Base Guards(tm) are the trademarked name of special shaped copper disks which you can make and swage to the base of lead
bullets, to protect the bore from fouling and the bullet base from hot powder gas damage. The disks, having sharp edges at
90 degrees to the bore, scrape leading from the barrel with each shot. Since they are attached by an "axle" formed as lead extrudes through a hole in the center of the disk and expands to make a rivet head (BG punch has a round depression that forms
this retaining head), the disks can lock into the rifling and turn separately from the bullet. This maintains a gas seal even
if the bullet skids in the rifling.
You can make BG Disks in any caliber, using the appropriate BG Maker Kit, and Corbin's .030-thick x 1-inch wide copper strip.
The strip is pulled through a slot in the die, and the press ram is stroked up and down only far enough to punch through the
disk and push it up into the top of the die, where eventually the stack of disks begins to emerge from the die top.