When you're building an AR-15, don't worry about who makes the stripped lower. It makes zero difference. In fact, don't worry about who makes the trigger assembly. I've shot so many different ARs and it makes zero difference who makes the lower. The difference is made in the construction of the upper and there are several important considerations to make.
1) Make absolute sure that it is a 5.56mm upper and not simply a .223 upper. What's the difference? Isn't the 5.56 the same exact caliber as a .223? Yes, it is. However, anything stamped 5.56 is a much hotter round than a .223 and the latter can not handle the hotter load.
2) Rifle Twist: Do you understand ballistics and know what the difference in a 55 grain round shot out of a 1 in 7 twist versus the same grain bullet shot out of a 1 in 9 twist barrel is? No? Research it and learn the difference. Know several things: do you want to shoot long range or short range? What grain bullet would you like to shoot (see previous question)? How big of a powder load? Do you want to shoot cheap and go with MilSurp ammo? Who is the manufacturer?
3) Bolt Carrier Group: Do you know the metal composition of the BCG? What holds up better against salt, or repeated quick firings? What is the difference between a full auto BCG or a non full auto BCG (contrary to popular belief, both are fully legal to own without ATF permission. The only full auto parts that you can not own without ATF permission are the trigger group parts), how much do you like to take apart tiny parts and meticulously clean them?
4) The top of your upper receiver: Flat top or A2? Want to mount your own flip up sights, or perhaps something different, like a Trijicon ACOG or perhaps night vision? Maybe you simply like iron sights and want to keep those (my favorite).
5) Accessories: The AR-15 platform is the most accessorizable firearms platform ever designed. Period. You can build a weapon that belongs in a Schwartzenegger flick or something that could belong in Steel Magnolias. Either way, they are all the same and all of them can be built on.
6) Barrel Length: Carbine length? Full Length? What would you like to do with it? Shoot deer? That's actually legal to do in some states. Shoot long range (in excess of 500 yards)? Go with a 24" barrel. Remember that accessories are different for the various barrel lengths.
The most important thing to do is to check your state's laws regarding firearms. This is the primary reason that I will never live in either California, New York, Massachusettes, or Illinois. I like gun friendly states. I do a lot of research to make sure that I will NEVER run afoul of either federal or state laws. I know them and can recite them. Know it front and backwards, even laws concerning ammunition. It is all relevant. Did you mistakenly turn your rifle into a full auto rifle because your friend had one that he did at home and you thought it was cool? Guess what? That's 10 years minimum in FEDERAL prison. I can not stress it enough. Know the firearms laws.
While I'm at it, know firearms safety first and foremost. In contrary to my previous statement, THAT is the most important thing to do.
Sorry for the rather long discourse.
Um Um Welcome to the forum?:dunno: