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This depends on what Super Lower you want to print. The single rib Super Lower will work with almost any AR-15 upper receiver. However, the double rib lower must be used with slick side uppers that do not have a forward assist. Mil-Spec uppers will work with the single rib lower, as will Aero Precision M4 uppers. The Bear Creek Arsenal side charging uppers will work with the double rib lower. As long as the upper is compatible with a standard “Mil-Spec” AR-15 lowers, and you consider the above mentioned caveats related to the forward assist, then you should be good. Any AR-15 caliber should work, including 5.56 x 45, 22 LR, 5.56 x 35, and 7.62 x 39, if the right mags are used.
Here are a few options for uppers.
I really like these Aero Precision uppers: https://www.aeroprecisionusa.com/m4e1-threaded-16in-556-mid-length-complete-upper-atlas-rone However, they are a bit more expensive because you have to buy a separate Bolt Carrier Group (BCG): https://www.aeroprecisionusa.com/5-56-bolt-carrier-group-complete-nickel-boron
BCA makes really cheap uppers that will work, but quality can be (is) lacking: https://www.bearcreekarsenal.com/guns-parts/ar-15/ar-15-complete-upper-assembly/556-16-parkerized-m4-barrel-1-9-twist-carbine-15-handguard-scg2-nrail-spiral-flash.html
And there is always PSA, they sell quality parts for a lower cost: https://palmettostatearmory.com/psa-16-mid-length-5-56-nato-1-7-nitride-13-5-lightweight-m-lok-upper-with-nickel-boron-bcg-ch-5165449606.html
Remember to always check to see if the upper come with a BCG, not all do.
And if you want to assemble your own upper from parts, KAK is a great company.
LR-308 parts are very confusing. There are a number of standards, and many of the parts are not interchangeable. I would only use parts that are specifically “LR-308” or “DPMS” compatible. Palmetto State Armory PA-10 kit’s are a notable example of a kit that is not compatible, the bolt hold open lever is the wrong size to work with my LR-308 lowers. I would recommend using lower parts from KAK, or just pick up one of my 308 Combo Kits.
LR-308 uppers are not as simple as AR-15 uppers. There are a number of standards. My LR-308 Super Lowers are designed to use the LR-308 / DPMS standard. Unfortunately, not all companies properly name their uppers, so care is needed. Bear Creek Arsenal, Aero Precising, and KAK all make LR-308 uppers that will work with my lowers. Bear Creek incorrectly calls them AR-10, and Aero calls theirs M5. True AR-10 uppers will not work. Below is a more in depth video.
Yes. 300 AAC Blackout does not place additional stress on the lower over 5.56 x 45. In fact, your gas system and buffer mass are much larger factors in how much stress is placed on the lower receiver. A heavy buffer with a lot of gas on a 5.56 gun will place more stress on the lower then a light buffer properly gassed on a 300 Blackout gun.