The NOE XCB bullet I have yet to shoot, but I designed the first one and have some bench time with it. The whole concept is minimizing the amount of free space for lead to go sideways, and matching the throat angle precisely with the nose angle. When working out the XCB reamer design and the AM 31-190X bullet that was to match it, I used the term "Morse Taper Fit", which will be familiar to any machinist. Matching tapers will always align themselves perfectly straight when pressed together. The limiting factor is they have to be pressed together in the static state, for there is very high contact pressure at two small points of it is even slightly crooked, and soft lead deforms very easily. Parking the bullet in the throat for a perfect fit means it takes a lot of pressure to get it moving, which puts a lot of pressure on the bullet base. My bullet had one, deep lube groove and had a nasty tendency to rivet in the neck due to the extremely high engraving pressure. The NOE design has multiple, small lube grooves which makes a stronger core and combats the riveting tendency somewhat. There is also more room between the three grooves for metal to displace as the bullet engraves, and the flow forces are broken up before they get to the core and distort the whole bullet. This is sort of a hybrid between the silhouette-style bullet with throat-matching nose taper and the Loverin bullet with small lube grooves the whole length.
What seems to be working the best with the NOE version of the XCB bullet is a very tough, malleable alloy with a good deal of tin in it to make it resilient and resistant to lengthwise flow. The whole idea being to deform the bullet as little as possible as it engraves, so it cannot slump or bump at all, just start out straight and squeeze straight into the bore without the nose achieving the plastic state, while the displaced metal from the back part of the nose and the driving bands flows into the lube groove areas without disturbing the core material which could cause the bullet to bend or the nose bump up more on one side or the other. Slow powders are the name of the game here, for if the pressure spikes up too fast while the bullet is resisting the throat taper, the base will rivet anyway and it won't do it straight unless the whole neck is filled up with brass and lead....then you have other problems.
When I cast my XCB bullets from too soft of an alloy (air cooled 50-50) and punched them with 13.5 grains of Unique, they left neat little lead rings at the end of the case mouths and shot for diddly. When I used 4350, they shot ok to about 1800 fps and then petered out while beginning to leave lead rings and lead streaks in the bore. When I used straight wheelweights air cooled, they shot great with Unique and didn't leave lead rings, but I was stuck at 1500 fps. When I tried 4350 I got to about 2,000 fps before the lead rings showed up and accuracy again went south. I kept strengthening the alloy and hitting it with different powders but always came up against a point where the base would rivet and the accuracy would fall off. Understanding that riveting was the issue due to starting against a load, I tried seating the bullet deeper to give it some jump. That worked out ok, but still didn't get the groups I wanted at any speed, with any powder, or any alloy I tried. In retrospect, Taracorp Magnum, water-dropped, with about .015" jump to the throat, and no more than one thousandth total neck clearance and with a high-stearate, stiff lube, just might have worked, but instead I decided to take the lessons learned from the bullet and abandon it in favor of an easier tack....which due to the throat shape ended up being the MP .30 silhouette. Due to the tight tolerances of the XCB chamber, I had to size the bullets to .3105" and turn the necks for a close fit, sort of "pre-slumping" the bullet. I also found I had to nose-first size the bullets, then apply checks and base-first size them to crimp and lube or the metal didn't move correctly. At .3125" as-cast the bullets didn't like getting sized that much, nor were they designed to be until fired, but I made them work and with a little jump, a relatively soft, ductile, 50/50 COWW/SOWW alloy with no additional tin performed admirably. Here are some groups I shot back to back at a hundred yards, without perfecting the workup, at just under 2700 fps from a 23" barrel:
View attachment 5304
Here is a pound cast of the XCB chamber, together with a loaded cartridge and a dummy that had been chambered and removed to show the engraving. I put the pound cast back in for some reason, so that's why it has double engraves on it, but you can see how close the static fit of everything, including the loaded chamber neck, is. BUT.... even then, it was still super-easy to screw it all up and get shotgun-pattern groups if the alloy and powder weren't perfectly selected. I don't seem to have a photo of a loaded MP 30 Silhouette for this cartridge, probably because I shot them all, stuck a fork in it, and moved on to other things.
View attachment 5305
Here's the AM31-190X first as cast, then tapped into the throat unsized at .311", then sized .3105" for the .3108" throat entrance and tapped into the throat. Looks great, don't it? What could possibly go wrong? Well, as I explained, lots. You STILL must get the static fit, jump, run, and bumping dynamic fit just right via powder burn rate and alloy selection or it will find a way to go south on you in a big hurry.
View attachment 5306