If plastic stock you might first want to drill a very small hole at end of crack to keep crack from travelling.
Its hard to get glue down into stock cracks, but if plastic stock, I believe I'd remove butt plate and attack from inside. Try to use a wedge to gently spread the crack (after drilling hole mentioned above!) to allow chosen epoxy to enter crack completely from inside. Pull wedge and clamp if necessary to close crack. As has been stated, plastics can be difficult, so not sure what epoxy to use, but JB Weld sticks to a lot of different materials and might work????? If it sticks, it's very strong.
Regards,
hm