Is An AR15 M4 Considered A Carbine?
I'm new to the AR15 and am looking into building one. My question is regarding the upper receiver (an M4) and the correct stock/buffer tube to purchase for it. I'm assuming I should get a carbine stock and buffer tube as opposed to a rifle stock and buffer tube, correct?
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The upper receiver is the same on the carbine and rifle versions of the AR15. The rifles generally have a fixed stock and 20" barrel, carbines have adjustable stocks and 16" barrels. So yes, if your building an M4 version, you'll want to use a carbine stock and buffer, and use a 16" barrel. 0 -
The upper reciever itself is the same, but the barrels are different length. The Carbine is 16, and the rifle is 20 inches. As such the hand gaurds, and gas systems are different lengths, and the extension tube, spring and buffer are different for each.
So you need to get everything rifle, or everything Carbine.
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The length of the barrel has NOTHING to do with the lower. The buffer and spring are different for an A2(solid) vs. a carbine(collapsible) but both will work with any common upper.
I have uppers in 16, 18, 20, and 24" and any/all will work on either adjustable or solid stocked lowers.
The gas tube and handguard are the only version specific parts on an upper.0 -
You can put any length stock & buffer tube(they must be the same length) on any lower receiver. Barrel length does not matter.
Want a 16" barrel with a rifle stock? OK!
Want a 24" barrel with a collapsible stock? OK!
Handguard length is more critical. Handguard length, barrel length, and gas tube length must be right. Otherwise, the gas block or frong sight housing can interfere with the handguard. Gas tube must be the right length to mate to the gas block or front sight.0 -
Answering in the simplest way possible, the M4 is a carbine, and uses a collapsible type carbine stock. If you want to build something like an M4 then yes, you'll want the carbine stock/buffer system.
Speaking more in general, as mentioned above, you can effectively put any correctly assembled upper onto any correctly assembled lower. (Whether or not you "want" to do that, being a different question).
In practice this means that if you wanted to you, could put a carbine upper onto a rifle lower or vice versa. The only thing is, the lower does have to be built correctly, which means the buffer, tube and spring have to be matched to the stock in question; not every tube will work with every stock, etc.
You didn't ask about building uppers, but if you're buying a pre-built upper, you don't have to worry about matching parts there; presumably whomever built the upper in question knows what they're doing and built it correctly.
If you're assembling your own upper FROM PARTS, then you have to match the gas system to the barrel and also make sure the handguards fit right.
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You didn't ask, but collapsible stock is useful if you need to reduce overall gun length for storage. Its useful if you may need to alter length of pull. EG, you need the same gun to be used by shooters of different heights, or perhaps the same shooter with and without body armor on. It can be useful for handling a gun in tight quarters (eg inside a vehicle). Its also a little bit lighter.
This is subjective, obviously, but personally, I like a fixed stock. Its stronger, and a bit faster to get in action.
IMO the standard fixed stock is a little more comfortable to shoot than the standard collapsible one, though there are all sorts of collapsible stock variants/upgrades that are pretty slick, too.0 -
Thank you all for your help, question answered. 
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