I did a few guns last year and have a bit of adivce to add. First tape off the outside of the stock with masking tape to avoid having foam residue stuck when it overflows. Second, spray short puffs of foam and let it expend for 30 seconds or so between puffs. Third stop before you think you need to, it will continue to expand for several minutes. Fourth, stand the stock up and let it cure for at least 24 hours before you trim the excess off.
