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build your own FN P90

9024 Views 56 Replies 12 Participants Last post by  TRX
Presented for your amusement:

I picked up a P90 magazine from CDNN last year, when they were on sale for $19.95. I bought it just to see how it turned the cartridges 90 degrees to present them to the breech. It uses a nifty helical ramp.

5.7 ammunition is, for some inexplicable reason, dirt cheap now. The "5.7" bullet's published diameters seem to be basically the same as "5.56", so you can use readily-available .223 barrels. You'd have to find a reamer.

As far as the bolt and breech are concerned, the P90 magazine works just like any other magazine.

You could easily build a tube gun, something like the SGN-9 or your own design, that would shoot 5.7x28. The P90 uses straight blowback.

Tube guns tend to range from "industrial" to "fugly" in appearance. If you wanted to make it not quite so industrial looking, you could wrap it in a plastic shell.

Where would you find a plastic shell? I found this FN P90 airsoft: Well P-90 Laser Red Dot Airsoft Electric: Sports & Outdoors

It's $26.95, $39 to your door. You could open it up, discard all the airsoft bits, and fabricate some bracketry to wrap it around your tube gun.

I'm not a fan of the P90's looks, though I admit it's rather striking. But as a lefty, the fact that it ejects down instead of my face or left armpit is a strong plus. Also, "cheap is a quality overcoming many faults."


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Cute idea !!!
Wonder what is the diameter of the 5.7mm case head ?? Same as 9x19mm ammo ie 10mm ???

The FN P90 seems to have a square receiver. Maybe a MAC bolt ???

the p90 uses DELAYED blow back, not straight blow back, there is a BIG difference!!
The 5.7 cartridge uses the same size shell holder as the .25 ACP.

The internals for the AR-57 may be a good starting point.
Hmm, interesting. The sites I hit early on all said straight blowback. Some others say short recoil. Others say both, which is par for the course... if FN Herstal's web site says anything, darned if I can find it.

Looking at the ballistics of the cartridge, I don't see any problem with running 5x7x28 in blowback mode, though.
There's a guy on YouTube who stuffed a Ruger 10/22 inside a P90 airsoft shell. Among other things that seem to have vanished with the new "improved" YouTube is the "link to this" URL they used to have, unless it's another casualty of their broken HTML vs. Konqueror.

Anyway, paste "Building instructions Ruger 10/22 FN P90 Replica Video 3" into YouTube's search bar and you'll see it. Most of the work was to handle the 10/22's side ejection and bottom magazine. None of this is relevant to a tube gun, but it shows the inside of one of the airsoft shells.
i luv this cartridge in my ar set up and it works just fine....
Pirate, I have read that it is delayed blowback but I took a buddies apart and I saw nothing to indicate that. A light bolt and 2 springs....
I've got a PS90 and a 57 upper.

If blowback is delayed I can't figure out how they're doing it.

Looks like straight BB to me which would explain why I get slightly OOB detonations from time to time on my 57 upper (full auto). The 57 upper does have the BC slightly longer to put the buffer under tension but if that's what they're calling delayed BB that's a stretch.
Pirate, I have read that it is delayed blowback but I took a buddies apart and I saw nothing to indicate that. A light bolt and 2 springs....
Looking at the bolt on one at the gunstore it has a sprung counterweight that looks like it is to hold the bolt back for a bit longer, delaying the return of the bolt into battery. Maybe that's the delayed part to it...
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've got a PS90 and a 57 upper.
How about taking it apart and posting some pictures?
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"
case 1.1285"
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
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This FN round seems very close to the MMJ-5.7mm ie Spitfire .22 cal US carbine clone, maybe a bit smaller case diameter.

This FN round seems very close to the MMJ-5.7mm ie Spitfire .22 cal US carbine clone, maybe a bit smaller case diameter.


That is what I thought, but the spitfire was more powerful and larger
#5 pic at the bottom is a little weight that I think is a type of anti-bounce buffer
There's the bit I was talking about! When I saw it I thought of some sort of rate reducer, but it could be an anti bolt bounce device.
FN P90

P90 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.
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...
I found that before, but darned if I can make any sense of it.
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