When Bore gets an opportunity to work, god damn it really works.
Last night I went to fight BNK3R for the second time, and instead of taking several minutes to take him down as it did the first time I fought him, he died near-instantly. When this happened, I was shooting up into his underside's weak spot with a fairly weak Miss Moxxi's Bad Touch, the bore 'penetrate' noise sounding heavily as his HP bar drained instantly.
What I assume happened is that I shot through both critical spots and caused immense amounts of damage near-momentarily. After a few more passes I think I've figured out where I need to stand to get the angle right, but I couldn't take him down instantly the same way I did the first time - about a quarter of his HP was the best I could manage.
I first noticed how much of a difference this skill made against Old Slappy, but I thought he was a special case because of how easy it was to score crits with Bore through his tentacles. l'll be taking another look at this tonight and see if I can figure out how to perfect instant BNK3R kills. Fun times.
The easiest way I've figured to do this is to aim diagonally through his body with a shotgun...about where the turrets on his sides pop out. That lets the buckshot fly through the hole in the centre of his body and causes massive crit damage when it reaches the other side, dropping him instantly.
I'm not sure what the requirements for this are beyond Bore, but I'd say that you could do this with pretty much any decent shotgun. Give it a try.