Law Tactical has developed an adapter system which allows any AR-15 stock to fold to the side. The system works for all commercial and mil-spec AR-15s.

The system requires an extension to the bolt carrier (which is supplied in the kit). The biggest problem with the kit is that firing the gun with the stock folded disables the gun.

The system sells for a reasonable $179.

UPDATE:

If the gun is fired with the stock open, the bolt will be locked, it won’t injury the user.

The Bolt Carrier Extension part, when installed correctly, will hold the Bolt Carrier Group in place.
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**The Bolt Carrier Extension will also stop the Bolt Carrier Group from flying out and causing any injury or loss of life if someone should accidentally fire the weapon with the stock folded. If fired when folded, the locking finger will bend, preventing further operation of the weapon until it is replaced. This is done to eliminate the possibility of catastrophic failure of the adaptor due to repeated misuse.

[ Many thanks to Paul at Accurate Shooter for the tip. ]