Maybe I'm just going to seem stupid, but does everything have to be FTB? Just fill a single-shaft ith TNT and blow it?
Have you ever actually tried that? If not, go do it and see what happens.
Back? No, I know you didn't try it and just looked down to the next part. What happens is when one thing of tnt goes off it send a lot of the others flying causing quite random damage and carnage - not that very precise. It's actually that function of TNT that makes TNT cannons possible, all they do is force the explosion to go in a specific direction, and even then it's highly inaccurate.