At the low end, we have the poly-urethane foam stock. Predictably, these are made of polyurethane foam material. Of course, a stock made of pure urethane foam is not strong or rigid enough. Hence, many manufacturers add chopped fiberglass strands to the urethane foam. In addition, aluminium rods are added in some areas, particularly around pistol grips and the recoil lugs. Solid urethane foam stocks are generally lighter than other stocks, but adding aluminium and fiberglass adds extra weight back in, which undoes most of the weight savings. These stocks are not sensitive to moisture like a wooden stock. Unfortunately, they share the same problems of temperature as a wooden stock. They are also the stocks that are most likely to break.
Then we have the hand laid fiberglass stock, as pioneered by Brown and Six from the 1960s. In this type, a cloth made of fiberglass, kevlar or carbon fiber is first laid out on the inside of a mold cavity. Then a polyurethane foam is poured into the mold cavity and allowed to cure. Alternatively, a polyester or epoxy resin may be used instead. With an epoxy resin, the epoxy has a 3-D molecular structure and is lightweight. It is extremely resistant to impact damage and once it is 100% cured, it has extremely long life. Polyester resin is cheaper in cost. However, the molecules have a 2-D structure and it only cures partway (upto 70%) in the manufacturing process and continues to cure for 7 years or so, after which is starts to deteriorate. The outer cloth of fiberglass, kevlar or carbon fiber adds the necessary strength to the stock, that make these stocks much stronger than a foam stock alone. After such stocks are manufactured, a bedding process to seat the barrel properly on the stock is needed. This bedding process may be done as part of the manufacturing process, or separately after the stock has cured.
These stocks are much more expensive to make, as it is a labor intensive process. The curing time can take a while and much of the strength depends on the manufacturing process getting the amount of cloth and resin just right. On the other hand, such stocks are very light, accurate and immune to the effects of weather and temperature. Hence, these are the stocks of choice for many sniper rifle models and hunting rifles.