If you are open to manufacturing your own suppressor you may want to look into a hybrid design that incorporates rear baffles. I have one that was made by Dark Horse, and a second one that I self-manufactured custom fitting purposes. Like you, I wanted a balance of minimal overall length and maximum barrel length. Using a hybrid design I was able to make use of 7" of barrel space for rear baffles, and only add another 5" to the front of an 18" .308. Depending upon the height of your sights or scope, you can also add more volume than most suppressors offer. In this case I custom made the suppressor to slip over the barrel of a bullpup rifle, which also gives me more maneuverability. I did all of this after obtaining a BATF NFA permit.