Honest truth about billet uppers

[Fighting Tenth]

What does the barrel attach to? The upper, if the upper is out of whack, that will effect accuracy. What happens if the upper is flexing?

Yeah, I suppose theoretically this could happen - I just haven't seen it in practice.
Does anyone (like perhaps the USAMU) have any hard evidence to support this happens?

[mstennes]

Yeah, I suppose theoretically this could happen - I just haven't seen it in practice.
Does anyone (like perhaps the USAMU) have any hard evidence to support this happens?

I do sorta, I guy here brought in his Model 1 parts kit to be built. After I had it up and going it never would group worth a shit, and made a funny noise. I took it all back apart and couldnt see a thing wrong, so I swapped in a barrel I knew that was good, same problem, so I kept looking and never could see anything, untill I started measuring, and low and behold it was off, and I also noticed under a magnifing glass it little stress fissures in it by the barrel area where it was flexing under load.

[MistWolf]

As the optics and/or rear sight is normally mounted to the upper receiver, if the receiver does not consistently hold the barrel assembly there will be a shift in the point of impact. The upper receiver could possibly affect barrel harmonics also causing inconsistencies in the POI

[Cesiumsponge]

Let me chime in from a manufacturing standpoint.

Forging creates a denser metal grain structure when the metal flows under forging pressure and orients surface grain flow to follow the curvature of the features forged into the blank. It increases strength. The same strength gains can be seen in other manufacturing techniques that plastically deforms the material being processed, such as hole mandrelizing, button rifling, thread rolling, cold-form tapping, etc. It's all run-of-the-mill processes used in commercial and aerospace manufacturing. None of it is unique to the firearms industry. You'll find a lot of folks in the automotive hobby tend to have a lot of wild ideas about billet parts too. While not a direct comparison, spun cast aluminum alloy wheels and forged aluminum alloy wheels have a huge weight disparity (and pricing too) because forgings can "do more with less".

Machining from a blank is nice because the shape is up to what the engineer and programmer wishes to do on the machining center. You can do smaller runs while keeping reasonable costs-per-unit because you're just investing in additional programming time and not new tooling if you change the design. Forging is a pretty expensive process to set up. Making any changes requires the manufacturing of a new set of forging dies.

The accuracy is up to the engineers. Mil-spec receivers are stuck to a specific shape and set of tolerances and cannot add anything extra without falling outside the print dimensions. Billets can be stronger than forgings simply because they can leave more material, can reinforce areas that are prone to deformation, and aren't limited to the blueprint shape of the receivers so it's not really an apples-apples comparison. If you have a forging of identical mass and shape to a billet, it'll be stronger than the billet. Forgings receive further machine work to clean up dimensions, true up holes, and whatnot so any meaningful tolerances in regards to flatness, parallelism, etc, is determined by the print specs and how tight the machinist wants to hold those specs. A tolerance might be -/+ 0.010" so a part held at +.0095" is just as good as a part that measures -0.0003" because it's within print specs. If it takes additional time and effort (which translates to money) to hold smaller tolerances than the print allocation, it makes no financial sense unless the vendor raises the price of the parts and the company is willing to absorb that cost.

How a company executes the machining processes used ultimately determines the product. Garbage in, garbage out. Dandy in, dandy out.