Latley, “improving” the AR platform by developing piston-driven ARs in lieu of Stoner’s original gas impingement design seems to be taking over the AR rifle scene. So why the rush to modify the 50-year-old direct impingement design in the name of improvement? The cynical answer is that this is what gun designers/companies frequently do in the quest for market share via promising a better product. The more positive response is that the quest to improve a design never ceases. However, this is not a new phenomenon for the AR; it began at its inception in the early 1960s. Soon after ArmaLite, then a division of Fairchild, spawned the AR-15, they sold off its rights to Colt. ArmaLite almost immediately turned to improving the AR-15 design, all the while being careful not to violate any patent rights.
Arthur Miller, George Sullivan, and Charles Dorchester were tasked to create an alternative to the AR-15 in the form of the AR-18. This was accomplished by turning to a piston operating rod action combined with using sheet stampings in the manufacturer of the lower and upper receivers. The AR-18 was equipped with a now all-too-familiar short-stroke gas piston located above the barrel. The AR-18’s bolt carrier rides two metal guide rods that double as return springs; this is different than the AR-15 design, which has the bolt carrier in contact with receiver rails and return spring located in the buttstock. This not only allows for the AR-18/180’s stock to fold for a more compact form, but it also creates greater clearance and tolerance to foreign materials in the action—not to mention the short-stroke piston prevents dumping hot gases directly into the receiver. Further differences between the AR-18 and AR-15 are evident in the charging handle on the AR-18 being connected directly to the bolt and reciprocating with the action, versus the AR-15 charging handle design. The more conventional AR-18 bolt allows a user more purchase when operating the weapon during loading, malfunction clearance, etc.
The U.S. military purchased a handful of AR-18s in 1964 for evaluation before settling on the AR-15/M16. Instinctually, the gas-piston ARs appeal to many by the very fact that hot gases and powder residue are not dumped into the action like the original direct impingement (DI) design. Anecdotal evidence of bolt carriers being handled soon after long strings of fire and merely wiped down compared to their DI cousins reinforces this. Current proponents of piston-driven ARs point to greater reliability in adverse conditions and less reliance on routine maintenance compared to the DI operating method. These same arguments were put forth in the 1960s as well, though to no avail, as the AR-15/M16 quickly became entrenched in the military supply chain.
ArmaLite responded to the lack of military sales for the AR-18 by introducing a semi-auto-only variant of the AR-18—the AR-180 in 1969. Looking back and analyzing the facts at hand, this seemed a sound idea with great potential due to little competition in the civilian market for “black rifles,” especially one priced as competitively as the AR-180. Furthermore, Colt had its hands full with producing M16s, with little time to consider introducing the AR to the civilian market. ArmaLite was trying to offer a reasonably priced military-style semi-automatic rifle variant. Indeed, they were ahead of their time in the 1970s. We take such models for granted today without a second thought because of the number of ARs/AKs/G3s/FN-FALs we have on the market.