You must be 18 to buy a rifle. The stock has nothing to do with it.
A collapsible rifle butt.
Yes, it is legal. Florida has no AWB in effect. However, anything you add must be US made.
100-1000 usd
720 mm (28.3 in) stock extended / 500 mm (19.7 in) stock folded.
5000-10000 usd
We can't answer that. It oes not violate US Federal law, but SOME states have their own laws, and you did not share what state you are in. A good start would be to check with the State Police or a gun dealer in your state.
To make a 56 assault rifle, use a fixed wooden stock and a permanently attached spike bayonet.
In MOST states it is 18 to own any shoulder-fired weapon. In order to own what the military considers an assault rifle, you also have to have a Class 3 Federal Firearms Lisence, but some states and cities definition of an 'assault rifle' includes anything that looks dangerous and NO ONE is allowed to own one there.
To make a 56 assault rifle, use a fixed wooden stock and a permanently attached spike bayonet.
5.56 x 45mm NATO is the caliber. The weight is 6.2 lbs, and with stock open, is 28.3 inches long. With stock collapsed, 19.7 inches.
You will have to check with the company that made it.
The M4 assault rifle has an overall length of approximately 33 inches (83.8 cm) with the stock fully extended, and its barrel length is about 14.5 inches (36.8 cm). It typically weighs around 6.36 pounds (2.88 kg) without any attachments or accessories.