(I wrote this a while ago. Below is my impression, not a scientific treatise.
During a recent photo shoot for Advanced Armament Suppressors, I had a chance to fire numerous modern weapons side by side. Among them were HK MP5 and G36C, FN P90 and an M16A2. Of these guns, only one, the M16 (in AR15 guise), has wide distribution among civilian users. Along with the MP5, it shares a long history of gradual refinements to the original design. P90 and the G36, on the other hand, have been in use only by police and military entities, and almost no civilian distribution. This difference may have accounted for the unexpected observations I made during the shoot.
M16 and MP5 were both extensively fired that day. We experienced zero malfunctions of any kind. None of the users ran into problems with sharp or slippery controls, excessive heat build-up, safety manipulations or anything else noteworthy. The guns felt like mature, refined designs. M16 worked the best, and the many civilian-inspired refinements added up to a very comfortable experience. The MP5 still had the infamous HK safety lever and a couple of sharp edges by the charging lever, but worked smoothly overall.
The G36, the vaunted improvement on the M16, came off poorly on many counts. For one, it failed to extract or eject half-dozen times. The small charging handle hidden below the optics rail proved hard to grab when malfunction clearing was needed. The lack of any heat shields within the forend made holding the rifle uncomfortable. The folding stock proved too long for most users. The visually impressive latches for clipping magazines together caused snags when we pulled magazines from pouches, and duplexed magazines were to awkward to lock in place. Moreover, the top round in the right-side magazine was repeatedly hit by ejecting empties. Iron sights were not usable through the red dot optic. While none of these problems would disqualify G36 from consideration, they reflect the lack of attention to the customer feedback. That is not unreasonable, given that all G36 buyers have been institutional: the end users have less influence on the manufacturer than do those who vote with their own wallets.
FN P90 has long been advertised as the next generation in submachine guns, an improvement on the MP5 and other pistol caliber weapons. Its magazine holds 50 rounds compared to the usual 32, and its cartridge penetrates soft body armor better than 9mm or 40S&W rounds. It is also very compact and has an unusual shape which appears ergonomic at first glance. In use, however, P90S showed some surprising and unnecessary deficiencies.
First and foremost, the weapon is not safe to operate because of the forward grip design. The back of the support hand touches on the trigger finger and actually presses on it if gloves are worn. That can become a contributing factor to accidental discharges, especially when the user moves over rough terrain or stairs. The magazine uses an interesting design for compactness, but it is very slow to load and any jams would be hard to clear. Previous magazines more complex than the Lee-designed box, including the Calico and Bizon helical drums and the Beta-C saddle drum, ultimately proved insufficiently reliable. The Russians have gone back to box magazines with the Bizon, and the US Army no longer recommends Beta-C for combat use. The rotating selector switch is hard to verify visually. Its small movement arc means that a gloved or stressed user would be unlikely to stop at the Semi position, although that is mitigated by the long trigger pull required for automatic fire even in the Auto mode. The charging handles are smooth, slippery and rounded. That kind of small, annoying detail would not have persisted on a gun with wider distribution simply because either the manufacturer or aftermarket parts make would have accommodated the customers. To top this off, the 5.7 cartridge has little use history by which to gauge its effectiveness on live opponents. The wealth of information available on the common rounds like .223 and 9x19 Luger simply does not exist due to the legal and marketing restrictions on its distribution. International Wound Ballistics Institute published test results in 2000 which suggest 5.7's terminal ballistics to be on par with .22 Magnum, an inadequate performance for combat. Whether this is true or not cannot be verified because almost no track record exists for other combat or hunting deployment of the P90 and the 5.7 pistol which shares the same ammunition.
Logistically, the G36 and its now-discontinued civilian variant SL8 are dependent on HK for service and parts. Unlike the AR15 and the AKM derivatives, these rifles do not enjoy wide aftermarket support. The same is even more true of the P90, of which no civilian version even exists. The availability of parts and of armorer expertise strongly favors the established designs produced by several manufacturers.
The situation is a result of two factors. The first is the obstacles placed by the US laws on the way of any company wishing to make or import modern small arms for civilian use. The severity of that problem varies over time but adds restrictions and uncertainty to any business plans. The second is the reluctance of some companies, notably H&K, to even consider the civilian market as important. In the US, that civilian market is large, diverse and, in effect, an extended beta test of any new or improved weapon system. Not selling to individuals cuts off an important source of feedback on the ergonomics and reliability. Gaming software companies understand that and market variants of the army training simulators to the public at large.
Some point at Russian and German designs as counter-examples. What is overlooked is the decades and the substantial monetary investment needed to fine-tune even the most common weapons. Even with all that effort, few would call AKM or G3 rifles ergonomic or SA80 reliable. The same is true of GPS devices, with off the shelf models improving considerably on the Army's official issue. Feedback from paying customers is only one of many channels for product improvement, but it is among the most responsive and cost-effective. Companies, such as Barrett and Advanced Armament Suppressors, which understand that civilian users are important have recently done better in securing institutional contracts than those who do not.
The openness of the overall weapon system should be one of the considerations at purchase time. Individual manufacturers come and go, but any product that allows for as wide an ownership as permitted by law is bound to have better aftermarket support than those that do not. Voting with money is always sincere, and American civilian shooters have a better track record of picking winning designs than the Army Ordnance Department.