People can get mighty comfy with dots and scopes. I know I have. I keep an old Inland carbine around just to hone my skill with irons.
I believe the 6.5 CM has gained it's popularity as a hunting cartridge. That's probably the reason most rifles have 20-24" barrels.
You might want to change your cartridge spec if you want more options with an 18" barrel. That's typically a carbine length barrel.