Long barrels is a relative term. Titans can happily use barrels a few feet long. 2 feet is still far too short for me, try 3 or even 4. Like Buff said, Titans have a shitload of volume and at ~30PSI they are seldom satisfied. Dart weight could also be a factor, try using three washers instead of one. That would add some exposed metal, but you're not shooting people anyway.
Thanks, I'll try your suggestions.