See also: P90 vs PS90This article is still under development
The question of which rifle is better, comes up periodically. The two rifle systems really shouldn't be considered competitors, as both are the correct answer to different questions. More "role-appropriate" comparisons would be the PS90 vs H&K MP5, and FN FS2000 vs the AR-15. However, these are not the questions which get asked. This article will compare and contrast an FNH PS90 with its closest AR-15 analogue, an M4gery chambered in 5.56mm. As always, this article is not going to pick which rifle is "better". The goal of this article is to bring up points in contrast so that you can evaluate which rifle's strengths appeal to you the most. However, I'll observe that I personally chose the best answer: buy both!
So far, PS90s haven't ever been available new under $1k, and are usually closer to $1.8k at late-2008 prices. That would buy two AR-15's in an M4 configuration, using mid-range parts, in late 2008. The edge here clearly goes to the AR-15.
Versatility: AR-15, no question
The PS90 is a very well-designed weapon for a specific role, however it is limited to one barrel configuration (two if you count conversion to a papered SBR, three if you count the two available SBR twist rates separately), one caliber, and one stock. Optical versatility goes to the AR-15 as well, as even the tri-rail version of the PS90 has a fairly short picatinny rail.
The AR-15, on the other hand, is rightly nicknamed "the Lego gun". Almost any accessory you can imagine can be fitted to an AR-15. Caliber can be changed by changing the upper, all the way up to 50 BMG/DTC. A plethora of pistol grips, bipods, optics, iron sights, lasers, tactical lights, quad rails, different stocks, drum-fed magazine conversions, and even a belt-fed conversion (the Shrike) are available for the AR-15 (if, in the case of the Shrike, you can find one).
Ammunition Choices: AR-15, dependent on role
For purposes of this discussion, the AR-15 will be considered to be a 5.56mm rifle. When folks ask to compare the AR-15 vs the PS90, typically they mean a 5.56mm-based rifle anyway.
This comparison is actually pretty interesting, given that the 5.7x28mm is functionally a shrunken-down 5.56mm round. The 5.7x28mm round is .224 caliber, same as 5.56mm. Both cartridges are bottlenecked, and the primary difference is that the 223 is a much longer case with far more powder capacity. 5.7x28mm chambers are rated at 50k PSI of pressure, while 5.56mm chambers are rated for 58k of pressure.
Commonly built rounds for the PS90 range from 28gr to 50gr. However, heavier bullets in 5.7x28mm tend to drop too quickly to be useful. 69gr armor piercing projectiles have been said to actually be visible during flight when fired out of the PS90!
For perspective, the heaviest load data I've seen for the PS90 has used 7gr of powder, while most AR-15 loads range between 19gr and 25gr of powder.
In short, because the PS90 is pushing the same-caliber bullet with less case capacity, the AR-15 will always have a much wider range of flexibility in the rounds it can fire. The AR-15 can fire heavier bullets faster than the PS90 will ever be able to, and it can shoot lighter bullets as well. If semi-automatic operation were not a concern, the AR-15 can either completely duplicate the PS90's muzzle velocity for all bullet weights, or exceed it.
However, the italicized part of the previous sentence is the PS90's one compelling advantage in this section. The smaller, lighter action of the P90/PS90 doesn't require nearly as much kinetic energy to cycle as that of the AR-15, so lighter loads will cycle the action just fine. This is important in law enforcement and urban fighting, places where over penetration can be a concern. An AR-15's heavier bolt and recoil spring impose a lower limit on the 5.56mm round, below which there isn't enough power to cycle the action. This threshold exists with any round, but it's much lower in the 5.7x28mm than it is on the 5.56mm.
Rapid Fire Performance: PS90, by a medium-sized margin
The design of the PS90 places the recoil towards the rear of the rifle, up against the shoulder, and in line with the stock. This design, when the stock is shouldered correctly, causes recoil to be almost negligible. The AR-15, on the other hand, has an axis of recoil located around the top of its stock, which encourages muzzle climb. An AR-15 can be carefully tuned with consistent ammunition to minimize muzzle climb, but the design will still have more intrinsic point of aim shift between shots than the PS90.
5.56mm is arguably far more lethal than 5.7x28mm. Folks may be able to make arguments for the polymer-tipped noses of the 5.7x28mm projectiles, but those arguments fall flat on their face when 5.56mm rounds loaded with identical projectiles are brought into consideration. The one and only area where the PS90 has an advantage over the AR-15, in lethality, is that a standard PS90 magazine will hold 50 rounds in about the same amount of weight as the AR-15's magazine, while exhibiting less muzzle climb for accurate follow-up shots.
Portability: PS90, hands down
At only 26.25" in overall length, the PS90 is shorter than even the shortest standard M4-style AR-15. This makes it more maneuverable in tight spaces, and more readily concealable. While the AR-15 can be shrunk a few inches more if one fills out the appropriate federal forms, the same forms can be filled out to legally cut the PS90 to an unbeatable 19.7" length. Weight is roughly comparable between the two weapon systems, at 5.9lbs for an unloaded M4-configuration AR-15 and 6.4lbs for an unloaded PS90.
Notes on the AR-57 Upper
One significant consideration is the existence of the Rhineland Arms AR-57 upper. This upper fits onto an AR-15 lower receiver, and allows one to use normal PS90 magazines in an AR-15 platform. Unlike the PS90, the barrel on the AR-57 upper is threaded to accept compensators, muzzle brakes, etc. Like the PS90, the upper is blowback operated and does not rely on a gas impingement system to operate.
Performance with Cast Bullets
Theoretically, the PS90 (and the AR-57) should both function much more reliably with cast bullets than the AR-15. The AR-15 has the disadvantage of using a gas system which could become clogged with lead particles, while the PS90 and AR-57 upper are both blowback operated and leave nowhere for lead to go but down the barrel.
-- SeanNewton - 20 Aug 2008