First off. . .Thank you to the folks that helped turn me on to working with this awesome firearm, the AR-15 that brings with it endless fun, excitement and fulfillment to my world and existing love of this sport, hobby and way of life! You know who you are. TOPIC: I worked my way into having a spare lower with no plans for it. It's a cheapy, but it works. Most of the parts were pieced together and I'm only into it for ~$115 +/- $5, which beat my goal of $140, as I settled for a 4 position buffer tube, which is fine for me as I always have the stock fully extended anyhow. This will obviously be a working type gun, which to me is a Mountain/truck/woods/loaner, that runs always. I have 3 options. (maybe more. . . ) But 3 main (obvious, to me) choices. .22 LR is not an option as I already have a rimfire AR available. 1. 7.62 x 39mm. -I'm stocked with ammo of ALL types. - This is an excellent round inside 200 yards and a great pair up for the .223. It basically has very many of the qualities (mostly penetration) that the .223 doesn't and vise-versa. -Since this is going to be a Mountain gun the 7.62 makes a lot of sense as it is better suited than the .223 for Bear Defense. - magazines could be an issue? 2. 5.45 x 39mm. - Stocked with some ammo and it is EVERYWHERE ! ! ! - Incredibly cheap to shoot, usually half the cost of 5.56 (.223) ! ! ! -I could set the upper up to train with for the use of another AR - fairly accurate - still makes a decent mountain gun 3. .223 (5.56 x 45mm) - have plenty of components for spare parts - have ammo, mags, etc. - familiar with maintaining, repair, etc. OTHER: -Possibly an economical (Adams Arms) piston upper. -Another (at least semi-) bear suitable dedicated upper -I'm all ears!!!