Cum is a word for semen. When a human male ejaculates, semen discharges from the penis, often several times in succession.

Due to the viscosity of semen and its high discharge velocity the “projectiles” often come out more as lines or “ropes” of fluid

So “rope blasting” is one of the many different ways of saying “ejaculating semen”

The characters are not fond of imagining a penis ejaculating semen everywhere every time the name of the gun comes up, so they react to it noticeably.

I think the “joke” of this meme is mostly just like “guess what the gun is called.”

I definitely needed the description to remember that even was a phrase, so this joke really only lands if you’re familiar with it being called that.

Hope this helps lol