My modern is 18.25", but it is something I had made up when I had nothing else to spend the money on and several friends and I wanted to experiment with minimum legal lenght barrels and to see what kind of patterns we could achieve. Sometimes I miss the days of having almost no bills cause of living in the barracks. BAck then, even the gunsmith said I was nuts and wasting money but he would take it if I insisted and didn't hold him accountable for any performance results. My short modern patterns as well as some of the best longer barrelled guns I have seen, but it is kinda stubby and handles like it.
So, yes, you can achieve good patterns with a short barrel, but it takes more work and range time to get there. I dont know when your season starts, but if it was me, with mine starting next week, I know I wouldn't have time. Having said that, it is after all your money so spend it how you want, but I am kind of curious why the insistence on such a short barrel? I mean, there is no added lenght due to the action as on a modern pump or semi, and it may be easier to achieve the desired patterns with a longer barrel. Just curious, but I'm using my .54 smoothy GPR with a 32" GM, so not much longer.