The second amendment is an interesting conversation for both sides of the isle and even those in the middle often have questions as to why someone needs to own certain types of firearms or munitions. As often with these types of questions, personally, I apply the ideology of the impact of a law-abiding citizen participation, through the lens of individual freedom.
In this case, if a law abiding citizen, expressing individual freedom, should own, let's say an AR15, what would be the risk to general society. The answer, in this case, would be little to no risk as they are law abiding, and does not justify government interference in an individuals freedom.
Government restraint is difficult at best, yet the job of a legislator is not to protect government from people, it is to protect people from government. As an individual I most often agree with Thomas Jefferson when considering government and his below quote likely does more justice to my position than I can.
“A wise and frugal Government, which shall restrain men from injuring one another, shall leave them otherwise free to regulate their own pursuits of industry and improvement, and shall not take from the mouth of labor the bread it has earned…is the sum of good government.”