Understood all, but none of what you described isolates the problem to the receiver flexing vs the barrel nut applying forces to a barrel that’s not completely firm in the receiver.
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Is bedding the AR barrel a myth? What's the best way?
- Thread starter KCode
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And they are beefier where you say the receiver is flexing.
Both appear as though they still have force applied to the barrel from the handguard through the female threaded area for the barrel “nut”. In pictures it certainly looks like it’s all still connected, just with admittedly thicker aluminum there.
Also, 0.4mil sounds pretty extreme. Especially for vertical flex compared to horizontal due to the shape of the receiver in those axis. Even with very heavy sling tension(far more force than pushing into a barricade), I don’t think I’ve seen more than 0.2 mil of shift. I’ll have to double check though.
I wholly acknowledge an identified problem; I just don’t think we can draw a conclusion about the source of the problem. There needs to be more testing with isolated variables. It seems the Larue upper truly has zero contact with the barrel nut. And I’m curious about the new Ridgeline rifle coming out.
I have been thinking of doing some testing with different specific weights attached to a few different handguards using the same receiver/barrel to check shifts at 200 with free float handguards. But that wouldn’t identify the cause, only the shift between different handguard/barrel nut designs.
It is a simple geometry issue which you clearly don’t understand. The extension protrudes beyond the weak point and having zero gap around the extension will stiffen it up and you stated that bedding wont help at all.
My girlfriend is a PE and can do the calcs and CAD drawings for her consulting fee of $300/hr if you want to put your money where your mouth is.
I just took my gas gun out to the range again yesterday to do POI tests on paper at 100yds. I zeroed with a bipod with a five shot group, then moved to a sand fill shmedium on the concrete table, supporting the balance point of the AR. Sure enough, I was shooting .4 - .5 low.
I just got a Wilson Combat .458 Socom billet receiver on Monday, so I'll try bedding the barrel and repeating the process, and see if it brings receiver/barrel extension flex down much. I was a bit surprised at how much the POI shift was, honestly. Its currently in a poverty pony upper with standard slip fit, using a PRI handguard and monolithic rail. I'd love it if bedding takes the variability down a bit.
I just got a Wilson Combat .458 Socom billet receiver on Monday, so I'll try bedding the barrel and repeating the process, and see if it brings receiver/barrel extension flex down much. I was a bit surprised at how much the POI shift was, honestly. Its currently in a poverty pony upper with standard slip fit, using a PRI handguard and monolithic rail. I'd love it if bedding takes the variability down a bit.
Gluing the barrel in doesn't make a meaningful difference in stiffness. Adding material between the barrel socket and the picatinny rail adds stiffness. The barrel socket still experiences bending moment behind the threads when the handguard is loaded on the end. If you mount the handguard in a picatinny vise mount, insert an MI receiver rod in the receiver without engage the barrel extension, and then lift up, push down, or twist, you can see the barrel deflection with the naked eye. You can measure it with a dial indicator too.
Someone who is a PE with mechanical background says otherwise.
What are your qualifications other than monkeying with a picatinny vise which is probably the source of the flexing?
My girlfriend is a PE dipshit. Someone who actually went to school for such things, passed the PE, and is an engineer.
I’ll take her word on it over someone who doesn’t know that plastic receiver blocks will allow the whole receiver to flex.
Why are you testing receiver flex by putting force on a receiver rod? That does not accurately represent forces applied to the handguard. You’re applying torque BEHIND the barrel extension.
Put the receiver in something like the vice block below. Then hang specific weights off the handguard. Use pullies if trying to apply upward force on the handguard. Put a level on the receiver picatinny if you want to check receiver flex. I doubt you’ll see any receiver flex.
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Why are you testing receiver flex by putting force on a receiver rod? That does not accurately represent forces applied to the handguard. You’re applying torque BEHIND the barrel extension.
Put the receiver in something like the vice block below. Then hang specific weights off the handguard. Use pullies if trying to apply upward force on the handguard. Put a level on the receiver picatinny if you want to check receiver flex. I doubt you’ll see any receiver flex.
Wheeler makes one of those with a picatinny rail clamp on the other side. Flip the receiver over instead of pullies.
But then you’ve changed the system. When shooting, the upper is secured by those pins, and not by the picatinny.
Pretty sure this has been done and documented already. I can't remember where I saw it but I'm almost positive that I've seen this testing with weights hanging off various hand guards done before and the deflection recorded for each different handguard. Now I'm not sure if there was any measurements taken for the actual receiver flexing *which I believe is what you're talking about* but they might have. I will see if I can dig this up.Why are you testing receiver flex by putting force on a receiver rod? That does not accurately represent forces applied to the handguard. You’re applying torque BEHIND the barrel extension.
