Here's a prototype of 308x1.25":
Next to a regular .308 Win. round
Next to a 45 ACP case
Used a 44 Automag case. It requires minimal trimming after forming. Forming was difficult as it gets too tight in the base. However, that that could be fixed with a custom reamer and I believe it would still work in the Giraud (w/ 308 Win. case holder), which is what I used to trim off the excess length.
I didn't have a 240 SMK, so I just sat a 190 SMK on top. The 240 is about 0.236" longer than the 190, so it's pressure ring would sit just above the neck/sholder junction.