But, I can't tell the difference in felt recoil between it and any other AR I have shot.
Have you tried shooting it side by side with a carbine or rifle? I never noticed the difference between the rifle and the carbine until I did that.
I think Coronach gave a great rundown of the situation (except for #3
). The only thing I would add to it is that most people, even institutional buyers, just aren't aware of the differences. If you asked 100 AR15 owners about their rifles, I bet 80% of them would be unaware that there is even a difference in different length gas systems.
As for why midlengths aren't more widely used, I'd just note that Armalite (the Mark Westrom version) was one of the pioneers of the concept. Armalite's Tech Note 48 (the website is currently unavailable; but you can find it in Google cache) described the problem as early as August 2003. Armalite had created the midlength gas system even earlier (around 2001 IIRC). For a long time, if you wanted a midlength you had once choice - Armalite.
Then the RRA guys split away from Armalite and they kept the midlength concept because it made sense, so now you had two manufacturers offering midlengths. In addition the popularity of longer gas systems in 3-gun meant you started seeing more of them offered from custom houses.
Now in 2006, you have Armalite, RRA, CMMG, Sabre Defence, and practically every custom AR maker out there offering midlengths. It looks to me like midlengths are gaining ground in the market, albeit slowly. I like my Bushmaster; but Bushmaster hasn't been a leader in new concepts for a long, long time now. They are usually a dollar short and a day late on new ideas for ARs.
A great anecdote illustrating why midlengths are slow to catch on (and why bigger manufacturers are often behind the curve) is the Bushmaster 1/7 group buy on AR15.com from several years back. At the time, about the only place to buy milspec chrome-lined 1/7 barrels was Colt (and the price was arond $400). You could occasionally buy foreign contract overruns from Bushmaster; but they were not a catalog item. AR15.com users begged Bushmaster to make a 16" or 14.5" 1/7 barrel. Most wanted an M4 profile but would have been happy for anything. Bushmaster, after much prodding, put up a poll to gauge demand. They had hundreds of responses indicating interest; but after six months, still no barrels from Bushmaster.
Suddenly a new company steps into the scene (CMMG) and they not only offer 1/7 milspec barrels; but they offer them in a variety of configurations (14.5", 16", M4, HBAR, etc.). About six months later, Bushmaster offers a limited run of 200 16" HBAR 1/7 barrels. Because many of the people who initially wanted a 1/7 either bought them at high prices from custom barrel makers or jumped on the CMMG sales, Bushmaster barely sold half of the run. Instead of reaching the conclusion that the problem was that they reacted too slowly to market demand and didn't offer enough options to maximize that market, Bushmaster reached the conclusion that there was not enough market demand for 1/7 barrels to justify production and continued to concentrate on 1/9 barrels.
In the meantime, two new companies stepped in to fill the gap (CMMG and Bravo Co.) and they offered milspec 1/7 barrels immediately. Not coincidentally, both were also quick to catch the midlength wave as well. Now both companies have grown large enough serving that non-existent market that several years later, Bushmaster is reconsidering its 1/7 decision and is once again polling on AR15.com to see if there is demand for another run. By 2010, they'll probably be making midlengths.
