Thread: build your own FN P90

Retrieved: December 07, 2013

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:

Amazon.com: 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."

TRX
04-07-2010

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.

TRX
04-08-2010

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.

TRX
04-12-2010

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.

TRX
04-15-2010

[description] Yes, but it's hard to make all that out from the crummy low- resolution pictures at the URL.

It wouldn't be difficult to float the barrel so it could move in recoil. I need to figure the case head thrust for the 9mm and 10mm, which were both used in straight-blowback arms, and compare it to the 5.7.

I really don't need yet another project, but I'm awfully tempted to buy that airsoft shell... with my luck it'd be "off" enough not to accept a real P90 magazine.

TRX
06-21-2010

Cool... but with a starting price of $420, it'd be more reasonable just to save my shekels and buy a real P90.

TRX
11-10-2010

I bought a magazine from CDNN last year, on sale for $19.95, just to see how the helical ramp thingie worked.

The Amazon page with the "Well" P90 airsoft has been up in a spare browser tab for months. Yesterday the price dropped from $30.28 to $28.95. What the hell, $5.99 shipping, I went for it. If it's as flimsy as reported I guess I can layer some carbon fiber inside it.

Green Mountain has 1:7 twist .223 blanks for $33. I guess that's the next step.

[486]
11-11-2010

Originally Posted by j427x
i wonder if 7.62x25 will feed from the mag? LOL!

No, too large of a base diameter and way too much taper. There was a guy over on weapguild that was thinking on that, but I haven't been back in a long time, my bets are that it didn't go anywhere...

TRX
11-11-2010

No, that thread died out.

For some demented reason brand new, brass cased, Boxer-primed 5.7 ammo is incredibly cheap now... less than what I've been paying for empty brass in some calibers. For reloading, jacketed .223 bullets are as cheap as it gets.

I like caliber conversions, but honestly, as cheap and available as 5.7 is, and the fact that it's the only thing that fits the unique P90 magazine, you'd be crazy to switch away.

There are some P90-ish variants that have a regular magazine hanging below the buttstock. I think I'd look into building a copy of one of those before I'd try serious hackery on the P90 magazine.

TRX
11-15-2010

The airsoft came in today. Sturdier than I expected.

One of the things I'd wondered about the P90 was its ergonomics. The blocky butt and the sharply angled grip, to start with. I regret to say they're about as bad as I thought. The corners of the buttstock are square enough to be uncomfortable just holding it up, much less actually shooting a real one. With CTS, my right wrist doesn't think much of the grip angle. And I discovered the reason for the odd oblong trigger hole is because the thumb of your other hand goes there. The fit is quite tight with my big hands; I don't think I could get an extra digit in there if I was wearing gloves.

On the other hand, my wife loves it. Go figure... I'll press ahead and money, time, and inclination permit.

TRX
03-20-2011

The magazine and the barrel blank are here. The next step is to turn the 1- 1/4" blank down into something that looks like a barrel.

Does anyone know what the diameter of a P90 barrel is in the chamber area?

TRX
03-26-2011

I forgot to mention, my wife decided she liked the P90, and said she'd pay for all the parts if I'd build it for her. I have GM barrel with the proper twist, a box of ammunition, three P90 magazines, a Well airsoft, and an Uzi recoil spring that ought to work with the 5.7 ammunition.

TRX
3-26-2011

Originally Posted by vz58
Link to airsoft gun?

I got it from one of the vendors on Amazon for $30-ish. I may have posted the link near the top of the thread.

The halves were screwed together, except at the muzzle where they were glued. A little careful prying for them apart, though.

TRX
03-26-2011

Originally Posted by gunnysmith
Be careful and remember the PS 90 is not a straight blowback.

Yes. However, the power of the 5.7 overlaps that of various common pistol rounds used in blowback SMGs. I found this chart (perhaps from the Dmitrieff SMG book?) on another forum, then looked up the muzzle energy of the various cartridges and the 5.7. The 5.7 is very close to the 9mm, and the bolt travel on the Uzi was compatible with the space available in the bullpup layout.

I suspect the main purpose for the P90's breeching mechanism was to allow very weak return springs, compatible with the small cocking knobs on the sides. The way the knobs are shaped, it would be no fun at all trying to cycle it if it had as much spring as, say, an M11/9.

I could work it okay with a slightly oversized knob, but I doubt my wife could. I'll figure something out when I get to that point.

TRX
03-28-2011

The sliding cylinder is some kind of anti-bounce doohickey, probably required by the weak return springs.

TRX
06-12-2011

Progess up until mid-April, when I broke my foot and had to take a break on anything that involved standing up. I don't have a follower rest for the lathe, didn't want to pay $125 for one off eBay, and didn't want to sidetrack the gun project by starting a steady rest project, so I just made about eleventy zillion .003-ish passes to keep the barrel from chattering.