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This is a Colt SP-1 rifle, this was one of Colt's earliest commercial production AR15 rifles after acquiring the rights to the design from ArmaLite. The AR15 was developed by Jim Sullivan and Bob Fremont, basing it around Eugene Stoner's AR10, which was designed around the larger 7.62x51mm NATO cartridge, the AR15 being scaled down to the smaller .223 Remington, later becoming the 5.56x45mm NATO.
This was quite a revolutionary rifle for the times, the receiver is all aluminum, which is doable because of how the bolt locks into a steel collar extension of the barrel, rather than locking into the receiver body itself. This would allow the rifle to be very lightweight, yet also rather sturdy, aluminum is also rather resistant against corrosion and oxidation.
The furniture is all plastic, and the way the gun is laid out has the stock be completely straight and have the cycling of the action 100% inline with the bore and your shoulder, ensuring what little recoil there is comes pretty much completely straight back, basically completely eliminating horizontal rising, something which people hadn't really been doing until then, not in any wide scale.
This allows for the rifle to be very easy to control in rapid fire and on full-auto, in spite of being so lightweight, and it's conducive to good inherent precision.
As the stock is completely straight, the sights must be raised up to your eye level, the rear sight sitting shielded in a 'trough' running on top of the carry handle, the front sight making up a tall and shielded post, the body of the front-sight housing also being the gasblock.
The rear sight is adjustable for windage, it has a large and small 'ghost ring' aperture for short and long ranges respectively, and the front sight is adjustable for range.
The ejection port is protected by a dustcover, as soon as the bolt starts moving back, the spring-loaded door swings open, staying in place until you close it manually.
