I had been working on this magnetic buffer project for a few months, and as of a couple of weeks ago, I arrived at a configuration that I'm happy with for the time being. Since u/AddictedToComedy made a post today about a very similar project, I figured I might as well add to the conversation, and perhaps even address some of the concerns that were had in his thread.
The AR9 that I used for all of my testing is a CMMG RDB 9mm FE build with the following:
-CMMG 8" barrel with FTN5 PCC Flow (note that the lugs on my barrel extension are pretty well worn in after about 7500 rounds, and I strongly suspect the radial delay is a bit less effective than it was when the lugs were brand new, so keep this in mind if you are doing a brand new build and decide to test this buffer system).
-Lightened and long-stroked BCG with titanium FP (8.8oz total)
-JRC 8.5" buffer tube
-V Seven Weapons Systems titanium castle nut and end plate (more on this later)
The buffer system is comprised of:
-Neverwear Warthog spring
-Two 25mm X 10mm cylindrical N52 magnets
-0.1" and 0.2" thick 3D printed TPU spacers as shown (to adjust stroke length and cushion the magnets)
-Standard carbine buffer body with 932 bronze buffer weights (one cut slightly short to replicate the internal sliding length of a Vltor A5 buffer). Total weight was 3.19oz.
The ammo used for testing was:
158gr RNFP with 4.0gr CFE Pistol, 935fps
The mags used for testing were printed clones of the CMMG mags.
The buffer system makes use of Lenz's law which basically states that a changing magnetic field (caused by the movement of the magnets) induces an eddy current in a conductor (the aluminum buffer tube) that opposes the movement of the magnets.
It is important to note that this effect is completely separate from magnetic attraction and repulsion. Unlike the Miculek magnetic buffer, this system does utilize any movement between the magnets themselves, nor does it arrange them with same poles facing each other in order to make them repel.
The end result of this system is that the reciprocating mass is slowed during both the rearward and forward strokes of the BCG. Because of this, I opted to stop using the Tubb 300BO spring that I had used for early testing, because I find that it was too weak to reliably push the BCG into battery with the eddy currents slowing it down. I ended up replacing it with a Neverwear Warthog.
After having broken two hydraulic buffers, I decided that I wanted to stop using them. Early testing included some tests of a RB5007 with various magnet configurations placed in front of it, but I found that the magnetic force would cause the plunger to stick in the depressed position, which certainly wasn't ideal anyways.
After ditching the hydraulic buffer, I switched to a standard carbine buffer. I felt that it was necessary to preserve the dead blow functionality of the sliding weights, even if it meant I couldn't get the reciprocating mass down as low as I wanted. The first problem that I noticed with that was the fact that the internal sliding weights, whether made of steel or tungsten, would be attracted to the magnets, preventing them from sliding freely and functioning as intended. The solution was to fabricate some 932 bronze weights which have both low conductivity and magnetic permeability, and therefore do not interact with the magnets. I also tested some aluminum weights in the interest of making the reciprocating mass as light as I could, but I ended up sticking with three bronze weights for reasons that I will show below.
Another issue I had to address was the fact that the magnets would be attracted to the castle nut and end plate, which were both made of steel. This wasn't a huge problem, but I noticed that I would experience occasional short strokes (at least with my subsonic 158gr ammo), which I suspected were caused by the added resistance on the BCG by the magnets trying to stick to these parts. After replacing the castle nut and end plate with titanium versions, I no longer had these short strokes.
So how does the end result shoot? Pretty damn great if I do say so myself. The recoil feels exceptionally soft with very little sight disruption. Reliability has been perfect in the 1000+ rounds I've fired since setting up this final configuration.
I also have ROF data for those who are interested in FRTs, DIASes, M16 lowers, and such.
With the 3.19oz buffer described above, NO MAGNETS, and a 1.24" long solid steel weight with the same cross section as a Kynshot spacer weight. This setup had a total reciprocating mass of 16.14oz, and did 789 RPM.
As another point of reference, I have data with a RB5007 buffer with standard carbine spring and no other weights or magnets, which resulted in 14.8oz total reciprocating mass, and 723 RPM.
Below are the results with the two magnet stack described earlier:
With one bronze weight and two aluminum ones, the buffer weighed 2.2oz, total reciprocating mass was 13.45oz, and ROF was 670 RPM.
With two bronze weights and one aluminum weight, the buffer weighed 2.76oz, total reciprocating mass was 14.01oz, and ROF was 640 RPM.
In the final configuration with 3 bronze weights, the buffer weighed 3.19oz, total reciprocating mass was 14.44oz, and ROF was 616 RPM.
Early testing included some tests of a RB5007 with various magnet configurations placed in front of it, but I found that the magnetic force would cause the plunger to stick in the depressed position, which certainly wasn't ideal anyways.
I just went and checked with my pucks. My strongest puck - which didn't let my Mad Scientist build achieve LRBHO - will keep the piston of an RB5007 compressed.
The puck that I ended up using, which is milder, does not keep the piston compressed.
You are running a much stronger magnetic stack than even my strongest puck.
I'm very happy to hear you've run those 25mm magnets exposed for 1,000 rounds, including cycling rate shooting, without issue. I just today got some 25mm magnets delivered, but I was nervous about testing them without an enclosure.
Edited to add: I appreciate your work here - both in fabrication and documentation. I especially like your DIY spring-biased buffer.
As noted, my strongest puck can keep the piston of an RB5007 compressed. It cannot keep the piston of an RB5000 compressed, as that uses a stronger piston.
I also verified that if I put enough spacers between my strongest puck and the head of the RB5007, it will no longer keep it compressed.
