Originally Posted By ARctangent:
Think of it this way:
All metals have a grain structure to them...vaguely similar to wood.
With a billet lower, in the process of machining a lower receiver, there are many opportunities to "cut across the grain" as it were.
When that happens, there are inherent weak-points where the possibility of a stress riser can occur.
In an attempt to counteract such problems, they thicken the lower up where they can in order to increase the modulus of elasticity (essentially stiffness).
In a forged lower, the billet block is heated, and then further forged in several steps of forging and finishing dies to achieve the final shape, and then through a final heat-treating.
This process essentially takes the grain structure and forces it to conform to the basic surfaces of the lower receiver.
This minimizes the chance of any internal stress risers occurring, and further increases the material density beyond that of the billet's state.
That allows the manufacturer to use less of a (essentially stronger and more durable) material.
Basically less of a denser material, or more of a material that is less dense...six-of-one, half-dozen-of-the-other to me.
The only way to really see a noticeable difference between the two would be in the abuse that a firearm would see in daily combat over extended periods of time.
Likely why the .mil prefers forged uppers & lowers.
Myself, I own both, but prefer forged...though forged is not as pretty as billet.
Just my $.02.
Right. For the military it's not really a matter of preference but of price; forged lowers and uppers are super cheap in quantity as machine time is vastly reduced as opposed to billet. Consider too that the government doesn't concern itself with cosmetics and the appearance of forgings, like many consumers do. And from a machining standpoint, a billet is much, much easier to achieve a level of precision that's frustrating to get with a forging. The fixtures and setup for forged lowers are a pain in the ass and unless you are maching a shit ton of them, billet might be a better solution for a small custom outfit (from a manufacturing standpoint that is). And billet lowers can incorporate all kinds of additional features including integral trigger guards, large magwells, ambi controls, etc. Of course you can do the same thing with a custom forging die (like Noveske has done) but machining can still be quite the exercise (probably where some of the chainsaw lowers are coming from) and you have to sell a bunch to recoup the cost of the die.
As far as strength, a forging is almost always "stronger" than billet but there can be situations where that's not the case (when forged grain flow is subject to stress counter its principal design load). But in an AR, particularly the lower receiver, you'll never see any difference as far as durability between the two.