Looking at building an AR-15 and I noticed that there is a few caliber sizes 5.56, 6.5 and 7.62 maybe even more, thats all they have to offer where i am getting it from. I know that part of it is what the end user is comfortable shooting. What ammo would be easiest to get. Is there plus and minuses to the different calibers. I am searching the net also but wanted to get some opinions from here also.