 The cartridge, upper receiver, and operating mechanism are all new designs by KAC.The KAC PDW uses a completely side-folding stock, unlike the M16 and M4 designs which have their main operating spring in a tube in the stock, and therefore can only partially telescope, and not fold sideways at all.The KAC PDW is over 10 inches shorter (19.5" vs. 29.8" with stocks folded) and more than 1 pound lighter (4.5 lb vs. 6.3 lb) than the currently serviced M4 carbine, a carbine in service with every branch of the US Military, and the barrel has been lightened with a new dimpling process.