barrel to upper receiver fit

[earplug]

Have built a couple of guns and notice that some barrel extensions and uppers have different levels of tightness. Drop in, friction fit or whack it.
Curious if there is any information about accuracy and the interface between the two parts?
I like to coat such parts with a good anti seize paste to allow future disassembly if it matters.

[SAnd]

An AR15 upper receiver drawing I have from the internet has the dimension of 1.000 + .002 inches. I pretty much trust this drawing.

A M16 barrel extension drawing that looks like an official military spec drawing has the outside diameter where it goes into the upper receiver as .9987 -.0008 inches.

Math makes my head hurt so I'll let you crunch the numbers.

Just remember that there is no problem that can't be solve with a adequate application of High Explosives. Or a Big Freaking Hammer.

[BPTactical]

Fits will depend on as you have discovered, variance of tolerances.
What you have encountered is normal.
This is part of the "Mil-Spec" shenanigans that many do not understand. Mil-Spec (dimension wise) simply means that the components as supplied fall within a given range of +- dimensions to ensure interchangeability between components of various manufacturers.
Manufacturers matter. Higher end manufacturers such as Noveske, Black Rain, Seekins and such tend to hold the tolerance range to a tighter standard.
Lower tier manufacturers generally accept tolerances over a wider range.
Maintaining those tighter tolerances is one reason for higher cost.

[Not_A_Llama]

Zediker is a proponent of loctite for this reason. I think it helps, but have found that I need the spray-on catalyst to get a cure with stainless barrels.

[fmj50]

BPtactical, Do you guys do bluing, like a real bluing tank?

[Great-Kazoo]

BPtactical, Do you guys do bluing, like a real bluing tank?


Not any more
https://www.ar-15.co/threads/158059-A-public-announcement-from-BP-Tactical-Services

[MarkCO]

Tight fit, better yet thermal fit, will produce the best accuracy over a loose fit all else being equal. With passivated metal, loctite does not help unless like stated in post #4, you use the activator.