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Another replacement barrel trunnion thread!

35K views 118 replies 26 participants last post by  Hkbeltfed 
#1 ·
The other thread is almost 2 years old without any progress, so I'm working on it myself. I gather there may be a few parties pursuing the production of a lightweight non-QD trunnion as many are as frustrated as I am. I figure, if someone beats me to it, I'll get one and I win. If they don't, I'll make one and I win. No downsides.

I'm about 90% done with measuring and modeling it. The goal is not to replica a stock trunnion to include tolerances it was made with, but to correctly assess the original planned dimensions and tolerances of which all the trunnions fall within. I'm happy with how the measuring is going and confident in the assessment of most of the measurements and their tolerances. That said, I'm also somewhat obsessive and would like to be certain of everything.

To that end, I wanted to have a sample set greater than 1 to be sure. Bushmaster is out of stock, so I'm asking other members. Will any one or two of you loan me your trunnion for a week? I can cover shipping and a loaner fee, or provide a free Magpul full auto ACR lower, or work out some other arrangement.

If your willing to do so in the interest of furthering this project and putting an end to the wait, please let me know. Thanks!

PM or email me at hkbeltfed@yahooNOSPAM.com just remove the NOSPAM part.

 
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#10 ·
I gave the machine shop the final file, tolerances, and ok to proceed on Monday morning. I only have the funds to do 5 now at about $300 each. It gets cheaper with quantity, but I'm not going to fund it. So, with one each for Big Dog and Anyonebutme, and one (maybe two) for me, that leaves one (two at most) for Gun Broker. If I do another run, it will have to be prepay, a larger quantity to get my cost down, and still sell them for $300+.

This is just for the trunnion in 7075 with a milspec hardcoat anodize and to some fairly tight tolerances. Compatible barrel nuts and wrenches can be inexpensively purchased from Aero Precision. Longer term, I have a locking barrel nut planned, but other than the features needed to incorporate it into the trunnion, I haven't started it yet.


The 3 stock trunnions I measured were all over the place and loose. I used the Ar15 ordnance print tolerances where applicable and fairly tight tolerances on the rest. That said, the shop said even if I gave them +/-.01, the finished result would be closer to +/-.001 anyway. They're also saying 4 - 5 weeks. The model has a few differences from the pics I posted and one of them is crucial as it's a real differentiator.


Wish me luck!

On a side note, that other trunnion thread is pretty disappointing with no progress after almost 2 years, what appears to be impractical features for machining in the drawings posted, and an unrealistic projected price (or a price based on an unrealistic volume or out of China)... unfortunately, it reminds me of the metal lower thread too :(
 
#11 ·
For locking I'd imagine a small set screw would suffice drilling up the bottom where the plunger would be, or use the stock plunger idea and make a nut that has sprocket type protrusions that interface with the plunger ramp. Even though I don't think the Aero COP upper has any type of locking system. We might also want to think about getting a better barrel nut wrench made, all the reviews I've found say the Aero wrench breaks if you try and remove the barrel because the outer lugs don't have enough support. Not a high priority for me because I never plan on removing it, but if I did it's only $12 if I had to take vise grips to it.
 
#12 ·
I’m not a fan of Remington's set screw into threads locking mechanism. Maybe if the set screw interfaced with flats or dimples, but not threads. I actually modeled the trunnion with a set screw as one of many revisions, even though I personally would not use it. I’d use safety wire instead. I decided on a plunger mechanism as it’s a great way to hollow out the trunnion. You can’t hollow it out from the front using a set screw as there’s not enough meat for the screw. One could do it from the back, but I chose not to.

I didn’t know that about the Aero barrel wrenches, thanks. I only chose the Aero nuts for their thread pitch which is common. There are probably other compatible nuts as well (not KAC’s), but the Areo ones are inexpensive and with inexpensive wrenches too.

My future plan is to utilize a ramped tooth, plunger style locking mechanism with the same pitch for the nut. The plunger part would be similar to the stock part, even dimensionally. The ramped teeth would be much larger than stock (ramped on the tightening side, flat on the other) but a fraction of the width of an Ar15’s barrel nut teeth. One would tighten to specified torque, if it’s on a flat, leave it. If it’s mid-ramp, tighten until the next flat, which wouldn’t be much.

