I don't have that much experience with the TNT. However I loaded some Horanday SX in a 220 swift. The SX has a thin jacket like the TNT for explosive results and too has a limit for velocity. The barrel we were using had a 1:14 twist and we were able to get the bullet to higher than recommended velocitys. However, later during the year we were shooting and they were tornadoing out of the barrel. Not sure what the reason was, I am assuming humidity had something to do with it but they were unwrapping in the air. You could see a gray streak in the air & nothing was making it to the 100yd target. Using that info and applying it to a different animal (have to love opinions) I think you can probably push them faster than the published velocity (depending on twist) but you may have problems down the road if you are sitting on the volital line. As usual (factories spending thousands of $ and being right) it would be best to hold them real close to 3400. You will not be able to tell a diff between 3400 & 3490 (or so) anyway.

Good luck! Let us know how they do.