Keeping this in consideration, are AR 15 lowers universal?
The AR-15 has an upper and lower receiver that is made from some form of aluminum. That being said, you will find that AR upper and lower receivers will vary in exact fit between different manufacturers. That doesn't mean that they won't fit. Only that you will run into some that are easier to put together than others.
One may also ask, can any AR 15 upper match with any AR 15 lower? In general, yes. In specific, not always. There are a number of lowers made by Colt that used a smaller pivot and takedown pin and those will only mate properly with an upper using the same pin size/spacing.
Thereof, are AR lowers caliber specific?
223/5.56, but it doesn't mean it's the only caliber available. In most cases, the lower receiver doesn't change and by simply mounting a different upper you can change caliber, barrel, and optics. The options are only limited by your imagination.
Are all AR 15s the same?
Today, nearly every major firearm manufacturer produces its own generic AR-15 style rifle. As Colt continues to own and use the AR-15 trademark for its line of AR-15 variants, other manufacturers must use their own model numbers and names to market their AR-15 style rifles for commercial sale.
