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Have never heard of an underwater assault rifle. In the US, you must be 18 to buy a rifle, and 21 to buy a fully automatic rifle.
18 to buy a rifle or shotgun, 21 to buy a handgun or assault rifle.
No.
You can buy a semi automatic Title I rifle at 18. To purchase a true, select fire (auto or burst) NFA tax stamped assault rifle, you must be 21.
You will need to define assault rifle, and where you live. A TRUE assault rifle is a machine gun, and transfers of them are tightly regulated by the Federal government. What most people CALL an assault rifle is nothing but a semi-automatic rifle, and in most of the US, does not require a license to possess.
A TRUE Assault Rifle is fully automatic. Federal law requires you be 21 to buy one of those in ANY state. However, what people may CALL an assault rifle (such as a semi-auto AK47) can be purchased at 18.
You must be 18 to buy a rifle. The stock has nothing to do with it.
18 for a military style semi automatic. For a true assault rifle, which would be an NFA Class III full auto weapon, 21.
Yes, provided you follow all applicable laws.
Maybe, maybe not. Depends on a number of things.
One cannot legally purchase an assault rifle anywhere in the UK. In the UK one may buy certain firearms, notably shotguns, but there are strict conditions attached to ownership.
It's not a real firearm, and, to the best of my knowledge, no replicas are being marketed.