This is the M4A1 stripped barrel, no front sight post, factory Colt, as used on the M4A1 Block 2 rifle and upper receiver group.
Charlie's offers barrel pinning service, with your choice of muzzle device. You can also get pinned and welded by a local gunsmith. I will tell you that our pin and weld service is the best in the business, because we searched long and hard and found the best shop, and we send our barrels out with muzzle devices to be pinned and welded so that your barrel is street legal at 16.1" or more. Our pin and weld service is also a little expensive, as it includes shipping to our pin and weld shop and return.
Charlie's Custom Clones will also attach your choice of low profile gas block for a modest fee, but we need your barrel nut sent to us, or buy a rail with barrel nut from us.
Note: The unpinned barrel is subject to all NFA laws and local regulatory oversight for a short-barreled rifle. Be sure you are familiar with all laws and that you can own and possess a short rifle barrel, or short barreled rifle.