Best overall COR gun, by far. The TP bonus adds much, much more accuracy to Slug Shot than coffinmaker does, and adds a nice chunk of damage to Detonator and a small bit of damage to Leaden Salute.
Should be using Martial Gun any time you plan on WS'ing at all.
All you ever need is this, Corsair's(+1) for soloing, and maybe a Peacemaker for level sync.
The delay on this gun does not matter. You are not shooting for tp, if you are, as the other comments say, coffinmaker is superior. This is for Joyeuse corsairs who tp with joyeuse and ws with gun. Similar to the sam/rng soboro concept, hope this clears something up for not only yourself and others who may read your comment and believe this gun is not good. But i'm not going to go into that, there's already enough comments about its worth and what it does.
I no longer use this gun anymore on my Ranger, however, I just wanted to add that vs HNM with high evasion, this gun can actually be pretty decent. It's also great to use if you are doing light skillchains since you will be using detonator/heavy shot. Hellfire +1 is great, but I just wanted to point out that when your slug shot acc is low, this is something you might want to try.