AK Rifles banner

Steel cased vs brass cased an interesting read.

20K views 47 replies 29 participants last post by  Texpat 
#1 ·
#40 ·
I can't find the original document, but I did find this data from an old US Army .30 caliber barrel wear study on another board and thought it would be useful to share here, since it focuses on combinations of bi-metal jackets, copper jackets, ball powder, and stick powder. All credit to Lysander for providing this information.

https://www.m4carbine.net/showthread.php?186725-Steel-Bullets-or-something-else

Here are some results of some US Army erosion tests done with 7.62mm Ball, M80:

Test firing.............Propellant..................... .....Jacket......No. of Rounds to.......Cause for
No...............................Type............. .....................Material....Disqualification. .......Disqualification

1..................................IMR 8138M Lot 2.................GM..............14,500.......... ...........V (1)
2.................................IMR 8138M Lot 2.................GMCS..........8,450............. ........K (2)
3................................IMR 8138M Lot 2.................GMCS........10,150.............. ........K
4................................IMR 8138M Lot 48..............GM................8,000........... ...........V
5................................IMR 8138M Lot 48...............GM................7,500.......... ............V
6................................IMR 8138M Lot 48...............GMCS..........7,850.............. ........K
7................................IMR 8138M Lot 48...............GMCS.........11,800.............. ........K
8................................WC 846 Lot AL44133..........GM..............17,300........... ...........V
9................................WC 846 Lot AL44133..........GMCS.........18,325.............. ........K

1) Velocity loss of more than 200 fps

2) Keyholing, defined as 20% or more bullets exceeding 15% yaw at 1000 inches (appox 25 meters)

GM = Gilding metal jacket (aka copper jacket)

GMCS = Gilding metal clad steel jacket

Partially in an attempt to explain the above, and also to test the usefulness of wear reducing additives, a second test was run, with stricter controls on barrel selection and more careful monitoring of various barrel parameters.

In this test several T65E1 machine guns (M1919A4s converted to 7.62mm) with chrome plated barrels were utilized. A 25 round belt was shot every 12 seconds until 500 rounds were expended. Then the barrel was allowed a 4 minute cool down period before the next 500 rounds were fired, again in 25 round increments. Bullet velocities and bullet yaw were continuously measured. Every 5000 rounds, the barrel was cooled to ambient, cleaned, measured, and samples of residue and bore fouling taken for analysis. The measuring consisted of measuring the diameter of the lands and grooves at 1 inch intervals. Then the process was repeated until another 5000 rounds was shot, or the barrel failed due to keyholing or velocity loss.

The findings from the tests described in this report are summarized in the table below.

Summary of Results
.................................................. .................................................. ..................Adiabatic
.................................................. .................................................. ..................Isochoric
.................................................. .................................................. ..................AverageFlame.......No. of
Cartridge...........Bullet........................ ..........................CaC03...Mo03..........Te mp..............Rounds to
Lot No.................Design..................Propell ant..............(%)........(%)..............(K).. .................Disqual.

LC-SP-1368.......GMCS (1)..........Ball WC 846...........0.15............0.............2884.. ................10,417
LC 12923............GM (2)..............Ball WC 846............0.58...........0............2790... ...............18,042
FA-42-73...........GMCS.................Ball WC 846...........0.47...........0............2831.... ..............13,342
FA-2115..............GMCS.................Double Base............0..............1.05.........2889.. .................8,625
.................................................. ............Extruded
.................................................. ............Propellant
FA-2016..............GMCS.................Double Base............0................0.............291 2...................6,333
.................................................. ............Extruded
.................................................. ............Propellant

_______________________
1) Gilding Metal Clad Steel jacket (aka, bi-metal), the total jacket thickness is .021" with an outer gliding metal cladding averaging .003" thick. The core is a lead-antimony alloy with 2% antimony, softer that the GM bullet design. (Note: this is about the same cladding thickness as used by Wolf, Brown Bear and Tula.)

2) Gilding Metal Jacket, the total thickness of the jacket is .026". The core is a lead-antimony alloy with 10% antimony.

Note: All GM jackets bullets came from the same production lot, as did all GMCS jacket bullets.

Calcium carbonate (CaCO3) is an additive used to neutralize acid during the production of ball propellant. It has also been shown to reduce barrel wear, unfortunately, it also leads to increased fouling. WC846 made after 1969 was made with reduced CaCO3 content (less than .25%) in order to be used in the M16 without fouling the gas tube. At some point, WC864 with extremely low CaCO3 content was split-off as WC844, and WC846 with increased CaCO3 content was introduced as WC864+CaCO3 for use in 7.62mm, Ball, M80 specifically to increase barrel life.

You will note from the above table that there is a better correlation between flame temperatures and wear than jacket material and wear, especially if you know that 8138M has a flame temperature of 2770 to 2820 degrees K. GMCS jacketed bullet are only slightly harder on the barrel than GM jacketed bullets, but the choice of propellant can easily make up for the difference.

