Off the top of my head, there's a takings clause argument under the fifth amendment, which is probably why grandfathering is so popular - the courts have sidestepped this issue.
Quickly Googling it, we find:
Thanks, LCAV! While we can assume this to be an utterly biased piece, which would not show any case law favoring 2A advocates, the rejection of a 5th Amendment prohibition against seizing arms en masse is pretty much sunk post-Heller
. The most recent (and arguably most significant) case challenging California AWB laws cited in this appendix is predicated on a finding that the Second Amendment does not confer an individual right. Now that we've finally acknowledged that it does, the given rationale fails.
In any case - the 5th Amendment's been incorporated, so in combination with the individual right conferred by the 2d, I think we can buy in reliance that, even if a ban occurs, large-scale, federal confiscation would be blocked by the Bill of Rights. I'm not suggesting the legal challenges to California law will be successful, I'm just saying that, on a federal level, any future outright ban giving citizens no means to place their property legally anywhere in the country or sell it to recoup their investment would go beyond the reasonable regulation left available by Heller
and rise to the level of an unconstitutional taking.
Further, by buying now, you're improving the odds that a view favoring ARs will be adopted. You're supporting the companies that create the jobs and do the lobbying. You're counting yourself among the voters who will fight for this right. You're increasing the total of dollars that would be stolen in the event of a ban or confiscation. Etcetera. And if we take it far enough, such that tons of AR/AK/etc. receivers are out there already, it takes the wind out of banning sails, because the sheer volume already in existence would suggest that the total elimination of these devices is impossible.