Cut and pasted from Ruger website Q&A section
"I have heard that Ruger has changed the 10/22 carbine trigger group components from aluminum to polymer. Is this true and, if so, why was this change made?
Yes, it is true that we have updated the 10/22 trigger group components and that they are now made of glass-filled polymer instead of the die cast aluminum parts. Ruger engineers are always looking for ways to improve our products, including evaluating new and evolving technologies and materials. The benefits of polymers are many, including improved impact resistance, an unmatched ability to withstand the elements, and of course weight savings. Accordingly, polymers are routinely used in firearms that are subjected to heavy use in harsh conditions, law enforcement and military applications in particular. The new trigger group features injection molded components of high tech polymer for an improved product with closer manufacturing tolerances. The firearm is as reliable and, because the color is incorporated into the polymer of the injection molded components, the finish will never wear off due to normal use or unexpected abrasion. Also, the heat stabilized, glass-filled polymer will withstand impacts that would bend or even break the die cast aluminum parts previously used in Ruger 10/22's."