From what I've read the barrel is supposed to recoil .030" before the bolt unlocks. I've watched some videos of the P90 firing, and I *think* I see that happening.
Given the relative mildness of the 5.7x28, I don't see where going straight blowback would be a problem. Recoil would theoretically be a bit sharper in blowback.
I've found some reloading information talking about swelling at the case head on fired brass, which is common enough on blowback actions.
The P90 magazine looks like any other magazine where it presents to the breech; the ramps and rollers that turn the cartridges are all in the magazine.
So far I haven't been able to tease any disassembly photos or diagrams out of Google or Bing. FN's web site has precious little information, either. Though none of it matters if you build your own innards from scratch.
I think there is some confusion here.
The pistol I took apart had a version of delayed blowback.
The p90 did not. I specifically tried to move/wiggle the barrel and could not feel a thing. I will try to get back up with the guy and look closer.
here are dimensions off an actual case.
HP OAL 1.5855"
rim .3025"-306" NOT concentric! poor quality?
extractor groove .2605"
case bottom .309"
Case just before shoulder .307"
Bullet diameter just before entering case mouth.222" BUT I think the ogee is continuing and it is a .224" projectile
File:PS90 breakdown.jpg - Wikipedia, the free encyclopedia
#5 pic at the bottom is a little weight that I think is a type of anti-bounce buffer
this states it is a blowback:
Modern Firearms - FN P90 personal defense weapon / submachine gun
This FN round seems very close to the MMJ-5.7mm ie Spitfire .22 cal US carbine clone, maybe a bit smaller case diameter.
FN P90P90 Operation
Despite the fact that the P90 Personal Defense Weapon fires a bottle-necked cartridge, which looks something like a downsized .22 Hornet, the weapon fires using an unusual method of operation that might be described as a cross between the short recoil and simple Bergmann-Bayard straight blowback principles. Upon firing, the 10.35-inch (26.3 cm) barrel and bolt recoil rearward for about 0.030 of an inch (0.76 mm), enabling the pressure in the barrel to drop to a safe level. When the barrel (which features a 1 in 9 inch rate of twist) stops its rearward travel, the bolt continues rearward in straight blowback fashion. FN Herstal SA seems to have developed a unique flavor of delayed blowback operation. Unlike the typical submachine gun, however, the P90 fires from the closed bolt to maximize semiautomatic accuracy. Recoil is brisk but very smooth, and cycling is reliable thanks in part to an anti-bounce weight in the bolt, which is operated by one of the main operating (recoil) springs. Polymers are extensively used throughout the P90 to reduce both the weight and the cost of the weapon. The human engineering of the weapon is outstanding and ambidextrous.
Gunco Member #10
The "original" Boltcutter Rivet Squeezers:
Project Pink - the Pink and Blue AK-74:
But there is nothing there to hold the barrel and bolt together?!?!?!? I just cant see it.
I guess I will call the guy and get him to let me tear it down...