6061 vs 7075 Billet Lowers

There have been lots of debates and mis-information regarding Billet Lowers vs. Forged Lowers and the use of 7075 aluminum vs. 6061.  This may clear it up for some of you.

The original design intent of the Stoners Direct impingement AR was for a 7.62 caliber and not 5.56, and was originally fabricated from steel.  After conducting field trials, however, it was determined that aluminum would be used rather than steel.  Due to this fact, the Military requested 7075 aluminum,  as no modern military firearms had been built from aluminum.  They wanted the highest strength aluminum available, which is forged 7075 material.  They had no testing data to assess if it was even feasible to produce a weapon from primarily aluminum.  Well, that was the 1960’s and we have learned much since then.  Stress testing on the aluminum AR15s was carried out on varying material grades and strengths.  Since then, aluminum has been the dominating material for AR-15 parts, due to its light weight-to-strength ratio, corrosion resistance, and ease of manufacturing/fabrication.  Most AR parts are machined from either 6061 or 7075 series aluminum.

Composition:  The composition of 6061 aluminum includes magnesium, silicon, iron, copper, zinc, titanium, manganese, chromium, etc.  The composition of 7075 aluminum consists of the same alloys but differs in the amounts of magnesium, silicon, iron, copper, zinc, titanium, manganese, and chromium, as well as additional metals, which make up the series.

For additional details and exact percentages, please view the following resources:

6061 – http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MA6061t6

7075 – http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MA7075T6

Key Properties 

In the 6061 vs. 7075 aluminum debate, it’s important to remember that the key properties of both types of aluminum are what make them ideal for a variety of AR parts.

• The properties of 6061 aluminum include corrosion resistance, weld-ability, medium to high strength, and good work-ability.

• The properties of 7075 aluminum are high strength, stress resistant, work-ability and improved stress:corrosion resistance.

Although 7075 aluminum does offer some additional strength over its 6061 counterpart,  6061 is entirely adequate for most AR-15 applications.

Strength 

Some AR-15 aficionados would declare 7075 to be superior when choosing between 6061 vs 7075 aluminum. This belief is due to the mechanical properties of 7075 aluminum.

Although the mechanical properties do differ between 6061 and 7075 aluminum, remember your AR-15 will most likely never be subjected to the extreme PSI’s listed in the following data.

6061 – Mechanical Properties 

• Ultimate Tensile Strength 45000 psi
• Tensile Yield Strength 40000 psi
• Fatigue Strength 14000 psi
• Shear Strength 30000 psi
• Hardness, Rockwell 40
• Machine-ability 50%

7075 – Mechanical Properties 

• Ultimate Tensile Strength 83000 psi
• Tensile Yield Strength 73000 psi
• Fatigue Strength 23000 psi
• Shear Strength 48000 psi
• Hardness, Rockwell 53.5
• Machine-ability 70%

Difference

When it comes down to it, 6061 will work just as well as 7075 as your billet lower does not see the stresses that the upper and barrel do.  It is important to take into account what your AR will be used for;   one must also consider the fact that thousands of AR builders have used 6061 aluminum without a single issue.

Beyond the type of material,  from which your AR parts are manufactured, how they are manufactured is the most important factor.  Knowing that the manufacturer pays attention to detail and provides quality control will ultimately determine the quality of product that is produced.