AR-15 question

[Ridge Runner]

If they are mill spec, and the same size pins, there were at least 2 differnt sizes made but can be used with an offset pin kit, they should all interchange except a few of the colts which mere made not to fit a standard lower.
The only company who comepletely manufactures they're AR's is Olympic Arms, all the rest buy parts and put them togather. Just decide how good of barrel you want and look into whats available.
J&T will sell you a complete upper, less barrel then you can get a custom barrel in say a 204 ruger, stick barrel on and you have a LR, HV varmint rig.
The possibilities are endless, anything that will fit down the magwell can be had
I have a 458 SOCOM, and a 25 WSSM chambered in the AR's, they are awesome hunting rigs.
RR