Mont Posted March 27, 2019 Report Share Posted March 27, 2019 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. Quote Link to comment Share on other sites More sharing options...
Matt.Cross Posted March 27, 2019 Report Share Posted March 27, 2019 You can shim the gap, but you'll want to know how thick a shim you'll need. I recommend using a "feeler gauge" such as can be bought at any automotive store. Quote Link to comment Share on other sites More sharing options...
Mont Posted March 28, 2019 Author Report Share Posted March 28, 2019 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? Quote Link to comment Share on other sites More sharing options...
Matt.Cross Posted March 28, 2019 Report Share Posted March 28, 2019 5 minutes ago, Mont said: 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? These folks have them: http://bisonarmory.com/ar-308-barrel-nut-shim-set/ Quote Link to comment Share on other sites More sharing options...
Mont Posted March 28, 2019 Author Report Share Posted March 28, 2019 Thanks! Quote Link to comment Share on other sites More sharing options...
98Z5V Posted March 28, 2019 Report Share Posted March 28, 2019 I've stayed out of this, but that's not gonna work here. Barrel nut shims are plentiful, but the work on the outboard side of the flange, before the barrel nut goes on, and they're only for timing of the barrel nut. That won't work here. You can't get a barrel nut shim onto the other side of the flange, to take up slack between the flange and the upper receiver - unless you remove the pin. It's not going on backwards, on the barrel, to take up that slack, unless the pin comes out. You can DO THAT, because the pin doesn't go into the threaded part of the barrel - common misconception. That pin doesn't lock the extension onto the barrel, like alot of people believe it does. If you want to do that, pull the pin, install a 0.006" shim, and reinstall the pin. Done deal. OP, if you're seriously trusting your measurements, and you're a serious 55/10-thou out on that gap, then either deepen the channel in the upper by 55/10-thou, or sand the back of the pin by 55/10-thou. Neither one is going to cause you any headspace issues, or bolt locking issues. If it was me, I'd cut the upper, and be done with it. Quote Link to comment Share on other sites More sharing options...
98Z5V Posted March 28, 2019 Report Share Posted March 28, 2019 26 minutes ago, Mont said: Hmm, that sounds so simple. Any reason why my gunsmith wouldn't have recommended this strategy straightaway? Your gunsmith isn't as well versed on ARs. ARs need a completely different kind of "gunsmith." AR15s don't count. This big-frame is a completely different animal, with all the variations (3 major), then all the screwed up aftermarket "manufacturers" that don't know what they're doing. This isn't common "gunsmith" territory. Quote Link to comment Share on other sites More sharing options...
Mont Posted March 28, 2019 Author Report Share Posted March 28, 2019 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? Quote Link to comment Share on other sites More sharing options...
98Z5V Posted March 28, 2019 Report Share Posted March 28, 2019 (edited) 8 minutes ago, Mont said: Bison Armory has this video posted where they cut the shim to install between the flange and the pin. Get after it, and see how it does - if it solves your problem, make sure to state that. I watched that video, that Matt linked, originally, before I made my posts in this thread. The only reason I'd think that's not a good idea is because it will support your barrel-to-receiver contact surface - except for the width of that cut in the shim. If you get some weird barrel harmonics, weird shot groups, stringing, whatever - suspect that thing first. It's worth a shot, literally, or many - in your circumstance. Give it a try. Let us know how it does. Who knows, it might be perfect. If that is a success, it negates all the people that say you have to "true" the upper receiver in order to have perfect, square contact between the upper and the flange. If you can prove that theory down, it would be EPIC against that practice. WAY EPIC. Personally, I'd like to see that. Too many people say you cannot possibly have an accurate AR unless you true the upper receiver, and "glue" the barrel in place - if you don't = never gonna be accurate. I've never trued one yet, not glued one yet, and I have some that are quite accurate. This test would seal the deal. Edited March 28, 2019 by 98Z5V Quote Link to comment Share on other sites More sharing options...
Matt.Cross Posted March 28, 2019 Report Share Posted March 28, 2019 27 minutes ago, 98Z5V said: The only reason I'd think that's not a good idea is because it will support your barrel-to-receiver contact surface - except for the width of that cut in the shim. That's possibility #1 in my mind. #2 is that you'll want to be absolutely sure there's no overlap anywhere on the shim if you cut it and it 'springs' down smaller than it's original shape. My final thought is that I suspect a minute gap in the shim resulting from a clean cut won't hurt a thing. However, that's given a clean cut; leave a burr anywhere on the edges of the cut and now you don't have uniform pressure behind the barrel extension shoulder. Something to keep in mind. Quote Link to comment Share on other sites More sharing options...
Mont Posted March 28, 2019 Author Report Share Posted March 28, 2019 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. Quote Link to comment Share on other sites More sharing options...
Matt.Cross Posted March 28, 2019 Report Share Posted March 28, 2019 It'll work out great, no sweat. Can't wait to see it come together! Quote Link to comment Share on other sites More sharing options...
jtallen83 Posted March 28, 2019 Report Share Posted March 28, 2019 4 hours ago, 98Z5V said: If that is a success, it negates all the people that say you have to "true" the upper receiver in order to have perfect, square contact between the upper and the flange. If you can prove that theory down, it would be EPIC against that practice. WAY EPIC. Personally, I'd like to see that. Too many people say you cannot possibly have an accurate AR unless you true the upper receiver, and "glue" the barrel in place - if you don't = never gonna be accurate. Better get @Lane, Mr. Science, on this! No way would a sample of one negate a time honored practice. As far as lapping and gluing goes it only takes a few minutes and I have only ever seen claims it helps, never heard anyone say it harms if done properly. Way to many professional builders and shooters follow this practice for it to be a fluke. Quote Link to comment Share on other sites More sharing options...
Lane Posted March 29, 2019 Report Share Posted March 29, 2019 15 hours ago, jtallen83 said: Better get @Lane, Mr. Science, on this! I had a reply all typed up, and then saw some others come in before I clicked submit; so I never posted it. My thought is; so long as you simply clip the shims and file the cut smooth, it shouldn't have any measurable effect on the coupling or barrel harmonics. Since you need all four of those shims to fill the gap; one could even rotate them so the cuts don't align; much like people do with gas rings. I would view this as yet a third or forth option in terms of bedding the barrel extension and facing the receiver. The shims should crush enough to provide a bit of both worlds, without actually facing or gluing. Wouldn't worry at all about springing down since they will be held open by the barrel extension. I would worry about squeezing out a bit; but the barrel nut should constrain them. Quote Link to comment Share on other sites More sharing options...
Lane Posted March 29, 2019 Report Share Posted March 29, 2019 On 3/26/2019 at 8:14 PM, Mont said: The Faxon Firearms barrel Since you are already working with nice things. Consider these... @Matt.Cross was eluding to this; and so was @98Z5V . When aligning the cuts in regards to harmonics; rotation does matter. If you consider the pin in the barrel; this b&w depiction should help tune it (barrel pin is the RF cap). If you want to buy a bunch of .001 shims; you might do ok with the color version with six well stacked and arranged in 3D. Seriously; don't be scared, you'll be fine with a pretty generic installation here. The coupling is really determined by the contact area (well above 99%), and the barrel nut torque. These other diagrams are all about splitting hairs. Quote Link to comment Share on other sites More sharing options...
Mont Posted April 2, 2019 Author Report Share Posted April 2, 2019 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. Quote Link to comment Share on other sites More sharing options...
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