I also vote for the Tirant 9. I love mine. I run it under a handguard on an AR9 as well. Barrel is 4" if I remember right. I had to go with the ALG rail to make it fit. It was the only relatively cheap option to fit the can under it. I think it sounds great, is easily serviceable, parts are easy to come by and cheap ish...
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What length handguard?