There are several types of synthetic stocks. The original, and best are made of layers of fiberglass, or kevlar cloth laminated with epoxy in the action area. They are then milled out for the action and barrel channel just like a wood stock. The buttstock and forend are made with hand laid layers of cloth much like fiberglass boats. The hollow areas are filled with expanding foam to deaden the sound. Expect to pay $500-$700, but they are the strongest and lightest. Kevlar stocks are the most expensive but weigh 16-20 oz compared to 32-36 for a typical wood or fiberglass stock
Companies like Bell and Carlson and HS precision use chopped fiberglass much like home insulation mixed with molten plastic and poured into a mold. The originals were heavy, and weak. The better versions now use an aluminum block for the action to bolt to with the fiberglass and plastic poured into a mold around it. They are not hollow. They do provide a stable accurate stock for around $250-$350. They are very heavy, and because of the construction can be quite chunky and thick. Hogue uses a similar plastic stock with a metal insert. Both types are extemely heavy, 36-48 oz.
Most factory stocks are injection molded. Molten plastic is poured into a mold. Air pressure is injected into the mold causeing the plastic to expand into the shape of the mold. Most are fairly inexpensive and can be purchsed for around $100 or often less. Weight is about the same as most wood stocks and often slightly more. You won't find a synthetic lighter than wood until you get into the $600 price range.
The injection molded stocks are hollow, but that is easily fixed by just taking off the buttpad and filling it with foam packing peanuts. Some use expanding spray foam, but the packing peanuts works just as well and is much easier and less messy. They actually shoot a lot better than they get credit for.
