Gun Control & RKBA

Trying to pass UBCs will be an uphill battle all the way. Right now a few states have them. Allowing general and free access through the local police or sheriff is something that could happen faster. Give each department some federal dollars to setup the system and an allowance for staffing. UBCs may be workable but at the state level. I believe giving the actual good guys with guns a chance to do something good will do a great deal to give both sides some common ground.

It wouldn't surprise me in the least to find out that there were billions of 20+ round mags for various semi-auto rifles in the US. The genie is truly out of the bottle.

If you're referring to the guns used by mass shooters, I think just about every type of gun has been used.
Without have a government run registry, this won't happen. There are maybe 80,000,000 gun owners, more people than those who voted for 45 in 2016. My guess is at least half of those 80,000,000 won't have anything to do with registration. I believe the ACLU is also anti-registration.

Red flag laws have pluses and minuses but I'd consider listening as long as court due process and appeal was part of the package.
The no fly list is problematic because the government won't tell who's on it. It took Ted Kennedy a bit effort, phone calls and flexing to get off of it.

Politicians won't like it but all of these measures should stand alone in their own legislation and not be wrapped in a huge package with other measures related or not.

Other countries are a mixed bag and mostly dissimilar to the US.