loaded ammo - and of the Presidents 100, only 22 were able to put all 10 shots in the 10 ring. If you gave soldier a new M4, 100rds of MK262, and told him to shoot a B27 at 300yds I would bet that target would get hit 100 times. If you then gave that same soldier a new M4 with a 14.5" SS match barrel and 100rds of MK262, and told hit to shoot a B27 at 300yds I would bet that target would get hit 100 times. I'm  not talking about an "X" count. I'm thinking, if armed, would that B27 not be shooting back? I wonder how different the groupings of the two targets would look? Do you think its better to upgrade to the match barrel, or upgrade to the match ammo and keep the standard barrel?  I think "ball" ammo will still shoot like ball ammo out of a match barrel, but good ammo will always shoot better out of a standard barrel.

Good points Steve and interesting observations.
I sometimes think that for the average combat soldier, less accurate ammo can be an advantage.
Most targets in combat are fleeting and it's a rare thing when you can take good aim on the enemy, especially if you are being fired upon and have been running and sweating, shooting from unstable positions etc, that you might stand a better chance of hitting your enemy if the ammo's being thrown around a bit. One of the reasons why machine guns aren't inherently accurate.

When the USNG and Army teams come over here to Britain to shoot the SR combat matches, they always bring M193 with them in preference to M855, and all because of the penetrator instability problems. They claim rightly that the 55gn stuff is more accurate and that M855 always has flyers out at 11.00.

Mark

Quoted: Quoted: I would have to pull the report on this, but the military testing of MTBF for barrels sets the *expected* service life of a 1:7 14.5 M4 barrels at 9600 rounds iirc. What did you go and say that for? Now every keyboard commando on here is going to start saying that his barrel is done at 9600rds

Come on, you know that most of the guys on here will never actually own a barrel long enough to get 9600 rounds through it... before some new trend comes along that they have to comply with.

Doodz, i got a noveske afgan barrel with KFH perm attached, only shot 2 boxes of WWB and cleaned like 600 times. Selling to fund my new build with the new UberShooter (tm) barrel

A couple of years ago I made up some stainless, 1:8, 14.5" semi heavy, continous taper, mid length gas system barrels -- I got a ton of messages from people that dropped all interest when they found out that stainless barrels are not chrome lined... that was the thing then, "go chrome or go home" was the tag line for every post here. Now it seems stainless in chic?

Quoted: Quoted: Quoted: I would have to pull the report on this, but the military testing of MTBF for barrels sets the *expected* service life of a 1:7 14.5 M4 barrels at 9600 rounds iirc. What did you go and say that for? Now every keyboard commando on here is going to start saying that his barrel is done at 9600rds Come on, you know that most of the guys on here will never actually own a barrel long enough to get 9600 rounds through it... before some new trend comes along that they have to comply with. Doodz, i got a noveske afgan barrel with KFH perm attached, only shot 2 boxes of WWB and cleaned like 600 times. Selling to fund my new build with the new UberShooter (tm) barrel A couple of years ago I made up some stainless, 1:8, 14.5" semi heavy, continous taper, mid length gas system barrels -- I got a ton of messages from people that dropped all interest when they found out that stainless barrels are not chrome lined... that was the thing then, "go chrome or go home" was the tag line for every post here. Now it seems stainless in chic?

Yeah, you're right about all that

For us it's all stainless. Chrome lined isn't available and parkerizing companies are few and far between.

Mark