What it does
The BF-10™ is a loading device used for reloading an AR-15's magazine while it is still in the firearm. It performs the same function as a stripper clip for an SKS or similar rifle.
Several states have passed laws which ban semiautomatic rifles with detachable magazines and certain cosmetic features (pistol grips, collapsible stocks, muzzle devices, etc). One means of complying with these laws is to remove those features or "go featureless". This results in a less functional rifle, that is less fun to shoot.
The other option is to keep the pistol grip and other features but use a fixed (non-removable) magazine. Until the BF-10™, this meant reloads would involve breaking open the action of the rifle and either inserting ammunition into the magazine or removing and replacing the magazine. With the BF-10™, the rifle action stays assembled, and the rounds are pushed into the magazine through the ejection port.
How it works
The BF-10™ must first be filled with cartridges. This is done by simply pushing them into the cartridge-shaped opening at the end. A retaining tab keeps the rounds securely held in the BF-10™.
When the last round is fired from the rifle, the bolt will lock back. The BF-10™ can then be inserted into the ejection port and "rocked" to lock it in the correct position. Then the rounds can be pushed into the fixed magazine with the natural motion of squeezing the handgrips and sliding them all the way toward the rifle. When the magazine has been filled, the BF-10™ can be rocked out of the ejection port. The bolt stays back, allowing the user to chamber a round via the bolt release paddle as per normal AR-15 operation and resume firing.