I used Green Dot to get ~1000 fps with ok chamber pressure and low muzzle pressure. There were actually quite a few other powders that would "work" in a similar fashion. However, many of the really fast powders (N310 included) needed over 55 ksi chamber pressure to get 1000 fps.
My use of 9 x 23 was entirely to show what an existing small capacity round rated at 55 ksi could do in the way of getting low muzzle pressures and useful performance.
For "testing" only, I am willing to run cartridges way over the SAAMI pressure limit if I know it is safe in a gun. However, I have figured out that I am not perfect and I can loose track of things. As such, I refuse to consider routine use of ammo loaded well above SAAMI in a common cartridge. This is just a hobby, and I do not want to take any chance that a "fun little toy" turns into an event that destroys a gun and/or hurts someone.
If I were to embark on such a rifle, I would most likely go with a 23 mm long "stretched 380" wildcat using 223 brass. It would have the same chamber ID as a 380, and I would need to figure out what kind of throat is needed to for all of the bullets expected to be used. I would "neck turn" the brass on my mini-lathe to get the brass thickness Ok for chambering ammo loaded with 0.357" bullets. This would effectively duplicate the 9 x 23, but with a near infinite supply of low cost (but high effort) brass.
Starting with a standard 380 or 9mm chamber might be a way to go. The "for testing only" ammo can be loaded up to both 38.5 ksi (9 mm +p) and to 55 ksi just to see how these pressure levels work out. If the 38.5 ksi pressure ammo is "good enough", the ability to use 9mm brass is a big deal. If the 55 ksi ammo makes a big enough improvement in performance, it could justify something like buying say 200 rounds of 9 x 23 brass from Starline or making the 23 mm long stretched 380 brass.
Note that I have dabbled in this kind of thing to some extent. At least 90% of my shooting is with handguns at a nearby indoor range. I made a 7.4" stub tube Contender barrel from a pulled 30-30 barrel. The chamber is pretty much 32 ACP but I use brass with the rim dimensions of the 32 S&W. My current batch of brass is made from 9mm cases. You could call it a rimmed 30 ACP. I love shooting this thing at 30' paper targets with a 55 gr tumble lube NLG boolit over 1.7 gr TightGroup. Velocity is around 890 fps. However, I also have fired a few rounds with the Lee 150. I do not remember the exact load but it was probably in the 800 to 900 fps range. It was a lot of "thump" with very little muzzle blast or noise. It looked wild as the exposed portion of bullet was longer than the case. I did something to extend the "nose riding" portion of the boolit down closer to the base so it would chamber.
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