Thanks.
Who makes quality lowers & uppers?
What barrel length is most preferred?
What about 300 Whisper/Blackout interchange?
I have done a 300 blackout build, as for interchangeability, you pretty much can just swap uppers back and forth as you wish.
For lower receivers, it is personal preference. Billet is nice, but not necessary. I have a few billet lowers and uppers. I can't say they are really worth the extra price though.
I have a few Anderson lowers, for the price they are great quality. There is also a company in Washington SSA, they have nice lowers, and also sell Anderson, Aero, etc. Their branded lowers tend to have a tight magwel, but it isn't horrible. Pricing from them is great. They had a deal on an assembled Anderson for a while, came ready to install an upper on. Think it was about $150 + shipping and transfer fee. Not sure what they have in stock now, Surplusammo.com
For uppers there are many companies. All depends on what you want. Only company I can say to stay away from is Palmetto State Armory. Bought 2 kits from them on an easter sale for a buddy. One has a completely loose barrel nut. Was supposedly test fired even. They refused to just swap it and had to send it back and wait about 3 weeks. They both are super tight, and keymod mounts don't line up correctly.
For assembled uppers, CBC industries has been great. Feed ramps in the receiver match perfect, are assembled CORRECTLY, and any issues they were quick to resolve. Had one that they sent with the wrong twist rate barrel, they had it back to me in about 5 days total from the day it went out to the day it came back. They included a free mag to compensate. They have a super light handguard from Hera, which is super light and has been top notch quality. It does get hot fast though.
If you want to build an upper on the cheap and don't mind having to clean up feed ramps, and waiting a loonnggg time for shipping, Delta Team Tactical isn't bad. Great pricing, just as I said, their upper kits need some tinkering to be perfect. Mainly just matching and polishing the feed ramps. They are about the cheapest around, if you like to tinker, and don't want to shell out a wad of cash for a good plinker.
If you want a kit where you can have several different uppers, BCA is pretty good too. They had a kit with 300, 5.56, and I think 7.62x39 all in one kit. Not sure if they still have it, it is a special that they emailed me about a while ago.
These are all the decent priced companies. There are a few that are midrange, but quality is about the same. Such as Apoc Armory. Then you get into higher end stuff and can spend $1000 on just an upper, like Bravo company.
I mostly build my uppers myself. Not hard, and I like knowing everything is assembled properly. Most assembled uppers I get are for buds builds. Of the companies I listed CBC seemed to be the standout for quality and service.
Uppers and parts do not require transfer/ffl, however lowers are the serialized part, aka the actual gun. Those need a transfer/paperwork to be done by a dealer.
Barrel length most common is 16". Shorter and it is an sbr and needs atf paperwork/stamp/fees. I have several 16", 18", and one 24".