I can see two areas of concern that are hardly mutually exclusive:
The security of the liberty of the people and their republic.
Individual freedoms.
If we look at things like an "AWB" or "semi-auto" ban, they have a severe impact on individual liberties, but also drastically effect the people's capabilities like continuity of fire. I realize that talking about the people's militia is mostly theoretical or on principle under present circumstances. I don't want to argue irrationally, but clearly, the purpose of the 2nd Amendment isn't limited to just "self-defense." A ban in these areas would have a consequence that would be immediately crippling to the people's capabilities.
Magazine Limits. It seems like this one primarily limits self-defenders, and interferes with individuals. As long as magazines remain detachable (they might not be under an AWB), the continuity of fire of a militia wouldn't be terribly diminished and under the kind of circumstances where the militia may be necessary, arbitrary magazine limits seem like they would be easy to circumvent. These are the same reasons why it can hardly be effective in thwarting the unlawful. So, again, this one seems mostly to interfere with would-be lawful individuals. This one is also the easiest to reverse, but capitulation would make a future reversal harder.
Red Flag Laws. This has been discussed in other threads. It seems like it could be very onerous on an individual basis, but not immediately a threat to the militia. However, this kind of infringement could easily grow in a way such that it became Green Flag law where people would be red-flagged by default except upon possessing the favor to be green-flagged.
UBC's - obviously a defacto national registry. This one avoids the most onerous interference with the militia and individuals at least immediately, but it sets up the opportunity for mass confiscation. In the short-term, it feels the most palatable (and voting seems to indicate this). But in the longer term, it may be more damaging than magazine limits or red flag laws.
Needless to say, none of these proposals will stop suicidal maniacs from mass and spree killings. Red Flag laws are the only of the listed options that even have a chance of thwarting some, but are more likely to be abused. Look, the Parkland shooter gave an abundance of warnings and they were all ignored. Having a Red Flag law won't stop anything if people continue to marginalize and ignore the people that need help, and there is no evidence those laws will change anyone's apathy. The Vegas shooter, to my knowledge, gave no warnings that were recognized or could have been expected to be.