Bore/Throat erosion - the enlarging of the internal diameters, both bore and groove, from all causes as a result of shooting ammunition.
Everybody has seen this THIS article from Lucky Gunner Labs, and while it is a good article and full of good information, many people out in internet-land tend to draw conclusions that are not correct. One of the most quoted incorrect conclusions is "…bi-metal jacketed bullets wear out barrel twice as fast as copper jacketed bullets…"
It is unquestionable that Wolf, Brown Bear and (probably) Tula steel jacketed, bi-metal jacketed ammunition will wear out a barrel faster than Federal XM193 ammunition, but the but the extra jump to "all bi-metal" vs "all copper jacketed" is not supportable.
Here's why:
From the earliest day of smokeless propellant, the problem of bore erosion and wear has been a constant head-ache to owners of large number of high use guns and very expensive guns, namely the military. So, not surprisingly, they did a lot, and I mean A LOT, of research into what causes bore erosion, and how to reduce it.
There are basically two causes of bore wear - 1) heat, the flame temperature of burning propellant is anywhere from 2500 to 3000 degrees K, depending on the actual propellant (for reference, the melting point of the steel used in the barrel is 1700 degrees K), and 2) mechanical rubbing between the bullet and the barrel. Of the two, the effects of heat are probably the most damaging.
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
The significant conclusions drawn from these results in the report this table was attached to were as follows:
1. Bullet jacket material (GM versus GMCS) does not appear to have a significant effect on barrel life. However, the GM jacketed lots all went out on velocity loss while the GMCS lots all went out on keyholing indicating that the mechanism of barrel failure was probably different.
2. WC 846 propellant is less erosive than IMR 8138M propellant.
One may note that test firings #2 and #6 differ greatly from #3 and #7, which is very puzzling as the components used were the same, and the propellant lots were the same. Also, the question of why did 8138M Lot #2 perform notably better that 8138 Lot #48, came up. In the report, the difference was written off to variations in the test barrels.
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.)
The result shown in the Lucky Gunner test, are almost the exact same results as the results shown in the above two Army tests, namely, the relatively cool ball propellant used in M193 (WC 844, the same stuff as WC846 but with less CaCO3) will wear out a barrel in 13,000 to 15,000 rounds, and the reason for rejection will be velocity loss (if you look at the velocity chart for the copper jacketed bullets, the velocity loss will be more than 200 fps in about that time). We can only assume what powder Wolf, et al. are using, but it would seem from the results to be a relatively hot extruded propellant (double or single based) which has shown to wear out a barrel in about half the time as WC846/WC844, and the reason for rejection of the barrels is keyholing.
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Originally Posted by lysander
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