This. The lower will go to an FFL, the upper direct to you, when you get home you pin them together and voila, a working rifle without paying the "hunting tax" surcharge to Uncle Sam.
Ruger uses proprietery parts which they will not sell on the open market, they install in their repair shop only, last I heard. Smith less so. A PSA is all military open source parts available from dozens of sources. It gives you the best option for long term use and any upgrades you might discover you suddenly need as "black rifle disease" creeps over you. ; )
As for ammo, a properly set up 16" rifle made to milspec will prefer NATO ammo, if you shoot white box or import steel case the powder charge is notably less and some folks have experienced malfunctions. Just the same as running cheap ethanol in a car meant to have premium - I have - it's underpowered and cranky. Most self defense and combat pistol makers recommend higher powered ammo, not cheap white box fodder. If it's important, nobody loads up junk for self protection, they carry good stuff. Furthermore, the military doesn't have dozens of loads for different purposes - they concentrate on one or two, those have to work in 20", 14.5" and 10.5" guns all issue. And no, it's not low contract bidder, they own the machines at Lake City and demand it adhere to specification. The idea that "it should run anything I jam in it" is not allowed or permitted when a soldier's life is on the line.
Sorry, its the internet, you ask a question and you get a third answer plus more information than expected.
