Will it work? I have a chance to pick up a Colt upper and will need a lower for it. I would like to mate it with an M&P or possibly RRA lower. A friend told me Colt uppers will only go on Colt lowers? He said something about the front pin being a different size. Is this true?

Older Colt sporters had a nut and screw type pin instead of a mil-spec push pin to hold the upper and lower together. A few years back Colt abandoned the screw pin and used the standard mil-spec from then on.

Depending on vintage, the Colt upper can be full mil-spec and drop right on any mil spec lower. If it is an older civilian upper, it may require and off set adapter pin that is readily available from any of the parts outfits.

Websites like AR15.com or m4carbine.net have sections explaining the differences on Colt made uppers and how to make the older ones fit mil-spec lowers.

Here is a pic of the Colt upper. The front pin mount appears to be pretty standard to me, but I'm new to these things............

smith mp15 receivers can be had for 90 to 120$ you can get a aero receiver for 70$ a lower parts kit for 60 $ to 80$ and your ready to go