If your making an open gun, then gas volume is your friend for making the comp work for you then, correct?
I know some load the .40 s&w long, and run it in barrels cut to allow it to chamber. That, along with magazines that would allow 10mm OAL to facilitate both reliable function as well as keeping the loads hot, but safe.
Truthfully, Ive always wondered why someone doesnt just run 10mm since many roll their own to begin with.... I suppose if I were to make a gaming gun I'd look at the .38 super/9x23mm type since the OAL length already fits the 1911 pattern well, and you can have a very efficient function for your comp.... and you get more capacity. And your not trying to make it run a shorter then optimal cartridge in such a "long" action.
Or Id make it in 10mm. Because it would be AWESOME.