I spoke with Paul at Hammerhead when they released their Tiberius barrels and he said all their barrels are .688ID except the one specificly made to .684ID. So my MoFo barrels will shoot the FS rounds the same as my Tiberius barrel.
As far as "The fins will never engage the rifling" That is BS! Grab a FS round and slid it in a Hammerhead barrel, it feels like they were made for each other!
Firing FS rounds from them is just plain amaizing! Minimal arc and they hold their speed much longer!
I have been looking forward to Rap4's D-Mags so I can fire FS rounds from my Phenom using my longer MoFo barrels since I do not own a T9.1 yet. Too bad Hammerhead isn't making anything for the older T9. At least I have my T8.1 and let me tell you even that 8" barrel reaches out! Unfortunatly you do not have precision aiming on the T8.1. :P
Lots of people like to talk about their "knowledge" on how the Hammerhead doesn't work, yet they don't even own one! They also forget you have to tune the barrels to regular paint balls!!! Also not all markers handle them as well either. For instance I fought with my Hammerheads on my Phenoms and still had a large grouping at range. Where my Trracer and MacDev Tactical Drone hold very tight groupings at the same Distance. When I used my Trracer in May (yeah I have not been out much this year) It was taking 3 shots with regular paint at long range, one for elevation, 2nd for windage, and the 3rd was for elimination.
The other teams paint was hitting the ground at least 5 feet in front of my and rolling past while I was still sending paint over their heads! I was chronoed high 260's low 270's. I personally love these barrels and 10" MoFo Scenario with M-50 and Snaggletooth Tip, A 14" MoFo (Non Scenario), Couple Sharktooth, and a Gold Member. I haven't had a chance to use the Gold Member though so I have no idea how the rifled brass will shoot.