The text below is a quote from the firearmblog.
High Tower Armory Ruger 10/22 PS90 Conversion Chassis:
High Tower Armory’s innovative HTA 90/22 stock is now shipping. The stock includes everything needed to make a standard Ruger 10/22 into a P90-ish bullpup and comes in three flavors; black, olive drab, and flat dark earth. The converted 10/22 has a working charging handle and safety in the same places as a real FN P90, but the translucent “magazine” up top is a fake, simply storing spare .22 rounds for looks. You’ll need to insert a regular magazine from underneath, where the actual P90 ejects its spent casings. The HTA 90/22 ejects its empties to the right using the standard 10/22 ejection port, which you can see peeking out from the side of the stock.
Some serious problem solving went into shoehorning the world’s most popular .22LR rifle into a carbon copy of the famous P90 shape without requiring any gunsmithing at all. The quality of the stock and its fittings is very high, unlike some bullpup conversions of the past. The kit comes with a fake silencer/suppressor that mounts on the stock itself and will work with either the standard barrel or Ruger’s optional .920in heavy barrel with no modifications needed. High Tower Armory states that they have had 14 year old kids successfully follow the installation instructions (which is going to make me feel real smart if I get stuck somewhere!).
One word of warning; to meet the overall length requirement of 26 inches and keep the 10/22 a Title 1, non-NFA rifle, you need an 18 inch barrel. Factory Rugers come with an 18.5 inch barrel, but if you’ve cut yours down to 16 inches, the HTA kit is a ticket to SBR city. The black version costs $229.99, and the olive drab and flat dark earth variants run just ten bucks more than that. Of course, you have to provide your own Ruger 10/22 for the stock to fit on. If you love the look and feel of the FN P90 but don’t want to pay the high cost of 5.7×28 ammunition just to plink with, High Tower Armory has all three colors in stock right now.