Here's one of the rarer Colt experimentation project. The Colt 703 is a rare piston-driven take on the AR platform, literally almost an AK but AR. The experimental 5.56mm rifle that traded the standard gas system for the AK style piston layout was due to some reports in Vietnam War regarding its reliability and fouling.
With selections of barrels and handguard, as well as its unique Stoner-63-esque upper, the rifle fires at a slightly slower rate (supposedly? Source required). The upper has access to a hidden top rail.
There are technically only two real life examples in the world.
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Model from Oxy and texturing/implementation done by me
