I've gone from having 1 old shotgun to now having 5 handguns and the SW MP15 Sport II in a matter of a year and a half.
That's nowhere near having an "arsenal". Breathe dude. Breathe.
Is there any reason not to just buy 4 or 5 (or more) of these MP15's just....because??!!
If that's what you want to do, then go for it but give it some thought. Get over the excitement of owning your first AR-15 before going overboard. I think you should build your next AR-15 from a stripped receiver and a stripped upper. There is no better way to truly lean about an AR-15 than building one from parts. You get to research and deliberate over every component in your AR-15. What trigger to use? What lower parts kit? What fore end? What barrel, twist rate, material, rifling type? Do I use an exotic coated BCG, full auto, semi-auto? Do I use an H1, H2, or Hydraulic buffer? Build your own AR-15 twice and either one of two things will happen. You'll either lose the desire to own more AR-15's or you'll build yourself a room full.
Variety is the spice of life man. IMO, don't buy another AR-15 until you've tried something different. Go get yourself a good, made in the U.S.A., AK-47 pattern rifle.
The general trend is to acquire numerous firearms, burn out, then decide you'd rather have a really really nice X instead of the three Y's. Then you start to shift toward quality over quantity.
Bingo. It's the predictable pattern of firearms enthusiasm. I'm no exception. Last year, I had more handguns and long guns than I could shoot. Some guns were only shot once a year. Some were just in the safe so I could say I owned one. Some were just there so I could say I owned Pistol X chambered in Caliber Y. I realized it was pointless to get mired in the constant acquisition phase (buy/sell/trade). I purged out inexpensive and expensive firearms. The overarching requirement for what I kept was that I truly have to enjoy shooting the firearm. The specific requirements:
Handguns:
1. Conceal carry tools: winter carry / summer & deep concealment carry.
2. A single affordable pistol that is stowed in my get home bag.
3. One fully decked out home defense handgun.
4. One range only pistol that is there solely for the pure enjoyment of shooting.
5. One pistol chambered for .22lr.
Long Guns:
1. A single .223/5.56 military pattern rifle that is compatible with AR-15 magazines.
2. A single AK-M in 7.62 x 39.
3. A single rifle chambered for .22lr.
5. A single pistol caliber carbine.
That's it. I debate and ask opinions on possible firearms purchases in the general forum. I have yet to buy anything I'm deliberating, because I already have exactly what I need. Getting out of the constant acquisition phase is like going through nicotine withdrawals. The urge for tobacco is inversely proportionate to the day I quit smoking. That first year going cold turkey was hell. It got better in year two. Even though it's been years since I've smoked, I still have the occasional desire for nicotine and a smoke but I can ignore it.