Put the receiver in something like the vice block below. Then hang specific weights off the handguard. Use pullies if trying to apply upward force on the handguard. Put a level on the receiver picatinny if you want to check receiver flex. I doubt you’ll see any receiver flex.
Upper Receiver Flex Testing - Part One [Archive] - M4Carbine.net Forums
This is the test that I originally posted on another forum, but was asked to remove. I had to split it up in two sections as the forum only allows ten pictures per post. More tests, comments and some follow up stuff are in the second part of the post... wherever it is. THIS IS NOT A TEST OF...
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Upper Receiver Flex Testing - Part Two [Archive] - M4Carbine.net Forums
Here is the rest... work continues on this, see the comments at the bottom. ------------------------------------ Test Two -- This was a duplication of the first test, this time with the upper receiver supported by "pinning" into a fixture that represents a lower receiver. Conventional...
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Looks like on a flat top upper, a 20lb weight hanging off the handguard, caused .0015" of flex at the receiver.
With a carry handle like a typical m4, the deflection was .0010" at the receiver. The carry handle adds some rigidity to the system. I suppose there are other ways to add rigidity to the system as well, though I don't know what effect shimming or gluing would have. I would think that IF, you could glue it well enough and that glue held at applicable Temps, it would add some level of rigidity to the system for sure, but not sure how much.
With a carry handle like a typical m4, the deflection was .0010" at the receiver. The carry handle adds some rigidity to the system. I suppose there are other ways to add rigidity to the system as well, though I don't know what effect shimming or gluing would have. I would think that IF, you could glue it well enough and that glue held at applicable Temps, it would add some level of rigidity to the system for sure, but not sure how much.
I would think (though id have to draw a force diagram and go through some steps to be sure i havent overlooked something in the system- ) Much like force in a beam, for the receiver to flex "downward", the top part of the threaded portion must pull apart so the speak, and the bottom part of the threaded portion must push together. If you could effectively attach the barrel steel to the receiver, it would provide a considerable amount of rigidity to keep that from happening, mainly in the top portion of the threaded part of the receiver, since that part would be in tension (if we are talking about pulling down on the hardguard and threaded part of the receiver). Someone correct me if I'm wrong.
I spent some time talking to Kino about this when Vltor first started machining the MUR (prior to casting). The ejection port compromises the M4 receiver even when adding material to the other side. The receiver experiences bending moment right behind the socket. There are only a couple ways of meaningfully strengthening the upper such that loading the handguard doesn't deflect the barrel; material change, integrating handguard to the upper (monolithic with bridge between the barrel socket and rail), and a handguard mounted to the rail (I mentioned this somewhere in this thread). Vltor went with the last option (so did ARMS).
I see, it just pushed the moment back to the next weak spot... yeah, that makes sense.
Seems like a steel rod or something could be put through the length of the thing to stiffen it up. Maybe material change is the easiest fix though not sure what that would do to the rest of the operating parts of the rifle ect....
Do you have part 1? I searched but didn’t see it come up.
Definitely interesting data, but it is really hard to follow without his first part. Like, what are his chamber gauge deflection measurements? What’s that setup look like?
Billet steel upper is the easy & obvious answer. I can't believe no one make one. Everyone shooting these in prs type matches is adding weight anyway. I would never buy another aluminum upper if this was an option. Hell if it was designed in Solid Works with FEA analyses it could be lightened to within 3 -4 oz of the AL receiver @ + 50 - 80% rigidity improvement. That's about the weight difference between a NF mount vs a M-Brace or Spuhr.
May as well just use solid works. The MRS was the best solution. Everyone else just ignores the flexing M4 receivers because it isn't all that significant when you start looking at how most AR's are used.
It's been done. Hard Times Armory did steel receivers in the 80's and 90's. Amalgamated did Titanium receivers in the late teens.
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I know it's been done, but why the hell is no one selling a steel or Ti upper?? The upper is really the key piece for positional accuracy in the comps we shoot.It's been done. Hard Times Armory did steel receivers in the 80's and 90's. Amalgamated did Titanium receivers in the late teens.
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If anyone finds a steel or Ti upper for sale please let me know.
Its easy. Preheat with a heat gun or torch and knock it out with a mallet and dowel
Using a computer model was one of my early recommendations to isolate receiver flex from barrel extension movement in the receiver.. But, I don’t have that software.
Edit: and run Monte Carlo testing for various tolerances around the barrel extension.
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Thanks!
I’m still not completely sold that it is (or perhaps is ALL) receiver flex. He noted less movement in the carry handle upper, but the two uppers don’t necessarily have the same diameter around the barrel extension. It’s another variable.
I did appreciate him heating up the area as another test.