In this test, measuring of the bore diameters did reveal something interesting. When a bore wears, enlarging of the throat tends to lead to velocity loss, as gas escapes around the bullet rather than pushing it down the barrel. Conversely, wear at the muzzle tends to lead to keyholing as the rifling loses its grip on the projectile before maximum velocity and maximum RPM are reached, therefore the bullet leaves the barrel with less spin than required to stabilize it.

The measuring of the tested barrels showed the GMCS Jacketed bullets seem to open up the muzzle more than GM jacketed bullets, which would explain why GMCS jacketed bullets tended to fail due to keyholing. Not only did the land diameter increase, the groove diameter showed a similar enlargement. Possibly indicating a gas erosion phenomenon as the bullet nears the muzzle?

In all cases, the addition of calcium carbonate in the propellant drastically reduced the progression of throat erosion. Molybdeum trioxide did reduce throat erosion, but the fouling residue was so bad it made continued firing of the gun difficult (It formed in the recoil booster and prevented the barrel from sliding freely).

These are just two Army published reports that show that flame temperature of the propellant has a very large impact on barrel life. In fact, as a result of their years of study, the US Navy has adopted the simple solution to barrel erosion is simply reducing the flame temperature of the propellant and live with the reduced performance. This is the thinking that brought forth NACO (NAvy COol) propellant, and reduced muzzle velocity, and subsequently range.

(It is interesting to note that ball propellant, even though it is a double base propellant, burns cooler than 8138M, which is a single base propellant.)
 
#41 · (Edited)
Comparing the above with other information from a police department test of an M14/M1A, data from Battlefield Vegas, as well as information from the Lucky Gunner test, I've drawn some additional conclusions as well as one that contradicts Lysander's conclusion from the above:

If usable accuracy, not velocity loss, is the prime concern, especially with high rates of fire using boat tail bullets (FMJBT, OTM, etc), then copper jacketed bullets are a far superior choice and will result in 2x - 4x the usable barrel life compared to bimetal jackets, regardless of any other factors including barrel construction. Bi-metal bullet jackets cause a different, more destructive type of wear that strips the rifling from the barrel all down the length of the bore. Powder burn temperature remains a significant factor in barrel life, bi-metal jackets will simply kill a barrel in a different way before powder differences become significant.

How different bullet jacket materials affect barrel wear with slower firing schedules isn't clear. It has been suggested that steel jackets raise barrel temperatures faster than copper due to differences in friction. What is clear is that, at high rates of fire, steel jackets wear the rifling all down the bore's length, not just starting from the throat and creeping forward as is seen with copper jackets.

AKs in 7.62x39 are known to fire accurately with bi-metal jacketed ammunition far in excess of the 5-7k rounds that caused keyholing in the AR15 barrels used in the Lucky Gunner test. There is evidence suggesting the reason for this is that most types of 7.62x39 ball use flat base (non boat tail) bullets. Flat based projectiles are often recommended by experienced reloaders when attempting to wring the last bit of useful life from a worn barrel. It's not that flat based bullets cause less wear, it appears that they simply perform better in worn barrels than boat tailed bullets do.

So, if you have a 7.62x39 firearm that you've shot extensively with bi-metal jacketed bullets and it appears to have lost its accuracy in only a few thousand rounds, check your ammunition to see if it is using boat tailed or flat based bullets. If it won't shoot with flat based bullets, you can safely assume it's time to replace the barrel.
 
#42 ·
Interesting post Aries. Over the past year or two I noticed my M70 starting to keyhole with Golden Tiger and Red Army Standard 8V3 that I tried. I know in years past it shot Golden Tiger from the same lot fine though. Fortunately stuff with an apparently normal bullet style like Wolf Military Classic, Tula, Brown Bear, Wolf Mil Spec and all that other good stuff has still been shooting pretty decent. I haven’t shot much of the copper jacketed russian stuff yet but it is nice that they have several reasonably priced options available nowadays. I will say I’ve shot my M70B1 that I put together, quite a bit over the years so if it is a case of the barrel beginning to give out I can safely say I got my value out of it, no doubt. But, like I said, so far it’s still kicking just fine with the non-boat tail stuff it seems.
 
#48 ·
Wow, amazing report, had not seen that before, stuff like this is why I joined this forum.

My two cents worth, sample size of 1.

1. Extractions issues with straight-walled cases like .223 are one of the reasons I adore 7.62x39 and it's tapered case. And in general everything about an AK lends itself to reliability and proper extraction, that tiny gas tube in AR's has always bugged me, I have a ton of those long pipe-cleaners in the shop and use them after every range session.

2. Looks like Brown Bear had some issues in that test, but the tightest group i've ever fired in my life came from Brown Bear .308 in a Remington SPS with a 26" barrel, .2497 as measured with my digital calipers at 100 yards. I posted that it did so online and got called a liar, so I posted the target and everything went quiet, but the next day when I woke up and checked my ammo supplier for it, after selling just a few boxes a week and having tons of cases in stock, it was all now gone, lol! I still have some of that very batch that I refuse to part with or shoot.
 
Top