If you are planning a new AR-15 build and are looking into the various uppers and lowers available, you are sure to come across the different types of aluminum used. There are two different types of aluminum that AR receiver manufactures use: 6061-T6 and 7075-T6. You might ask what’s with all the numbers?
Without getting deep into science, cast alloys are given a four-digit grading system that denotes the composition of the alloy. 6061 is Aluminum/ Magnesium-Silicon while 7075 is Aluminum/Zinc-Magnesium. The T suffix stands for a thermally treated temper and 6 means that the aluminum was solution heat treated and artificially aged.
The numbers and temper come into play when you look at the trade offs of each type of aluminum.
Back in the day, M16 receivers used to be made out of 6061-T6 but around 1968 the military changed the specification to 7075-T6. Why you may ask? Well, during the Vietnam war, the M16 had a plethora of problems including flash suppressors getting caught on vines and jamming problems due to lack of cleaning. Another problem that M16 rifles in Vietnam had was corrosion due to the hot and humid temperatures of the jungles. That, combined with sweat, caused the rifle’s receivers to exhibit irregular corrosion. Per Eugene Stoner’s suggestion, the military made the switch to 7075 due to irregular corrosion on the aluminum receivers, NOT due to strength.
Basically 6061-T6 is easier to work with especially when milling and welding and has a higher resistance to corrosion. 7075-T6 is harder than 6061 which makes it slightly more difficult to work and weld with and it also has an average resistance to corrosion.
Most aftermarket billet receiver manufactures use 6061 because it is easier to work with in terms of hardness; most aluminum handguard rails are made of 6061 as well. So why does 6061 take a lot of flak for being not as strong if it is so commonly used? Most of the hate comes from armchair elitist who will say stronger is always better and to never use lesser materials. When it comes down to the wire, stronger aluminum can’t hurt, but 6061 works, and has worked, for decades.
What does this mean for you?
Go ahead and buy that cool billet lower that is made of 6061 and be happy with it. Don’t worry about it breaking, despite what all the non-scientist elitist say. Just don’t take it to Vietnam.