Hello! I’m new to these builds. Can you drop the 11.5 in completed upper onto any lower? Will it fit? Will I need to do anything else to it? Also, Will it still eject shells the right way or will I have to adjust that too? Thank you!
Welcome, Bronco. It will fit, and almost certainly function, but you need a lower with a pistol brace rather than a rifle stock or it will be considered a short barrel rifle which is regulated by the National Firearms Act. It is also concerning to some of us that the ATF (or AFT) has new guidance which may judge it to be an SBR anyway (that issue is discussed and linked elsewhere on these forums).
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A lower in pistol configuration is correct. You may need a heavier buffer (an h1 or H2).
Bronco, will it fit….yes it should……but keep yourself on the safe side (legal) of modification and use of your modern sporting guns.
An 11.5 inch is considered a pistol length barrel and you good to go mounting it on a designated pistol lower.
It will fit and operate on any AR-15 lower….rifle or pistol. We are not the Forum Police here to tell you that you should not put a pistol length upper on a designated Rifle lower receiver.
If your new to AR-15 rifle & Pistols……and you want to add a 11 inch upper…….YOU have an obligation to know the things to keep yourself out of legal trouble.
I don’t know of anyone who has ever been in this sort of trouble……but with firearms and the ATF….the stakes are pretty high……immediately. So know where the line is….don’t cross it….but if you do cross it……cross it correctly.
Many forum members here can give you first hand advice to do it correctly.
BTW……Welcome 
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