One more thing about all this . . The idea of "Rifle Drilling" or "Gun Drilling" axles really got popular back in the early 70's in Drag Racing because the guys in the, then new, "Pro Stock" class were doing everything they could do to lighten up their cars . . Axles weren't the only things they drilled . . At one time, early in the game, most of the big names, Glidden, Sox and Martin, Warren johnson (just to name a few) were drilling holes in anything that they could do it to, including boring all screws and bolts that were in tension or compression. Anything dealing with shear, they backed off from . . But the axles are kind of a direct carryover from that period and technology.
And one might be surprised at how much weight the centers of bolts and screws make up on a car. Warren Johnson (way back before he was famous) told me once that it took up to 10 pounds off a car if done right . .
Now 10 pounds may not sound like much but back then it was a bunch . . And in some cases, it still is . . especially if it's on your wife's butt and she asks you about it . .