The cavity in the survivor stock has two cavities separated by a partition. The larger cavity is aprox. 1" wide by 2" long by 6 3/4" deep. The smaller cavity is 1" x 1" x 6.75" deep. The partititon is 5.5" long from the bottom of the cavity to its top which leaves a full space in the stock near the buttpad of 3 1/4"L x 1"W x 1.25"D.
Depending on caliber you could store a lot of rounds in there along with fish line/hooks/sinkers, rolled up space blanket, matches, small compass, etc. Mine is on a 357 Magnum rifle so it will hold a lot of ammo.
The forend has 3 compartments that are: Forward compartment 2"L x 1"W x 7/8"D, Middle Compartment 3.25"L x 1"W x 5/8" deep at the barrel mount recess to 7/8" at the forward partition, Rear compartment 2 7/8"L x 1"W x 5/8"(barrel recess)-7/8" at rear channel.
I can get 21rds of 357 ammo plus room for fish hooks with line wrapped around them and a small compass in there plus you could probably stuff some paper/cotton/kleenex and or matches in there for kindling and to reduce or eliminate rattle in the forend alone.