I’d love to see a repeat of the test using the same upper, but using two different barrels. One with a standard barrel extension and one with an oversized extension requiring a thermal fit. That way you keep the receiver the same. For testing the carry handle upper, I guess one would just have to try and find one with a nearly identical inner diameter where the barrel extension is inserted.
Still though, great data and thanks for sharing.
There's so many variables it's hard to speak in absolutes with any of this stuff. Material change is the best answer and it's kind of hard to understand how that isn't more available. I guess most folks have all the precision they want out of a solid ar as is. We are just always pursuing the last bit of precision we can squeeze out of any system and most of us are bolt gun guys who are then moving into the ar platform, so our expectations are very high.
I'm going to go to my buddies tomorrow and hit him up to make some steel uppers and see what he says. He already mills uppers himself for a ton of folks so maybe he will dona run for us?
Would that not simply be the matter of a tool change to accommodate steel if he already has the programming?
That's over my head but it seems like it would be for me. If we had enough of them I think he'd do it but not sure what he'd charge or anything. How many guys would want one or more than one? If we got a decent number then I think he'd do it. He has been milling them a long time and makes suppressors and all that stuff. He got pissed at the put of spec uppers he got years ago and so he started making them himself among other things.
Put me down for 2 x steel uppers. If they were straight up same dimensions as a std upper I'd be happy. DLC on a steel upper would be tits.
If your buddy is all set up for Aluminum I'd gladly take a heavy wall 7075 like the old Mega uppers.
Mega 3/8 heavy walled upper below right. These were like 2013 I think.
I will talk to him and see what he would do. I'd like to have a few myself so maybe we can work something out. I need to go by there anyway and see em.
If they’re billet, probably. If they’re forgings, no. Unless he has a source for forged steel uppers.
Pretty sure the ones he normally does are forged that he then puts in his cnc and mills but I'm not sure what all he can get or will do. He "can" do about anything, but it's always a question of if it's worth it. He's got a machine operator thats ridiculously tedious about everything he does. He's a little weird but he's a good dude and a legit fantastic machinist. I just don't know if my buddy will agree to it for a small batch. Depends on how backed up he is probably.
Sorry to hear that.
I work with a lot of PE dipshits. Hopefully she makes up for it in other areas.
Sorry couldn't resist.
I have a poor shooting upper and going down the rabbit hole of how to improve it. The BE to receiver fit is the loosest I have seen. I ordered .001" shim stock but worried that won't be thick enough.
Can I just stack 2 pieces of .001" shim instead of buying .002"?
Can I just stack 2 pieces of .001" shim instead of buying .002"?
Maybe, but I would just use the .001” and some 620 loctite. That’s what I did to the last two I worked on, both shoot great now.
BCM demo receivers were and still may be about 50 bucks. They have a nice tight fit with my RRA lowers and are thermal fit on the barrels. I just thermal fit glued a really good shooting Valkirye barrel into one. I have a receiver that is not thermal fit to test barrels in before thermal fit and glue. Check your mounts and stuff too. I have Rainer ultra match 6 arc I was about to complain about. I tried a few bullets and powders. I ended up changeing the scope riser and found the one on it was not very tight.
I tried .001" and it was still loose, tried .002" and that was a no go. Rather than using .001" and 620 loctite, I just went with the .002" and some heat and slipped the barrel in, I did have too tap it in the last little bit as the upper had cooled down. In the end had an excellent fit and very nice grouping. I would do it again if the need arises.
There's a company called "Triggershims" that sells 3 packs of shim stock for under $15, including shipping. The most common pack includes 1 ea of 0.001", 0.0015". and 0.002" thick shim stocks, but you can mix and match up to 0.007" thick.
You can also have the upper Cerakoted with an emphasis on adding material to the barrel extension tunnel, but masking off the carrier raceway.
Whenever I come across an upper that is loose-goosey, I either throw it in a bin or have it Cerakoted as described above.
You can try. I doubt that SS foil doubled up will play nice. I keep a box of .0015" and .001" around. Probably used the .0015" 5 to 1 over .001" until I started using reportedly undersize uppers. I seem to remember using .002" once. It must have been a hotdog down a hallway set up but can't recall which gun it was....
As someone else mentioned, a BCM is not very expensive (when they're available) and you know it will be a thermal fit unless you have shrinkage on your extension.
2 double entendre's in one post.
Thanks, I placed an order at triggershims.com. very nostalgic lol.
This upper is using a proprietary BE and receiver, so upgrading the parts is not an option.
Shimming seems like less trouble and expense than cerakoting, plus its already painted.
This upper is using a proprietary BE and receiver, so upgrading the parts is not an option.
Shimming seems like less trouble and expense than cerakoting, plus its already painted.
Looking on triggershims but cannot find the assortment pack you bought, can you post a link? Thanks