Some are asking about buying a trunnion, but other than 1 or 2 extra when this run is done, I won't have any for sale unless I do another run as per my last post. I could sell those 1 or 2 here, but I hope to recoup some of the funds I’ve spent on Gun Broker.

Some are asking about the weight. Projected for the trunnion is 2.07oz. The Aero nut is 1.34oz. This combo should be 5.29oz lighter than stock. That’s 1/3 lb! My planned barrel nut and locking mechanism may be lighter still as it will be all aluminum. If nothing else, it will be close.
 
#16 ·
I'm funding this run of 5 myself as I have nothing in hand, not even a prototype. If I made a mistake on the file, or the shop screws up, or whatever, I'd be in trouble. If all goes well, maybe I'll do another run like I mentioned. It would have to be prepay and a large quantity, like 20 or more, to get the costs down so there's something in it for me.

I'm all ears if someone has a way to do them for $150.
I've sunk enough money into this that my personal trunnion will probably be the costliest of all of them. I had to buy some specialty micrometers to measure certain things, pay or compensate for shipping both ways on 2 member's trunnions, take time off work to visit shops, etc. Speaking of which, considering how sloppy the stock trunnions are, I'm very glad I didn't pay to CMM one! Granted, I like this stuff, but if I just looked at the monetary aspect, I'd probably never have started it. I was naive to believe the other thread. I doubt a machinist with thier own labor and machines could make and sell them for $150. Hopefully for for members sake, someone proves me wrong.

I just want one for myself and I was tired of waiting. Mine is so kick ass too that if someone else comes out with one, it won't matter. Not only am I very proud of the lengths I've gone in ascertaining the original dims, and to avoid building the loose tolerances from a single sample into the model, but also the changes I've made. Two key distinguishing features, one of them crucial. I'd like to tell the forum, but I can't yet :(
 
#18 · (Edited)
I have no problem scaping together 300 or so each. I'm a firm believer in buy once cry once and since these are going on match guns, it's worth it.
 
#21 ·
Whatever's left over, either 1 or 2, will likely end up on Gun Broker in hopes of recouping some of the funds I've sunk into this. Or I'll post them here for a very high price (to comply with the rules) and accept the best offer. Either way, I'll keep everyone posted.
 
#23 ·
Count me in for one or give me a heads up if these hit Gunbroker! Thanks Hkbeltfed for your contribution. If you ever need any help with anything CAD related I run Autodesk Inventor all day long at work and am certified in Solidworks, I'm a Designer/Drafter. I just wish I knew more CNC people myself!
 
#25 ·
I was asked about the other 2 items via email, so I'll post it: It's 10 ambi charging handles out of 6061. Machined "bent", meaning not flat and bent later, and leaving them in the white. They're machining the cut out at the top rear of 8 full auto trigger boxes to eliminate the auto sear hole, just like the stock semi boxes and black mil-spec hardcoat anodize. These are for the last of my full auto marked lowers. Plus the 5 trunnions in 7075 and black mil-spec hardcoat anodize. Unfortunately, it's two separate batches of anodize. I hoped to do all 3, but it's 3 different types of aluminum and they won't do them together. Or if they did, they'd still charge them separate. Hopefully all goes well!

I thought my ACR was pretty trick before, but it will be a lot cooler in a couple weeks! In addition to the above, I'm working on another barrel & "different" gas system too. Plus, the locking barrel nut & plunger for my trunnion. Plus, an ambi safety for my Geissele 3 position trigger. Sheesh, at the rate I'm going, I'm not sure if I'll ever finish it!
 
#27 ·
I was asked about selling the trunnion cad file via email: I'm not willing to sell the file at this point in time. I don't even have a trunnion in hand yet. If I only do 5 and do not wish to pursue it further, then I'll consider it. Even if one had the file, you'd have to do a short run as it would cost you almost the same anyway. You pay for all the machine setups, which take longer than the actual machining, whether you do 1 or 5 or 50. Same for the anodizing.

Looking at the poll for the other thread where they're suggested to be only $150, there's not much interest. I'm assuming much less interest at $300. So, I'm skeptical on another run.
Besides, all is not lost for 1 or 2 of you as I'll have at least 1-2 for sale, albeit for as much as I can get for them.

When these 5 are done, and others can see the differences I've incorporated, I'll post an interest thread. Then, we'll see.
 
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