Buffer tube question and SOTAR Larue Upper review

[Disciple]

FCD castle nuts are now Class 3 and a vltor tube will likely be class 2 or better.

How about a BCM tube?

[pag23]

Well I called Larue today on the status of my order...they are back logged 4 to 6 months! So I guess the original person mispoke when they told me 4 weeks.

I am just going to have the lower built and keep the Larue parts when they come in as spares...

[Curlew]

What’s the minimum pitch diameter for a class 2A buffer tube thread? I’m guessing around 1.140”, but if someone could sanity-check that, I’d be grateful.

[curious1]

I think the numbers after finish are the same as the standard for a 1 3/16"-16 UN 2A. So 1.1403" is the chart number in the one I looked at. Numbers are different before it is anodized.

[Curlew]

...1.1403" is the chart number in the one I looked at.

Thanks for that. I finally managed to dig up a table myself, and it agrees with what you found: for class 2A, the PD should be 1.1403 min, 1.1454 max.

Over the years I’d noticed (by feel) a lot of variability in thread fit, and I’d even taken to measuring the buffer tube with a thread micrometer, but I hadn’t known what the actual spec is. But using 1.1403 now as a reference point, it seems that many of the examples I have are on the small side, and they don’t necessarily line up according to the general reputation of the manufacturers.

Two that seem to meet 2A specs are from Colt (no surprise) and Bushmaster (!), but both only barely, at around 1.140 to 1.141. Three others are are in the range of 1.138–1.139, so a little too small, and these are from manufacturers that you would normally think of as making good stuff. Two more are so small (1.134–1.135) that I don’t think they even meet 1A. But the best fit of all: an old commercial tube from Olympic Arms, which at 1.142 lands squarely in 2A. Go figure.