Setting aside the debate over gun control (too heavy a topic for me and my allergy ridden head) they are fighting a dumb battle.
For one thing the bill seems to be increasingly minor, mostly background checks and similar items, not any real gun control.
The bill is no better than an even bet to make it out of the Senate with a simple majority and even if it does the House is even less likely.
If it does pass it will be challenged in the courts and given recent rulings might well be tossed, or at minimum will be on hold for years.
So the pro gun side has pretty much already won the battle but by blocking the vote the GOP loses the war by looking insensitive and stubborn.
Doing things the damn silly way again.