So called bi-metal jacketed bullets do not wear rifle bores any more than gilding metal jacketed bullets. The gilding metal overplate on the steel jacket is thicker than the depth of the rifling. There use to be a common belief that AP ammo also wore rifle bores excessively and extensive testing indicated that this also was false. In fact, AP ammo seemed to offer a barrel life advantage over standard ball ammo. Bi-metal jacketed bullets in some cases do not exhibit the accuracy of gilding metal jacketed bullets due to a lack of jacket uniformity but that is not always the case. It is sort of a lot to lot phenomenon - some lots are good, some are not. Rick
The US design for the Gilding Metal Clad Steel (GMCS) jacket has a total jacket thickness is .021" with an outer gliding metal cladding averaging .003" thick. The core is a lead-antimony alloy with 2% antimony, which is softer than the 90/10 lead-antimony alloy used in the Gilding Metal (GM) bullet design. (Note: this is about the same cladding thickness as used by Wolf, Brown Bear and Tula.)
With M80 and M62 bullets with GM jackets, the total thickness of the jacket is .026", per print.
The copper cladding is not quite as thick as the rifling depth of a standard M14 barrel. If you dig out a shot GMCS jacketed bullet you will see that the rifling just barely cuts through the gilding metal, but only at the corners most of the time.
They do wear the bore more, but not to any appreciable extent, Choice of propellant has a greater effect on barrel life than the bullet design.
EDIT [1]:
There is a report done by BRL comparing GMCS and GM jacketed bullets (where the above information came from), it showed that W846 propellant with high percentage of CaCO3 (calcium carbonate), had barrel lives in excess of 13,000 rounds for GMCS and 18,000 rounds for GM. Extruded propellant with low percentages of CaCO3 had barrel lives around 8,000 to 10,000 rounds. The current typical propellant choice used in M80 Ball is W846, so you can expect a fairly good barrel life with either GM or GMCS jacketed bullets.
EDIT [2]:
Oh, there was one other thing, The nature of the barrel wear was different between GMCS and GM. All of the weapons firing GM jacketed bullets were condemned for velocity loss (200 fps velocity loss from muzzle velocity reading from the first string fired), all of the weapons firing GMCS jacketed bullets were condemned for keyholing (20% of the bullets exceeding 15 degree yaw at 1000 inches from the muzzle). Further measuring the bore and land diameters down the barrels showed that barrels shooting GM jacketed bullet showed large amounts of throat erosion, with little muzzle wear, whereas, barrels shooting GMCS jacketed bullets showed the expected throat erosion, but had large amounts of muzzle wear.
This indicates that there is a major difference in how the two bullet jackets interact with the barrel...
