Assuming you are in the US, and subject to US law- yes and no.
The TRUE CQBR is a military weapon, and not for sale.
However, you CAN buy a civilian equivalent to use on an AR style rifle. There ARE other changes to the rifle (gas rings on bolt, extractor, etc). HOWEVER- this change will result in a rifle with a barrel shorter than 16 inches.
As such, under Federal law, this is a Short Barreled Rifle, regulated much like a machine gun under the National Firearms Act. The transfer or making of such a firearm must be approved by the BATFE, registered with them, and a $200 tax paid.
There is no, "CQBR MK16" version. If you are referring to the "MK18" version, then yes. It has a version 2 gearbox that is all metal; however, the hop up is plastic.
You can buy a plastic m4 or you can buy a metal m4
M4 Commando? Is that some video game weapon? The M4 carbine and variants (such as the CQBR) use the 5.56 cartridge. Standard is 62 grains, although heavier grain bullets are recommended for shorter barrel lengths. Colt did market a 9x19mm variant of the CAR-15 under the name "Commando", but it's not considered an M4 variant.
The M-16 Carbine or M4 use a 5.56x45 Nato round or the civilian version .223 Remington
AR-15 = Semi-Auto civilian rifle M4 = Select Fire Military rifle
A semi-automatic M4 derivative can cost as low as $800 from ImpactGuns. But for a civilian to acquire a fully automatic, military grade M4, be ready to spend up to $3000.
You wil have to specify if you are looking for military, police or civilian.
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The M4 carbine is chambered for the 5.56x45mm NATOcartridge. It can also fire the civilian .223 Remingtonround.
M4 is a nomenclature which was coined by the military. Is is a select fire weapon, with both semi-automatic and either burst of full auto firing modes. Civilian legal lookalikes which fire semi-auto only can be purchased, and some (such as Bushmaster's) are even marketed under the name "M4". However, most of these "M-forgeries" do not meet the specifications prescribed by the Department of Defence for the actual M4 carbine.
Buy the best you can afford and like.
if you've got the money then id say go for the benilli m4.