In many countries, the sale, possession, and use of model rocket motors is subject to governmental rules and regulations. For example, in the
United States, high-powered motors (above 160 newton seconds) are restricted to citizens who possess the requisite certification and permits.
from wiki
looks like above a G requires permits
ClassTotal Impulse
(Metric Standard)Total Impulse (Imperial Standard)
A 1.26-2.50 N·s 0.29-0.56 lbf·s
B 2.51-5.00 N·s 0.57-1.12 lbf·s
C 5.01-10.00 N·s 1.13-2.24 lbf·s
D 10.01-20.00 N·s 2.25-4.48 lbf·s
E 20.01-40.00 N·s 4.49-8.96 lbf·s
F 40.01-80.00 N·s 8.97-17.92 lbf·s
G 80.01-160.00 N·s 17.93-35.96 lbf·s
H 160.01-320.00 N·s 35.97-71.92 lbf·s
I 320.01-640.00 N·s 71.93-143.83 lbf·s
J 640.01-1280.00 N·s 143.84-287.65 lbf·s
K 1,280.01-2,560.00 N·s 287.66-575.30 lbf·s
L 2,560.01-5,120.00 N·s 575.31-1150.60 lbf·s
M 5,120.01-10,240.00 N·s 1150.61-2301.20 lbf·s
N 10,240.01-20,480.00 N·s 2301.21-4602.40 lbf·s
O 20,480.01-40,960.00 N·s 4602.41-9204.80 lbf·s