A mil spec lower is just that.you can buy milspec parts and they will fit. CMMG is mil spec. Now the extention can be mil spec or commercial for the collapsing stock and the plastic buttstock dia. Again the milspec is a stronger design by the way the threads are done where it screws into the lower reciever . Magpul has a discription of both
http://spreadsheets.google.com/pub?key=p...amp;output=html BCM products are the best you can get. I have ben through 6 ARS since my first one in 1976 3 Colts operated perfectly, 1 oly build POS I sold it quick and 2 builds with BCM uppers Awesome like the old Colts. Milspec is what you want I have a build on a CMMG lower it works !!!
Most rifles are overcleaned. The AR if quality parts are used in the MFG. parts aren't parts, should go more rounds than you normally would like to go if kept well lubricated.
I found that I could neck size even with an auto "223 and 30-06" I don't run max loads. That saves on trimming. I would not cut corners on self defense loads though.
If you want an AR you can depend on with your life look at Bravo Company. Get quality and make sure the barrel is 5.56 NATO instead of 223.This link may help with the AR choice http://spreadsheets.google.com/pub?key=p...amp;output=html
IMO the heavy frame is nicer for shooting magnums. I have been able to shoot a Python enough to say how nice they are to shoot but I have only used wad cutters.
Actually the whole truth would be yes I bought the 40 XD tactical ,but my Son and I went out to shoot it and well basically my son has an XD40, same thing happened with the XD compact 3 years ago!!!.It is really OK though.