Re: 223 WSSM and 69gr SMK
What's the elevation and temperature extremes where you shoot?
What is the twist and length of your barrel? What COAL does your rifle's magazine alllow?
The 223 WSSM cartridge is rated 64542 psi with 53.7 grains case capacity and a COAL of 2.360". A 220 Swift is 62366 psi and 47.0 grains case capacity. That's the closest common commercial cartridge. The 223 WSSM is between the 220 Swift Ackley Improved and the 22-284 wildcats.
I haven't tried shooting anything heavier than 55 grains in my Win 70 223 WSSM.
It has a 10" twist 22" barrel. I Belivee it would shoot the 69 SMK ok at the mile elevation where I shoot. I could probably shoot the 77 grain SMKs too. The questions are whether heaver bullets will fit in your magazine and throat with appropriate seating depth and whether they'll stabilize in the environment where you shoot.
Quickload shows that 69 grain SMKs driven with 90% SAAMI max loads of VV N560 or IMR-7828 SSC, velocities around 3400 FPS from a 22" barrel can be expected. Yes. Its a hot cartridge.
Don't expect great barrel life, but if you own it you might as well shoot it. I'm hoping for 1000 shots, similar to a 264 Win Mag or 7mm Rem Ultra.