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Well that sucks.

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Out of a FN 4.8" pistol barrel: MV of approximately 1560 fps and ME of 216 ft. lbs. for the 5.7x28mm 40 grain

22 WMR out of a 6.5" barrel: Fired from a 6.5" pistol barrel the 40 grain loads register 1480 fps MV and 195 ft. lbs.

I couldn't find data for an equivalent barrel length

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What? The LCP and LC9 are some of the best selling pocket pistols on the market.

Ruger has their shit together.

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SS198 is 2,200ish FPS out of the Five Seven for around $.50 - $.60

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5.7 is fairly miserable to reload in quantity.

The brass is fragile. Very fragile. The 57 pistol is a LOT gentler than the PS90, but the PS90 absolutely mangles the brass. The shoulder requires a (relatively) enormous resizing process to get it back in spec. After the second reloading my cartridges in relatively warm loads in a PS90 come out damn near straightwall.

Trim length is fairly critical to good function. Thin brass, (relatively) high pressures, and a hefty resizing operation means trimming is fairly critical because its going to stretch a good deal.

The vast majority of the brass has primer crimps. This isn't that big of a deal but the size of the case and the lack of reasonably priced progressive loading options means you have to remove the crimps individually- I can't just toss it in my 1050.

Cases are relatively easy to crush if you aren't careful. I use an M die, chamfer the case mouths, and then use a die with a sliding sleeve to avoid this.

I've personally found that some dies are incapable of setting the shoulder back correctly. Or rather, a combination of dies and shellholders. I am completely incapable of setting the proper length with hornady dies and lee or hornady shellholders. With a redding shellholder I am able to set it. I i bought one of the cutaway Sheridan gauges and it was helpful. I also have an Elite Ammunition gauge and it is too tight. It just shows that manufacturers aren't in perfect agreement about the 5.7.

The hubbub about it being super intolerant of powder charge and it's going to blow up if you go 0.1 over is dumb so don't worry about that so much, as long as the loads are reasonable.

I don't know if the fuss about the lube on the cases is important or not. There is a lot of fuddlore as to what purpose it serves. Some same it is needed for ps90 extraction, some say for ps90 magazines to be reliable. The pistol is considerably more tolerant. I've only gotten 4 loadings out of a piece of brass before I was so suspicious of it I tossed it, and I never bothered to try and clean it. Those who do use simple green and an ultrasonic.

The 5.7 is safe to reload... it's just a gigantic pain in the ass for the sort of gun you're going to be pumping tons of rounds through.

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The P90/PS90 & AR57 are straight blowback, and it's unsurprising the necks blow forward.  The FN 57 is delayed blowback - I would assume the Ruger would be short-recoil operated, both being easier on brass.

The polymer coating  on the 5.7x28mm case is almost certainly for extraction from the P90 chamber, given that 5.7x28mm is likely the highest pressure round ever successfully run in a straight blowback action.  The hottest Elite loads are specifically not for use in the P90 or PS90 for that reason.

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Says the guy with that...thing in his avatar

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My PS90 barrel has just the slightest bit of play.  So I would not call it straight blowback.

The bullet would pull the barrel forward when fired.  Then, the barrel and bolt, held together by pressure on the fired case, would start to recoil rearwards.  The barrel would come to a halt, and the bolt would travel rearwards, with the extractor pulling  the case out of the chamber.

I am surprised that this type of action hasn't seen more use.