Basically, can some one please explain how and why the AR15 rifle (i.e., M16) and the AR15 carbine (i.e., M4) differ from the end of the lower receiver backwards?  Specifically, why do you usually only see rifles with fixed stocks?  I'm sure this has something to do with the receiver extension, buffer spring, buffer, and maybe other facets of the AR15 system.  Please give all the detail you can.  I've scoured the internet to no avail.

Also, if you can, please explain how AR15 pistols/SBRs and mid-length gas system fit in to the above question.

Thanks a bunch!

Traditionally, 18" and longer barrels are typically mated with a fixed rifle stock. 16" and shorter are mated to collapsible carbine stocks. Barrel length and stocks are independent of one another, so you can mix and match for your personal preference. The stocks mount to the receiver extension. Carbine extensions are shorter, thus a shorter spring and buffer than a rifle setup. Mid-length refers to a gas system that is between a carbine and rifle. It has nothing to do with which stock you pick. Pistols can't have a stock, so they just have a buffer tube only. SBR's refer to barrel length, so again, you can use which ever stock you like. I don't think I have ever seen a SBR with a rifle stock, but you certainly could if you wanted to. I hope that all makes sense.

Carbines are by nature, compact, thus the collapsible stock.
Rifles are by nature, not compact, so one doesn't mind a longer, fixed stock.

I don't know how much absolute definition I would place on either rifle or carbine based on the butt stock configuration.  Maybe it used to be more of a fixed stock/rifle category, but with the advent of so many variants of the AR platform in small and big calibers, and the array of available butt stocks, I think the butt stock has become one of the many preferential elements of choice rather than dictating what the definition of the rifle might be.  Rifle and carbine may be better described by the barrel length if one is simply looking for a more descriptive element.

in a nutshell: it's all just personal preference.

20in with carbine stock


Carbine with rifle stock:

Just a quick question, probably been asked a gazillion times on here and elsewhere, but what are the dimensional differences between the "mil-spec" and "commercial" buffer tubes? Also, what is the "H" buffer? Is it lighter than, say, a "H2" or "H3" buffer? Thanks in advance for your help.

http://www.ar15fornoobs.com/mil-spec-vs-commercial-spec-buffer-tubes

Soo, according to the pictures posted, the DPMS one with the slant back on the buffer tube is a "commercial" tube then? I noticed the difference between mine when I took off the stock assemblies from the buffer tubes.

That's my general understanding too, however, there is an occasional mention that this has some very infrequent exceptions...the commercial version not "always" having the slant.  I've not seen it mentioned where a milspec unit had the slant as an exception.  It's a better bet to buy a cheap digital vernier caliper from Harbor Freight to be absolutely sure.

Slant is not a defining characteristic.

Generally, it's easy to tell the difference just looking at the threads.  If the tube is slightly larger than the tips of the threads, it's commercial.  If the tips of the threads are larger than the tube (and almost always will have a section of threads with flat tops), it's the mil size.

 
14.5 BCM ELW setup as a rifle, and yes it is a stamped lower.