When we decided to start making vent liners at Lehigh Bullets we made two versions. A flat version and a domed version. We focused more on the domed version cause they looked better, and the pressure was directed away from the flame channel in the vent liner.

http://www.lehighbullets.com/products.asp?cat=25

But after some unplanned testing, and testing bullets I found out the domed ventliner also lasts longer. And it seems pretty significant as well. I have the same treated flat and domed liners. The domed are pretty significantly better. I used to track shots but don't do that anymore.