Q:

Why won't this wall break?

I found a secret wall in Enter the Gungeon. I know the starting gun can't uncover secret walls, but I found a handy mermaid shotgun that can, and did, reveal the breakable wall to me. However, the gun doesn't seem to want to actually destroy the wall. 
I've tried my only other gun, the one that specifically says it can't break walls, but other than that, I'm out of options, since rolling into it does nothing.
How do I get through this thing?

A:

Fire off a blank, one of your blue bullets.  Using a controller that's clicking LS and RS at the same time.

