JB weld has two major products and a few others. They have a 4-minute product and you better use it in the first 2 minutes. This is not the best for most gun work as it has a lower heat release temperature.
The regular JB Weld (JB Industro Weld is best) is best for most gun work including bedding and gluing. Don't waste time on it either.
If you are just bedding and the regular JB is too thin, just add some finely chopped fiber glass or use one of the other good products like AccuGlas.