If you mean for terminal performance, for that length barrel, I might research what people use in subguns. Longer than a handgun, shorter than a carbine; subgun- or PDW-optimized ammo might be a place to start. Then again, if you have access to a chrono, you might (or might not) find that that barrel length makes a standard-pressure load act like a +P version of the same load.
One thing I'd be conscious of would be over-driving a given bullet - pushing it faster than it can tolerate and actually reducing performance thru added velocity.
If you mean accuracy performance, I'd start with any decent 124's or 147's and see how they do. Range-wise, you'll likely be able to clang 150-200 yard gongs (24" or so) if everything is working as it should. That's only 12-15 MOA; should be doable from most shoulder guns once you get the particulars worked out.