10" 223... ...if it starts a 90 gr vld at 2200 fps
...If it starts the more common 69 gr smk at 2500 fps..
And
...if I were younger, richer, and better-looking... I'd be Brad Pitt.
(I love QuickLoad)
Firstly, a 10" .223 will launch a 69-grain bullet at roughly what you claim it will do with a 90-grain bullet; hitting 2275fps or so with 23.5 grains of N140. It will not launch the 90-grain at that velocity. (Actually it
can launch the 90-grainer at that velocity using 22.4 grains, but you'd be running more than 70,000psi in a cartridge rated for 55,000psi, which no sane person would want to do on a regular basis.)
If you're willing to go to a grown-up size carbine with a 16" barrel, you can hit 2250fps with a 90-grain bullet. But not with a 10" barrel, sorry.
And to get decent accuracy with that bullet, even the manufacturers will tell you you need at least a 1:6 twist, preferably a 1:5. So now your gun is horrid for use with the rimfire adapter that you say we must use. (.22LR ammo is made for use in barrels with 1:16 or so twists; about one-third the twist rate you need for these .223 uberbullets.)
Why go to all kinds of weirded-out custom-fab stuff to try and force the .223 to equal what a .300whisper/blackout does every day of the week with off-the-shelf factory stuff...? Instead of a 69-grain bullet at 2250fps (775 ft/lbs), the 10" .300bk gives 110 grains at 2260fps (measured from my 10" gun, giving 1247 ft/lbs), and that's with plain-jane over-the-counter Vmax ammo. And it does it in the same size gun, same size magazine, etc. You spent years praising the .300 whisper when you thought nobody else knew about it, and you've spent years trying to make the .223 do what the .300whisper/blackout easily does. Why fight reality? I'm personally okay with either a 10" or 16" gun, but if a person just 'had' to have the shorty, the .300bk wins in every performance category we care to measure. Every one. Period.
So why - as in "what is the reason for" sticking with .223, when .223-level performance obviously isn't what you want. I like the .223 and it's the caliber my primary gun is in, so no .223 hate here. But I'm also okay with actual .223 performance. I'm not trying to force the caliber to be something it's not; but you constantly are.