A personal defense weapon (PDW) is usually defined as a light, compact and easy to use firearm. However the definition varies depending on context and who you ask. Many will still say it is a Short-barreled rifle (SBR). These are regulated in the United States by the ATF. Many SBRs are available as selective fire (can switch between semi-automatic or fully automatic fire). The United States Department of Homeland Security defines a personal defense weapon as an M4 carbine (or a Colt AR-15 civilian semi-automatic rifle). Some governments' definition of PDWs includes handguns. An example is the Czech Republic which has proposed a constitutional amendment that would allow its citizens to own and keep PDWs as a way to discourage terrorists. Regardless, a PDW is usually agreed upon as being somewhere between a submachine gun and an assault rifle.