So I have an M&P 15. Can I buy an after market upper and put it on the gun?
I don’t really plan too, but I could see wanting a shorter barrel and a free floating barrel in the future. Really just curious how it works.
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If it was bought as a rifle/carbine it must stay in that configuration as long as the overall length stays over 26 inches and barrel length is over 16 inches you should be fine.
Same goes for pistol length AR’s if purchased as a pistol it must stay a pistol.
Home builds it will all depend on how you filled out the paperwork when buying the lower.
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Dang, thank you. I never realized this. I’ve heard it mentioned but never really thought about it. I know a shorter barrel is supposed to be better for moving through structures, but I didn’t even think about ATF 
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ATF definitions make 0 sense.
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Yeah it’s frustrating to say the least. You can run a 14.5 and pin and weld a longer flash hider or whatever you wanna run and it will count. But either way it’s still gonna be 16 inches.
In my opinion it’d only be worth it if you’re looking at running a can. That’s where you can have real gains or losses depending what you want the rifle to do
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I’ll probably leave it and just work on the furniture for now. Might look into a free floating barrel in the future, I think a new upper would make that upgrade a lot easier. I’m pretty happy with my M&P out of the box, it’s just fun to play around with upgrade ideas.
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Someone please call me on it if I’m wrong but I believe you can’t have a true free float barrel on an AR because of the gas block. Maybe piston AR’s are different
You can however do a free float handguard which would help accuracy, however using a factory barrel and standard ammo I’m not sure your accuracy increases would be worth dollar output
Not trying to burst your bubble just saving you from pains I’ve endured. Your rifle your choices
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I believe you are correct. I guess I meant free float hand guard. I’m very limited because the gas block is connected to the front sight post for me. I’ve seen videos of people sawing it off, ( sight post not gas block). but I don’t want to do that. I’m happy with the stock upper (though I added a magpull hand guard). Just wondering what I could get out of a new upper I guess.
That being said, the money would probably be better spend on a trigger assembly or ammo. But it’s fun to play with ideas.
I wonder if I could potentially get is a lighter upper 
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Your handguard can absolutely help reduce weight and give you real-estate for a flashlight or hand stop/vertical grip whatever your flavor is.
Bang for your buck a trigger would be where you’d see the best results in accuracy
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I was wondering something similar. Are stripped down AR-15 lowers interchangeable? Especially if you finish an 80% lower are there limits on which upper you use?
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Nope, you are right about the barrel. Also right about the handguard. @Scoutbob, @Sheepdog556 has you good to go brother. I have the M&P 15 and absolutely love the thing. Enjoy, it is a solid carbine.
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Technically once you put a pistol upper or rifle/carbine upper on it, It’s supposed to stay that way. So as long as your not carrying around a rifle and pistol upper just to swap them out it would be very hard to prove what you did first to it. Something to think about when taking/posting pictures of firearms.
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Beyond the legal implications, remember that the upper and the lower function as part of a system. If everything, old and new, adhere to standard NATO specs you should be fine in theory. However, if there are OEM proprietary components you could run into issues, again in theory. The most obvious would be buffer tube mass &/or spring rate impact to timing. The effects could be as simple as a change to the trajectory of the discharged brass, or something less benign like a failure to reliably cycle.
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The answer to your question is YES.
As a demonstration I have a friend that has a registered SBR lower and a literal rifle case filled with uppers to attach to it. 7.5", 10", 14.5", 16" 18" 20", 22" and even a 26" tube. His lower is full monty tricked out with the Gisellie (sp) trigger, retained pins and every high function widget you can think of. The uppers are a spread of $200 to $1200 and in various flavors of cartridge and all are run off the same lower.
For us non tax stamp folks you would want to make sure that if you have a pistol the barrels keep you under the legal OAL limits and for rifles you stay over 16" and never let the twain meet. Same for stocks and braces, never get them confuzzled.
If you get into different cartridges I have found it handy to buy a dedicated gun case per cartridge and cut the foam to fit at least the upper and the mags that are specific to it. Similarly I have a “pistol” case just for pistol length uppers just so that there is no question as to what goes with what both for me and any observing LEO. Could I be in “Constructive Possession” if I have a rifle case and my pistol case? Possibly, but I think that by spending the $$$$ to ensure the separation and segregation of the two platforms a “reasonable person” would determine that I had no ill intent or intention to swap the opposing parts and took pains to ensure it didn’t happen.
Cheers,
Craig6
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@Scoutbob The AR-15 platform does not offer unlimited universality. Disregarding legalities, takedown pins can be a different diameter. An upper gas system needs to be tuned with the buffer assembly i.e. carbine length gas tube needs a carbine buffer tube, spring and buffer. Some uppers will not even match the contours of your lower. Been swapping bits and pieces for years and not all are created equal. An area I have had issues with are the bolt catches and magazine releases. Food for thought.
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This would be a very unpleasant surprise 
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I see a lot of answers about legalities. I thought the question was about mechanics.
To that end, the best answer I have is… usually. Seems like most AR-15 platform manufacturers use similar specs for their base models built for the same ammo, but there are definitely exceptions. I wouldn’t drop cash on a random upper without doing a little homework or validating that they fit and work together, but if I found a really good deal at a trade show I might be tempted to take the chance.
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Same goes for pistol length AR’s if purchased as a pistol it must stay a pistol.
So if I put an AR pistol and did not register it, why would it matter if I took off the 7.5" upper and replaced it with a 16" upper? I didn’t think it mattered.
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That’s the grey area in the law. So if you have a pistol and just swap the uppers you would be legal, SO long as you DO NOT have a stock you could swap as well. That shows the potential to have a SBR. I know this sounds ridiculous but its the rules. The safe road is once you make it a pistol or rifle leave it that way.
That being said what happens in your home is your business just don’t go out with a bunch of “parts” that can be swapped out to make an SBR and there wouldn’t be an issue. But with parts to potentially assemble an SBR a motivated LEO could make trouble.
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