I mistakenly bought an Aero Precision M5E1 upper/enhanced handguard set thinking the M5 was just an updated or upgraded M4. Oops!

Rather than send it back, I've decided to do a .308 build using the M5 upper. Until I realized I had a .308 upper, the only thing I knew about AR .308s is that parts interchangeablity is lacking. Now I need to start learning the particulars.

I own one .308 rifle that uses FAL metric magazines. It would be nice if my new lower did the same, but I'm certainly not going to make that a deal breaker in choosing a lower. RRA lowers use FAL mags but I don't know if an RRA lower will work with my M5 upper. Waiting to hear from RRA on that.

I think I'll go with a 20" barrel for ringing steel at distance. It's to be a range gun, a plinker.

From reading, it appears that DPMS pattern seems to be the way to go for more choices, parts-wise. Does this "DPMS" standard apply to barrels and such as well as lower parts? Heck, is my M5 upper even DPMS compatible?

One thing I am wondering is: Who makes an ambidextrous magazine release for the .308 lowers? I shoot lefty and appreciate a mag release I can actuate with my left forefinger. All my builds get an ambidextrous mag release.

I've built 5.56 and 300 BLK weapons before but never anything using the larger parts. All helpful advice appreciated.

Here are a couple of threads that will help you a lot.  
The Aero M5E1 will make an excellent .308 AR, but lack of standardization across the platform means you need to to some homework before buying parts.  

https://www.ar15.com/forums/t_3_121/413443_-308-AR-FAQ---Your-help-and-knowledge-is-requested-.html

https://www.ar15.com/forums/t_3_121/662611_Aero-Precision--308-M5-Everything.html


ETA - My advice, stay away from RRA anything for this build.  Most of their stuff only works well with their stuff.

Save the headaches... stick to Aero for the lower, BCG, and LPK. Mixing various mfg. parts can be a source of aggravation.