Totally forgot I had this from an AR-15 I used to own. Gutted as it doesn't fit the S&W 15-22 or I'd be using it! Its an original/steel model Redi-mag which makes magazine changes ridiculously quick and easy, simply hit the mag release button and both the mag in the gun and one in the Redi-mag drop at the same time, drop the fresh mag into your hand and slap it in the now empty mag well.
Here's a video explaining its use from a someone who clearly likes Wolverine;
It will work on the vast majority of AR's, including .22 versions as its been specially cut to allow the higher feed lips some mags have (e.g. Ceiner style). This will NOT fit on a 15-22
or any non-milspec lower receivers (such as the SGC solid billet version). The only place i've found them for sale in the UK is at Brownells for £113?!
http://www.brownells.co.uk/epages/UK.sf ... /100004455
I'm looking for £50 posted