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Just curious, is there a recommended clearance for the ID of a barrel nut and the outside of the barrel where it meets the upper receiver? I measured what I have and the clearance is about .020, would .010 be better for accuracy, would it have any affect at all?

The clearance that matters is the od of the barrel extension and the id of the receiver where the extension slides in. The nut just holds the shoulder of the extension against the receiver. Ideally those faces are perpendicular to the axis of the extension/ barrel and make full contact.

I haven't looked it up but i would think .001" difference would be ideal but you would have to hold your tongue right to get them to slide together.

That makes sense, and I guess is the reason for lapping tools.

Yup. I have a tool and compound if you need to borrow it or lap it real quick in the DFW area. It takes less than 5 minutes to do if it is even needed. Most uppers rarely need to be lapped

I know I'm backwards on this but I went ahead and slapped it together yesterday morning, we'll see how it runs when I get it finished out. One problem I know I'm going to run into though is that the barrel is heavy enough that I have to file down the screws on MLOK inserts in order for them to not touch the barrel when tightened down. No biggie, all I needed is a QD sling point and bipod.

kayt00 wrote: Thu Aug 27, 2020 6:16 am I know I'm backwards on this but I went ahead and slapped it together yesterday morning, we'll see how it runs when I get it finished out. One problem I know I'm going to run into though is that the barrel is heavy enough that I have to file down the screws on MLOK inserts in order for them to not touch the barrel when tightened down. No biggie, all I needed is a QD sling point and bipod.

I have a lapping tool, and I’ve assembled them both ways, with and without lapping. I can’t say as I’ve seen much difference in performance, but only one was what you’d call a “precision” rifle to begin with, and I used a machined billet receiver for it anyway that was already square. The amount of metal removed to square any of the other receiver uppers I’ve used has been so negligible that I’m honestly not convinced that it’s a necessary step for most AR type rifles, unless you’re maybe working with a “factory seconds” discount unit. Even then, the “seconds” uppers I’ve bought from BCM have been square right out of the box, so it probably depends on who made it.

I’ve also disassembled factory assembled uppers, and exactly zero of them were lapped at the factory....unless they were lapped first and anodized/painted afterwards. So, unless overdone, it doesn’t hurt if you have the tool and the time, but if you’re building a basic carbine from quality receiver components, it’s not the end of the world if you don’t do it.

Well, if it comes down to it and I feel like I need to do something else to it I'll start at lapping. It's nothing to yank an upper apart. As it stands everything I own shoots better than I do so there is that.