Mitch won’t let their silly bills anywhere near the floor of the Senate. The average person, those who aren’t as politically involved as many here, see the headlines: “Congress passes Bill XYZ doing this or that”. It dies in the Senate. Joe Average never hears/understands that part and assumes another piece of useless legislation passed that changed nothing. How many times did idiot House Republicans ceremoniously repeal Obamacare knowing full well it wouldn’t go anywhere in the Senate or that Obama would veto it? What happened when they did gain power? Let Dems try AWB2.0, particularly something along the lines of banning all semi auto rifles like the WA State bill, Republicans will win back majorities in both houses plus the White House.
I think you overestimate the importance of guns to most Americans. They don't see the issue as you do. How people feel about guns can pretty much be broken down into 5 groups:
1. gun ban zealots. Only .gov should have guns.
2. almost a zealot. Only .gov should have semi/auto guns. Muskets and single shot rifles for the plebs.
3. Fudds. They don't care about gun laws as long as they can have their Remy 870 and dove/duck/deer hunt. They don't even know the difference between AR and an AK.
4. "I support the 2nd amendment" folks who are fine with most laws on the books (NFA, no machine guns, SBRs, etc.), for the most part. But don't want another AWB, AR or mag bans, etc.
5. The folks who feel all weapons laws are unconstitutional and the common man should be able to have nukes.
Most of America falls into #3. The "extremists" on either side tend to be in #2 and #4. That leaves #1 and #5 as the fringe and very small minority.
That being the case, gun control isn't an issue that could flip the House, Senate and Presidency all to Rs. It's just not. It may help win some regional house races and some state senate seats, like in TX. Those races are important too. But nationally, it doesn't motivate the R base nearly as much as I think you believe it does. You're probably in #4.... but most of America isn't there with you.
If it were that potent an issue, the Ds wouldn't be trying for it knowing it would keep them out of power. Hell if it were that potent an issue I have no doubt you would see granny Warren running around with an AR and ammo belts strapped across her chest like Rambo.
At the end of the day, they're all about power. They chase it, seek to acquire it, and ultimately impose it. If guns carried that much water with the electorate they wouldn't be advocating for more "control".