ArmaLite's SPR-A1 is a 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. That's why the SPR-A1’s three o’clock, six o’clock, and nine o’clock rails are detachable. Users can change 1913 rails to fit their changing needs for sights, lights, lasers, grips or swivels. Each SPR-A1 comes with three extra rails standard at no extra cost and includes a a 1913 rail with a quick detach sling swivel hole. Each rifle also has a low insert for a bare rail with a low profile, and a plain insert with a quick detach sling swivel hole. Its one-piece construction offers a continuous sight platform. This one-piece construction gives the sighting platform extra rigidity. It’s a good, strong backbone. That means your sights will stay zeroed in the most trying conditions.