ArmaLite Special Purpose Rifle (SPR) Mod 1 is forged, one-piece upper receiver/rail system with exclusive detachable side and bottom rails. ArmaLite knows you and your firearm often have more than one mission, and you want options. ArmaLite gives you just that. The SPR Mod 1's three o'clock, six o'clock, and nine o'clock rails are detachable. That's right, you can change your 1913 rails to fit your changing needs for sights, lights, and lasers. ArmaLite takes that versatility even further. Each SPR Mod 1 comes with three extra rails standard, at no extra cost. Its one-piece construction offers you a continuous optic platform (COP). There's no gap to bridge between receiver and upper rail when you're installing your scope or sights. That means your sights will stay zeroed in the most trying conditions.