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So I got two of the Bison Armory shim kits today, each has 4 shims, 2x0.002 and 2x0.001. Well I put all 8 + 1 extra 0.001 shim on and it just closes the 0.0055 gap. Either my feeler gauge is mislabeled or the Bison Armory shims are not quite as advertised. Anyway I tried to install my Brigand Arms barrel nut and I just couldn't get it to square away and I'm afraid to go beyond 45ftlbs with an aluminum jam nut. So I figured maybe I'll remove one of the smaller shims and see if that gives me enough. Nope, I can't get the barrel out of the receiver to remove a shim. It comes out about 1/32 inch and then something hits/grabs and it will not come out any more. I could pry it off but then I'd ruin the shims. I tried smashing it out with a polymer AR15 upper receiver gas tube alignment jig but the polymer has too much give and won't break the barrel free. The barrel is really locked down. Wondering if I hadn't fully closed the gap and maybe torqued the barrel pin and created a bulge. But that don't explain why it moves freely for the 1/32 inch. Any ideas? And the option that I'm already working on is to cut and add another shim... but I need more shims. Not sure how google didn't pull up these other options last week, but I found that Aero and Spike's both sell shim kits. I ordered the Spike's because they specify the sizes. https://www.spikestactical.com/collections/upper-parts/spikes-308-shim-kit/. If I were doing it again I'd start with the Spike's as it has a big dog, 0.025 which would have probably saved me a bit of trouble.

Good points. Thanks for all the input. I'm new to building. This is only my second upper build and my first big-frame. The point of it being light enough to use for still hunting hogs. Shots won't ever be outside of 200 yrds. With a 16" pencil barrel and budget receiver I'm not expecting or needing too much accuracy to begin with. So I'm willing to roll the dice on accuracy as long as I'm not doing something that runs the risk of literally blowing up in my face. And I like the idea of trying something that's easily undone before taking path of no return. I'll update when I have data. Probably won't sway anybody, but hopefully I'll still be able to get 6 inch groups at 200 yards.

Bison Armory has this video posted where they cut the shim to install between the flange and the pin. Is this a viable option or is there a reason why this is not a good idea? I'm worried about removing any material from the receiver or the pin because the feed ramps match exactly with the pin pressed against the receiver. If I removed any material I'm afraid that the barrel extending over the feed ramps would cause feeding issues. Thoughts?

Thanks!

Hmm, that sounds so simple. Any reason why my gunsmith wouldn't have recommended this strategy straightaway? The gap is ~0.0055 inches. Do you have a recommendation for where to source a shim set?

So I saw this article on Shooting Illustrated and figured hey I should recreate this. https://www.shootingillustrated.com/articles/2018/12/27/building-a-lightweight-ar-10-on-a-budget/ I'd always wanted to a lightweight .308 but had always been too afraid of mis-fit parts. But I figured the article to be a road map. Ordered up all the parts I could afford at the moment and got to building...and immediately ran into trouble. The Faxon Firearms barrel does not mate properly with the Live Free Armory upper receiver (pics attached). There is a gap between the barrel ring and the upper receiver. The indexing pin is stopping the barrel from going in any further. I thought about filing down the receiver indexing slot but if I did the barrel extension would overrun the feed ramps in the receiver as currently they are perfectly aligned. Live Free Armory states 0.5 inches from the receiver body to the indexing slot and 0.6 inches from the receiver body to the end of the threads. This checks out with my receiver. Anyone have another brand upper to confirm/compare? My query out to Faxon hasn't yet been answered but I get 1.23 inches from the ring to the end of the barrel extension and 0.145 inches from the ring to the far side of the indexing pin. Are these measurements to spec? Has anyone ever met with a similar mis-fit issue? And if so how did you move forward? Is there anyway to shim the gap? Thanks.