I bought a S&W M&P 15 in 5.56 and wanted to ask about what other uppers would fit on this gun? The reason for asking is that the 5.56 ammo is getting hard to find and is pricier than .223. If I would buy a less popular caliber that costs even a little more I will be able to find ammo more readily available. I already have a M&P 15-22 so I don't need a .22. What is out there that won't make the ammo too much but more available?