Precision and weight and upper choice

[JediGuy]

Help me out with this, if someone would.

The MUR and Mk2 either add mass or rearrange it on the upper. The standard BCM and Mk2 both have a “intereference” or friction fit, as does the Bootleg lightweight upper. The CMT and FCD uppers both seem to add some mass and redesign.

I get the principle of a friction fit, and it makes sense to me. Movement between upper and barrel = no good. Same for truing and lapping.

But, how much precision gain is there from the redesign or increased mass of an AR upper receiver? I understand it increases rigidity of the upper. But, assuming a return to aim at the exact same point at 100 yards, with a tight fit from barrel to upper...is the benefit there?

Along the same lines, particularly if the general belief is that upper rigidity helps, is there any decrease in effective precision when using an upper without a forward assist? Clearly, there is reduced mass.

Hopefully that’s understandable. I am absolutely not an engineer.

Reason for asking: I don’t want a separate complete rifle, but I do want to rework some things to have a nice, precise upper that I can toss on my SBR lower at times.

[JediGuy]

https://www.m4carbine.net/showthread...sting-Part-One

https://www.m4carbine.net/showthread...sting-Part-Two

[MistWolf]

Less mass does not always equal less stiffness. A stiffer upper does not guarantee improve precision, but it does eliminate a variable. A flexible upper will move with less force and may not return to its original node after moving around and, as the sights are mounted directly to the upper, the flexing could be changing POA/POI.

The VLTOR MUR is a good, stiff upper and only weighs about an ounce more than a standard forged upper. I have a forged MUR upper with a 20 inch Lothar/Walther barrel that shoots between .5 and .7 moa with good ammo.

Truing the upper and a tight barrel fit are other steps that do not guarantee improved precision, but again, they eliminate variables. If the barrel shifts in the upper, POA/POI shifts.