Maybe because of the flash hider on the muzzle, the barrel length, and the ten-round magazine?
I don't know if your son has fired the thing, but look out! When you reduce the barrel length and the weight that much, but keep the cartridge the same, you get a very impressive amount of recoil. It also has an accuracy problem (probably due to the reduced stock allowing the barrel to move).
It had the shortest in-service time of any standard British military rifle, and was removed from service due to inherent flaws in the design.
I've got one anyway! It's not considered an "assault rifle" in Georgia . . . whatever that is. :-D
Correct on all accounts WRT the #5 jungle carbine.
http://www.surplusrifle.com/no5/index.asp
The Navy Arms version in .308 apparently didn't correct any of those problems.