My reading of the 2nd amendment is that it prevents the Federal government from making ANY restriction on civilian firearm ownership. What it does allow is for a state to decide whether it considers its civilians to be members of the state militia (not reserves) and as such arm itself as a last line defense force, and what weapons they should purchase for themselves (regulated militia).

The recent history during the 17th century had countries requiring able bodied civilians train with the weapon of the day in case of need in the defense of the realm - example the UK required regular practice with the longbow, a law which was repealed in the late 20th century! So it's not surprising that the US founders were not in support of creating a regular army (which could be used for offense) but should allow the states to call up civilians as an irregular defense force - and BYO gun.

Is the 2nd amendment an anachronism? Quite probably. Is it a total mess today? Quite probably, since we have legal challenges to state restriction on firearm ownership (something the 2nd allows) and we have the Federal government setting restrictions (something the 2nd is supposed to prevent).

Bottom line (in my view), does someone NEED an assault rifle? No, in the same way someone doesn't NEED a car that will drive over 180 mph. But both allow someone to do something considered illegal, IF the purchaser chooses to. (I'm not trying to equate murder to speeding). Heck even buying a handgun allows someone to be able to commit murder, should they so choose.

Personally I agree with you that I don't see a need for anyone to own an assault weapon. But I also don't feel a need to say what someone else is and isn't allowed to do, so long as anothers rights are not infringed. But the proposed laws as they stand do not eliminate the problem, if you want to ban something you need to then remove it from the field of play, if you don't the threat is still out there.
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I used to think it was terrible that life was unfair. Then I thought what if life were fair and all of the terrible things that happen came because we really deserved them? Now I take comfort in the general unfairness and hostility of the universe.