OK, you stated you wanted
maximum reliability. So I am going to take that approach; but keep in mind that we are discussing fairly small differences in performance here.
In doing research on 14" barrels with permanently-affixed 2" muzzle devices, I've found several folks claiming that the higher pressures present in the shorter carbine gas system can potentially adversely impact reliability and service life.
There are links in the Reading Library at the top of the forum that explain this in great detail (complete with pressure curves and technical diagrams). Essentially the shorter gas system affects reliability by trying to cycle faster than the system was originally designed. When the rifle is cold, carbine gas system delivers a more gas, more quickly so that the cyclic rate is much faster than the rifle version. As a result, the magazine has less time to push the next round into position before the bolt comes back. As the carbine starts to heat up from prolonged firing, the second thing that happens is the carbine starts to try to extract before the brass has shrunk away from the chamber walls - this can result in torn rims, the extractor slipping off the cartridge, or the carbine using most of its energy pulling the case out and not having enough left to cycle.
The shorter gas system also affects service life due to increased pressure. Basically, it comes down to the pressures on the bolt and internal parts of the carbine being about 1.5x as much as the original rifle design. As a result of that, a recent NDIA Powerpoint noted that M4s subject to a heavy (SOCOM definition of "heavy") firing schedule developed micro-cracks in the bolt as soon as 3,000rds and even with a moderate (again SOCOM definition of "moderate") firing schedule, almost all M4 bolts display these cracks by 6-10k rounds.
With the carbine gas system, you can still adress both of these issues. On reliability, you can add a heavier buffer to slow the cycle time and lower feed ramps so that rounds that are in marginal position feed better. You can also use good quality magazines with stiffer springs and download them by a round or two. Second, you can add a stiffer extractor spring insert, a stronger extractor spring and an O-ring buffer to make sure the extractor stays on the brass.
To address the service life issue, there are several "enhanced" bolt designs that reengineer the traditional AR15 rifle bolt to better withstand the pressures produced in the carbine. Most of these designs also redesign the extractor as well to make the second step mentioned above unnecessary. However, these are all newer designs and may not be fully tested. If you want to stick with what works - buy MP tested bolts that have been proof-fired and replace them every 6-10k.
However, the easiest way to address these issues is just to move the gas port forward about 2" - this causes slower cycling and drops the internal pressure on the bolt. This little change makes most of the upgrades unnecessary (and sometimes undesirable in the case of heavier buffers); but you can add the extractor, mag spring, and feed ramp upgrades and they will perform well in this platform too.
However, now that we have discussed the theoretical limitations of reliability and service life, let's discuss real world practical use. SOCOM uses its weapons pretty hard. During pre-deployment training a Navy SEAL platoon, each man might fire 20,000rds in six months. In addition, there will be a fair amount of full-auto and even suppressed fire. Both of those things are hard on carbines. They can literally shoot a brand new carbine to pieces just during the workup. Very few of us will reach that level of training so it is important to understand that we aren't discussing the difference between reliable and unreliable as much as we are discussing the difference between 99% performance and 95% performance (those aren't actual numbers - just an example).
In my own practical experience, I ran a carbine length gas system with a regular strength extractor spring, regular buffer, no feed ramps, and black follower used GI mags of ancient vintage for about 10,000 rounds. During that time I had no parts breakage of any kind and the number of stoppages that I had that weren't related to ammo or magazines could be counted without taking off my shoes. Using good magazines and good factory ammo designed for the AR (cannelured bullets), will solve most of your problems right away.
Here are the desired features which I'm told may influence reliability:
14" barrel (1:7 twist) with 2" flash hider permanently attached
collapsable stock
I don't like permanently attached muzzle devices. They limit the amount of work you can do on the AR (can't remove the front sight base). They reduce velocity a small amount and the .223 relies heavily on velocity to be effective. Finally you only get about 1" reduction in length over a 16" barrel, which doesn't make much difference in handiness for me.
no preference on gas block size or positioning (supposedly mid-length gas systems are best in carbines?)
I'd prefer to have the rifle "complete" from Bushmaster, Armalite, Rock River Arms, or Colt, to avoid any gunsmithing snafus.
If you are looking for maximum reliability, I would drop Bushmaster from that list (and I own and have been very happy with several Bushmasters) because their carbine doesn't come equipped with all the things I mentioned earlier (heavier buffer, upgraded extractor, feed ramps, MP proof tested bolt) and they do not make a mid-length gas system.
Both Rock River and Armalite offer midlength gas systems; however like Bushmaster they don't include many of the upgrades I mentioned in their carbine gas system offerings.
Colt doesn't offer a mid-length gas system; but the LE-series carbines they do sell have all the appropriate mods we discussed earlier.
That said, are any of those features problematic in terms of reliability? If so, which changes would those of you in the know suggest?
All of the manufacturers you mention are good manufacturers and all of them put out the occasional lemon. I believe that the dealer is more important than the manufacturer when buying an AR.
I don't own any carbine gas-systems anymore. They are all 16" midlengths. I find the midlengths to be smoother in recoil and I am convinced (though I don't have complete personal experience equal to the carbine yet) that in the long term they will have better service life and be more reliable as well. Having said that, I got excellent reliability out of my carbines and had no issues with parts breakage or service life in the 10k rounds on the most used carbine.
Much more important than the gas system or the manufacturer are the ammo and magazines. Those are really the key to making this particular design run. Being in LE, your friend may chamber and clear ammo quite a bit before firing it. In that case, ammo with hard military primers and a cannelured bullet is important. The military primers prevent the repeated light indentation of the free-floating firing pin from setting off the round and the cannelured bullet keeps the bullet from being set back into the case and creating unsafe pressure levels. If his department doesn't already specify a particular load, I would look at the law enforcement offerings from Hornady, Federal and Winchester.
The second thing is good mags - if you are buying new magazines, you want USGI magazines, green USGI follower or Magpul self-levelling follower, and new chrome-silicon springs. Even then, you will need to try them out to make sure you didn't end up with any lemons; but once they all test good all your will need to do is keep them clean of major chunks of debris. Remember that mags are designed to be disposable items. When one starts giving you trouble on the range, ditch it and replace it. Currently, a lot of people offer "high reliability magazines." I am skeptical that some of these offer an improvement. Some of these, like the H&K HRM, have offered improved feeding, easier seating of the mag, and stronger material but have suffered from new issues caused by the change in design (corrosion, spines cracking). The basic USGI mag is cheap, available and a proven item that has worked well for me, so I would probably be reluctant to try many of the "super reliability" magazines in a scenario where I might really need the rifle until I had a lot of range time with them.
The 6920 has an M4 barrel profile (a little heavier), and a detachable carry handle. This will make it easier to attach optics.
