208 a-max with 5774 works +6db louder than 1680. although I do not have a load for such a long barrel, I do have loads for 16" 1:7 ABC micro 7..220smk as well as 240smk are my best suppressed loads. Shooting 57db-61db, 1040-1050fps 1 1/2"group@100yrds.. but the rounds are also my most expensive. Costing approximately .92 cents just to make. .208 a- max with 5774 is a VERY good round for hunting. ..I do bump up the load to 1090fps,which in Arkansas is just below supersonic. The 5774 powder is a little louder ,67db-71db, than the 1680, but still very effective.. all of my db ratings are at 2" in front and 6"below front of barrel. 50yrds....virtually no sound,i am kind of long winded but, hope this helps