I called Barnes today.

I didn't read this before I did or I would have asked about the hardness. I can do so.

First; According to Barnes, the driving band type was developed for a couple reasons. One was that they found accuracy was improved with the driving band type to the tune of better than 50% reduction in group size {2-inch groups became sub-1-inch groups}. You guys that shoot them can verify that or not. They incorporated a flat meplat also, as they found thru testing and customer etc, recommendations that it penetrated deeper/straighter than the older RN type. I think this is well-established fact now.

As for pressure, the driving band type was said to produce somewhat LESS pressures, about 2,000 psi on average, which is something on the order of the range in variation in other bullets tested, so peaks were similar to some other bullets, tho average was greater for the non-banded.

Huvius: from my knifemaking experience I can tell you how to anneal a gilding metal bullet {I do not know if they are sold "dead soft"}. Heat it up to glowing red and drop it in a bucket of water. Opposite of steel hardening. But I can also suggest that the probability is you will get warping and deformation of the bullet if you do. Maybe it wouldn't matter at close range. The surface will oxidize, so you'll have to tumble them if you want pretty bullets.

Copper alloys are normally measured on the Rockwell B scale, and harder materials like steel on the C scale.

Rifle barrels normally register about 24-to-low '30's on the C scale. Brass alloys are normally measured on the B scale for softer metals. It is possible to harden brass or bronze to exceed pure iron in hardness tho I've never done it, that is, had it tested, as pure iron is hard to come by. What passes for "iron" is usually mild steel. Brass runs about 93RB and that correlates to about 16RC, very "soft" for steel. Knives run in the high 20's for some bayonets {soft spring}, mid-high 40's for machetes to low 60's for "short" knives. A knife that goes RC62 is a very hard knife. I heat treat my own to 50RC for hard use bolos to 57RC for hunting knives. Those I donate to servicemen I heat treat to about 52-54 for good hardness with excellent toughness, and I apply a differential hardening as well to prevent breakage. In simple carbon steels, in a general sense, the harder the knife blade, the less "tough" it is, i.e. the more brittle it is.

Theoretically, a VERY hard, oversize bullet could "egg-in-the snake" its way thru a thin, soft steel gun barrel. What that would do to the rifling is unknown to me and certainly no one here has provided any proof of damage though we can all imagine some sort of damage, mostly at the breech end and at solder joints along the barrels. Hard steel jacket, tungsten- or hard steel-core military ammo can produce excessive rifling wear as cited in various military ordnance sources and by experience. Pressing the rifling to the outside of the barrel? I don't know, but I do know it would take pressure, and lots of it. And for it to push the rifling out similarly at the breech as at the muzzle is, well, almost impossible for me to imagine if that is what it is said to do. Nay, impossible, in the absense of any proof, which, of course, as always, has not been provided. Daryl is right. It takes pressure. LOTS of pressure at the thick breech, and, indeed, quite a bit at the bayonet end.

Just how an engraved, undersized bullet {Barnes are sold undersize & I'm sure all double owners slug their guns, right...?} could push rifling out to show on the outside of a barrel near the muzzle is unknown to me and not demonstrated here by anyone. Indeed, it defies credulity.

For an undersize bullet to engrave, NOT obturate and then press the rifling out anyway seems, well, you all decide. In fact, the suggestion that an undersize Barnes {or other} bullet does not obturate is a recommendation for it, not a condemnation in this instance.

As for boring them out, I submit that MIGHT cause obturation depending on the wall thickness of the remaining bullet and, correspondingly, greater pressures...

Maybe we should fill the void with something. Like lead?