These are custom setups for open class in USPSA/IPSC with special feed ramps bolted to the underside of the shotgun. Now this one might be able to be retrofitted to your shotgun fairly easily, although I don't think this is the exact same kit you see in the video. Not sure about pump guns, as I've never heard of anyone putting one on a pump shotty.
Reference: http://www.tigervalley.com/pdf/3gunrules.pdf Section 6.7.4(a) + (b) which calls for Speed loaders to have a Primer Relief cut. Does anybody know if the ArmsTech Speed Loader (as seen in the initial post video) incorporates the "relief cut"? What is the spec for the "Relief Cut"?