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Posted: 11/4/2007 10:44:30 PM EDT
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Ok so before I go buying parts that don't fit together, I'd figure I'd ask some questions. Whats the differences between the A1, and A2? What makes it a M4, is it just the feed ramp in the upper and barrel? What makes it a carbine? And what are the major differences between the barrel types, lightweight, heavy, bull...? Any thing else I should know about the variations? |
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As far as uppers go: M16 - Slickside receiver, no forward assist or brass deflector. Rear windage adj. only  A1 - Rear windage adjustment only, tear drop forward assist, no brass deflector  A1/C7 - same as A1, but w/ brass deflector  A2 - Rear elevation and windage adjustable, brass deflector, round forward assist  don't have time for barrels right now. |
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You can do some reading on the differences between barrels here. Technically, a carbine is a shorter version of a rifle used by cavalry. So, basically, we have the M16 rifle, and the M4 carbine which is a shorter version but otherwise just about identical to the M16. There are minor differences, like feed ramps, grenade flats, and such, but a shorter M16 is the basic of the design. |
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IMHO, an M4 is has an M4 profile barrel (with the M203 notch), in 14.5" or 16", with a fixed FSB, on a flattop upper, and an A2 lower with a collapsible stock. Other parts can be swapped out, but I think that those features are what fundamentally creates an M4. Now, granted, the earliest M4's were made with A2 uppers, and I would not begrudge someone who has an M4 upper on their M16A1 lower from calling it an M4, I believe those features to be a good baseline. Now, for a while there, Colt was trying to claim that they owned sole rights to the name "M4," but lost that one, what makes a Colt M4 is a whole other ballgame, though most of the above still applies. The upper differences have been covered pretty well already, though you should know, if you go into the Retro forum, you'll find that there are more different kinds of uppers than you ever imagined. That said, you can dig into those if you feel the need to build a clone. Other than that, the only other thing you need to know about uppers that I didn't see mentioned is "big" and "small" hole uppers, which is a much more important difference functionally, than what makes an A1 or A2. The reason for this is that the first Colt AR15's sold to the public, known as SP1's had non standard front pushpins, slightly larger in diameter, and the upper receiver was cut to match. Now, SP1 or "Sporter" uppers are know as "big hole" or "large hole" uppers, and can be found in any of the various upper styles out there, even flattops. The easiest way to tell an SP1 lower, other than the rollmark, is to look at the right side of the magwell, which goes in to the various different kinds of lower available, a much larger selection very recently, thanks to NoDakSpud LLC, and their collection of "retro" lowers. On the SP1, there is no raised area around the magazine release button, and is called a "slabside." Now, however, NDS has released a "slabside" lower with standard sized pivot pin holes, known as the Model 601 lower receiver, based off of the original Colt AR15 model 601, the very first production variant, purchased by the Air Force primarily. The next type, available only (in production version) from NDS is the "partial fence" or "XM16E1" lower. It is similar in shape to the above lowers, it has only part of the raised area, the part that holds the spring and detent for the pivot pin, but not around the magazine release. Finally, the A1 lower has a "full fence," that you're most likely used to seeing, however, like the above three types, does not have as much structural reinforcement around the stock and front pivot pin. NDS also makes a line of these, and there are many older lowers out there made from this same forging, however NDS is the only company who currently produces A1 lowers. However, the easiest to find, and most common lower out there that you will find is the A2 lower, introduced with the M16A2, it has full reinforcements around the stock tube, and a more radiused area around the front pivot pin, and is far and again the most common one out there, used on anything from the M16A2 and beyond. M16A2 lowers also have a couple of different variations in the forgings, but nothing anywhere near as drastic as either the difference between the A2 and A1 lowers, or the various A1 lowers themselves (SP1, 601, XM, ect.). In addition, there are several companies which produce proprietary lowers, usually machined billet, Sun Devil and POF being examples. All but the SP1 will have standard sized pivot pins, however. One more caution, make sure to buy a forged, or possibly billet lower, there are many cast lowers floating around from older manufacturers, and should be avoided, unless you have no other choice (i.e. in a ban state, and it's the only preban available). As for a carbine, in the AR world, a carbine generally seems to mean anything with a barrel shorter than 20", as not all of them have collapsible stocks (605 and 652). An argument could also be made that a 20" barrel with a collapsible stock could be a carbine as well, and this is technically correct, since it's just a shorter version of a full length AR15. HTH, ~Augee |
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