Say you are shopping for a new lower receiver at your local gun store or online. You come across several highly-rated receivers, but they are labeled as 7075-T6 or 6061-T6. What do those numbers represent?
Both lowers contain magnesium, silicon, copper, iron, and zinc, though the 6065 version contains less zinc than the 7075 receivers. Because it has less zinc, 6065 is easier to weld, though its strength resistance suffers significantly.
How about the T6 at the end of each designation? T6 is the grade of tempering. The scale ranges from T1 to T10, with T6 as one of the most common among lower receivers. T6 is strengthened via heat, then artificially aged for a strong, durable, and resilient receiver.
7075-T6 Aluminum Lower Receivers
Durability. Strength. These two descriptors are perfect for 7075-T6 lower receivers. A 7075 lower is built of high-grade aluminum, often found in aerospace applications, marine craft, and transportation projects. It is light, durable, but also expensive.
In a lower, 7075-grade aluminum is less-than-ideal unless you’re a large machine shop. It’s too expensive and too strong to cut and mill from a blank receiver easily. If you attempt to drill using average drill bits and endmills, as most of us will, it will wear them down significantly quicker than usual. The cost of new drill bills and other equipment will quickly add up.
7075 Mechanical Properties:
- Ultimate Tensile Strength — 83,000 psi
- Tensile Yield Strength — 73,000 psi
- Fatigue Strength — 23,000 psi
- Shear Strength — 48,000 psi
- Hardness, Rockwell — 53.5
- Machine-ability — 70%
6061-T6 Aluminum Lower Receivers
Alternatively, the 6061-T6 receiver is made of “lower-grade” aluminum. But in this case, the term “low-grade” is a misnomer. We’re not talking about subpar quality aluminum. It’s still high-quality, durable, reliable, and aerospace grade — everything you want in a firearm’s lower receiver. However, it’s not as durable as 7075, which, in this case, is acceptable.
When building out an 80% lower, you honestly want 6061-grade aluminum, as it is softer and more cost-effective. It’s easier to cut and mill as a softer grade metal, which means you won’t waste endmill bits or drill bits in the process.
Furthermore, just because 6061 isn’t of the same grade as 7075 does not mean it isn’t strong. It still holds up to tens of thousands of pounds per square-inch. You can literally drive trucks over it, without risk of damage. If something manages to destroy your 6061 lower, it’s probably destroyed you as well.
6061 Mechanical Properties:
- Ultimate Tensile Strength — 45,000 psi
- Tensile Yield Strength — 40,000 psi
- Fatigue Strength — 14,000 psi
- Shear Strength — 30,000 psi
- Hardness, Rockwell — 40
- Machine-ability — 50%
Which Lower Receiver Grade is Better?
In the end, we typically opt for 6061 aluminum for 80% lowers. Because it’s slightly softer, it allows for easier milling and lets your tooling last longer. Plus, 6061 is more resistant to corrosion, which matters when you have a raw & uncoated trigger pocket after you finish milling
Sure, personal preference will play a factor in your purchase, but don’t ignore the clear savings associated with 6061 receivers. While some drill bits are relatively cheap, they do add up over time.
We highly recommend picking up an 80% receiver to start your journey. Building an AR-15 is a rewarding experience, full of opportunities, that concludes with an exceptional new rifle in your collection.