Which One is Better? Well, It Depends and Here’s Why.
This is an age-old debate and as always, the answer is it depends on use and mission. If you are familiar with the debate, I’m sure you have heard the phrase “thermal expansion.” If you have been following along and have read my rant about thermal fitting you will also know that’s how we attach our barrels to the uppers. Good ol’ science strikes again. Now the question is does it apply to the barrel nut and does it affect anything.
The Use of Aluminum Nuts
So, Aluminum expands when heated up. That’s how we thermal fit our barrels to our uppers. We have to heat up the upper to expand the metal in order to slide the barrel in. Once it’s cooled it shrinks the metal and locks the barrel in place. We must get it to a temp of at least 150 F to expand the metal enough.
The barrel nut is what keeps the barrel secure in the upper though. Unlike the barrel it has threads that have to be set and torqued to make sure the weapon operates safely. In theory, you could have an aluminum barrel nut that can get hot and expand to the point that you loosen your torque and could potentially back off over time. In order for that to happen, the barrel nut would have to get hotter than the upper. Since most of the heat from the gun comes from gas system inside the upper this is highly unlikely to happen, but science says it’s possible however unlikely. An Aluminum barrel nut is stronger than its regular steel counter part and lighter. So if your building a light weight build go for it.
Why Don’t We Use Aluminum Barrel Nuts All the Time?
Ok let’s get nerdy for a moment. So thermal expansion in 7075 aluminum, which is what the upper is made of, is about double the thermal expansion in 4140 steel. That means aluminum will expand twice as much as 4140 steel at the same temp. So, to put that into perspective if you had a mile of 7075 aluminum a 40 degree temperature increase would cause it to extend 32 inches. The same test for 4140 steel would only expand 16 inches. If you want to nerd out on this go here
Why Does That Matter?
Well with your upper being made of 7075 aluminum you are going to see it expand when it gets hot. When your upper expands it has the potential to create a loose connection for your barrel. In a non-thermal fit connection, your barrel is not super secure from movement like a thermal fit upper is. So, if that upper expands even more it can create a lot of movement. When I say movement, I’m talking about fractions. Just enough to create a deviation in accuracy and possibly reliability in extreme situations not enough to create any danger.
Now if your upper and barrel nut are the same material, they will expand together so the likelihood of torque breaking is very low. That is when danger would come into play. Now if the barrel nut is made of a material like 4140 that doesn’t expand at the same rate as the upper it will continue to lock that barrel seat in and prevent the barrel from moving too much. When the barrel is thermal fit in there it will prevent the upper from expanding to a point where any movement is possible thus maintaining extreme accuracy in extreme situations. Since 4140 does still expand though it prevents the increased thermal expansion on the upper from creating too much stress to both the upper and barrel nut to the point it causes stress fractures.
Well, there you go. That is why we use 4140 steel barrel nuts and not aluminum. Whatever you choose I don’t think scientifically it will matter for safety reasons, it just may degrade accuracy during your mag dump. Our rifles are just built with extreme use in mind, so we plan accordingly.