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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.
The Vision Minor 22IDEX or Funmat HT would be good options, but they both cost well over five grand. How you tune your printer is more important then the machine itself. I use the Prusa i3 MK3 because they are reliable and easy to use.
We use Prusa i3 MK3 printers for most parts. I also use an Ender 3.
There are a lot of considerations here. Nozzle size and material are the biggest considerations. I have used tungsten carbide nozzles with apparent good results, it printed PLA+ as good as brass. However, I did have clogging issues when printing with carbon fiber Nylon, I think that the rough internal surface catches the fibers. I have better results with the E3D NozzleX, though they conduct heat more slowly and can’t print PLA+ quite as well as brass, but they still do fine. 0.4 MM nozzles are a good all around option, but a 0.5 MM nozzle will clog less with carbon fiber Nylon while still maintaining detail. My newer lower designs are optimized for using 0.5 MM nozzles, be sure to check the README to see if the design is compatible.
Short answer, no. Long answer, it won’t hurt until you need to replace or remove the ejector. On my AR-9 designs the ejector is 3D printed, it presses into a pocket on the lower. This may be concerning to some users, as it appears to only be a friction fit holding the part in. However, if you look closely you will see that the upper receiver holds down the edge of the ejector when the upper is installed, this locks the ejector in place so that it can not escape. I do not recommend glue.
As of now, 3D printing your own firearm is legal under federal law. However, a number of states do regulate or outright ban the home manufacture of firearms. You will have to check your state law.