Regarding barrel wear from SE bullets. The VDM material I have shows a lot of concern with cycling issues on the MP40, but apparently showed no concern for barrel wear, so it must not be significantly greater than the mE bullet so that it concerned anyone. Remember, this work started in 41 or 42 on the all SE bullet so there was lots of time for testing.
Barrel wear, as I understand the process is a function of five things.
- the relative velocity of the two surfaces
- the coefficient of friction between the two surfaces
- the force vector normal to the surface forcing the two surfaces together
- material properties of the barrel (ie yield strength, thermal properties, etc probably best captured as yield strength as a function of temperature)
- geometry of the contact-the internal construction of the barrel
With a given barrel and a given load, some of these variables drop out.
The SE load has a lighter bullet and a somewhat higher velocity than the mE bullet and will therefore cause some increased wear.
The force normal to the surface is a function of the malleability of the bullet contact surface. The mE bullet has a GM coated steel jacket with about 1/16th inch of lead betweent the jacket and the core. I suspect that this is probably almost as malleable as a lead core bullet since the deeper part of the core will add relatively little to the surface malleability. Jean-Pierre is right, it is the “softness” of the bullet.
The SE bullet is not solid iron, but being made by a sintering process it will have tiny voids between iron particles, and apparently used some sort of binder, but it is not clear if any of this was retained in the bullet after the sintering process. The bottom line is that a SE bullet is more malleable than a solid iron bullet but I have no idea how much more malleable the mE bullet was in the bore than the SE. The SE bullet also had a surface finish to prevent corrosion and likely to reduce barrel friction.
I would expect the SE bullet would cause more barrel wear, but the fact that it apparently was not an issue in the test process, I also suspect that the increased barrel wear was not significant, at least it wasn’t to the German army at the time!
Relative barrel wear from a SE bullet would increase in 7.92x33 and 7.92x57mm weapons because of the higher velocity. This may be why we see less use of SE bullets in 1944 and 1945 in the rifle caliber cartridges.
If someone has indications that the German Army was concerned about barrel wear with SE bullets, particularly in 9mm P ammunition, I would appreciate hearing about it.