Sorry, I've no hardness data for Xtreme bullets.
My Lee U die ID measured an average of 0.4622" which is about 0.003" smaller than my newest RCBS die. Some limited testing with that die on weak (thin) R-P brass and Hornady #45177 bullets showed a dramatic 73% setback reduction when compared to reloads using a 0.003" larger RCBS case sizing die. For the three rounds tested from each die, setback with the same amount of impact energy went from 0.034" down to 0.009".
Why? Smaller initial case ID with the U die means more case stretch and thus a tighter grip during bullet seating.
Tell your friend to chamfer the insides of his case mouths and flare the case mouths barely enough to keep the bullet from tipping during seating. That'll mean less remaining flare needs removal and an even better case grip on the bullet. Too much initial case mouth flare means more residual flare needs removal after seating the bullet. Cartridge brass is not perfectly elastic and the squeezing in of the case mouth to knock down that remaining flare - even just a little bit of it - means the case gets pried away from the bullet just below the case mouth. Pried away because of leverage. Pried away there means less case grip on the bullet there. Up at the case mouth, crimping/deflaring may squeeze the case mouth against the bullet (or even into the bullet) but upon removal from the crimp die the brass springs back out a bit but the bullet metal stays put meaning less case grip up there too.
Cartridge brass is a bit of a metallurgical miracle. As the proportion of the alloying metals are changed, most other two part alloys see their hardness increase as their ductility decreases. But with brass, hardness and ductility both increase up to the point where the mixture is 70% copper and 30% zinc - cartridge brass.
If your friend happens to be an electrician or into electronics he may be thinking that if crimping solderless electrical connectors onto wires works for that combination why not do the same for ammunition? The only reason it works for electrical wire is because the wire used with most all solderless (crimp) connectors is stranded wire - not smooth, solid wire. The action of the crimping tool forces the metal of the connector barrel into the indentations between the strands. In addition, this action deforms the round bundle of strands into a roughly oval shape preventing the connector from unscrewing from the wire. Hence, the wire nut, split bolt and several other types of electrical connectors for solid wire.
When you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind. [Lord Kelvin]
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