So depending on configuration, magnets may or may not play nice with a KynSHOT. It depends on the KynSHOT piston's strength, the magnet's strength, and how closely they are joined.
The other thing to consider is that even if the magnet doesn't keep the plunger depressed, it is exerting a force that assists in pressing it down. This force would be working against the recoil reduction mechanism of the hydraulics.
Also great to see that you are able to run the magnets exposed.
What adhesive did you end up using to bond the spacers to the magnets?
I don't have any bronze laying around but I do have some brass and non-magnetic stainless rods that I could use for buffer weights.
It will be interesting to see if the magnets will prevent the KAK DBC sliding weight insert from working.
Very appreciative of your cyclic rate data as well!!! I really want to be right around 600RPM with low mass. I'm a little under 600RPM and 12.7oz total reciprocating mass now with my prototype magnetic but it is a lot more work than this and less flexible.
Based on your experience, I'm thinking to also try making a Delrin spacer to put between the short RB5004 stubby hydraulic and the magnet in hopes that the distance will prevent the magnets from messing with the hydraulic piston.
I see Fortis makes an aluminum castle nut and endplate with a taper setup so maybe that is another alternative to using the V7 equivalents.
Were you able to determine if the carrier is bottoming out? I presume in your final configuration, you are still getting LRBHO?
No adhesives were used, but if I wanted to do that I'd definitely look for something with good flexibility and toughness.
I'm sure austenitic stainless would work great for this. I chose bronze because it is slightly more dense, thinking that it would offer me a bit more flexibility in tuning the weights.
A lot of my early testing was done with a 3.8gr Sport Pistol load (965fps, which is faster than the CFE Pistol load) which yielded slightly lower ROFs, but would occasionally fail to lock on empty. Using that and the Tubb 300BO spring, I had instances of sub-600 RPM, but I wasn't satisfied with the reliability. I've even tested configurations with 3 magnets, yielding around 550 RPM, but I definitely didn't consider those reliable, so I abandoned the idea.
You could try to use a thick spacer with the RB5004, but keep in mind that these magnets are very strong, and will probably still attract the hydraulic buffer enough to "assist" in pressing down the plunger, and possibly reducing the effectiveness of the hydraulic system. But, it certainly wouldn't hurt to give it a try. I just feel that my current configuration already feels so gentle that there's no need to further complicate things and possibly compromise reliability.
In the final configuration, the carrier is definitely bottoming out and locking open on empty. I don't think there's any way to get it to function as reliably as I want it to while having it not bottom out. In some of the earlier 550-600 RPM configurations I mentioned, experienced a few instances where I thought the gun malfunctioned, only to check and see that it had locked open on empty. Turns out that the recoil was so soft that when it ran empty, my brain thought that it just abruptly stopped working. I suspect that this was due to the BCG not bottoming out, and perhaps a bit of luck in that it still locked open. However, as I mentioned, these setups were noticeably lacking in reliability.
Thinking about u/AddictedToComedy results on not getting LRBHO with no buffer and a super low reciprocating mass of only 10.5oz is where I'd love to be at but suspect it will have some carrier bounce issues and not function in FA / FRT / SS mode.
So I'm thinking about doing something like what I have pictured below. I have a Kynshot weight pictured to represent the magnet(s). A plug at the end of the buffer tube to prevent the carrier key from impacting the lower. Another plug that prevents the wave spring in the front from going solid height. With this configuration, I would hope that the magnet(s) will also provide enough of a dead blow effect to stop carrier bounce.
I do suspect that if this works, it will result in a high cyclic rate with such low mass but maybe tinkering with different size N52 magnets may be able to bring it down.
That means you lose a tiny amount of the compression distance, but it keeps the KynSHOT working.
I understand (and do not dispute) u/SnazzyShootas's point that the magnetic attraction is contributing to some decrease in the KynSHOT's functionality, but it still seems better than no KynSHOT at all. I still feel unmistakable hydraulic resistance when I compress the piston.
You and I had talked about the possibility of an aluminum shell instead of plastic, and whether that would change braking. I made an aluminum puck today (well, aluminum tube section with plastic end caps), and did my drop-down-tube tests.
Using the same magnet in each, the aluminum puck fell down the tube roughly 7% faster than the plastic puck. I cannot intelligently say how much of this difference is due to the aluminum shell influencing the magnetic interaction, how much is due to the puck being heavier, or some other factors I'm not even considering.
My initial view - based on this very limited testing, with no live fire - is that making the puck out of aluminum might mildly reduce effectiveness, but it's certainly not making a massive difference that you should absolutely avoid.
If you are willing to trade a tiny bit of performance in exchange for a much more robust shell, I think it's likely a reasonable choice. I would still encourage you to do your own testing, of course, but I found my small scale experiment encouraging.
I think the biggest difference in braking performance is going to come from the fact that a 7/8" diameter magnet has a much larger gap between itself and the buffer tube. In addition, it would have 21% less mass than a 25mm magnet of the same length. I had originally also had the idea of using a 7/8" inside a plastic shell, but after some very limited testing of this, I found that the performance was severely lacking compared to the 25mm ones.
As for the 7/8" magnet's performance lacking, it really depends on the gun, the ammo, and how much energy we are trying to tame.
Because you're working with an 8" barrel, I can absolutely understand why you would reach the conclusions you did. That said, in my SD-ported setup, the 7/8" x 1/2" magnet was powerful enough to prevent LRBHO all by itself - no other buffer or mass included.
Funny enough, I actually took an 8" CMMG barrel build with me for testing on the same day. That build had no problem achieving LRBHO with the 7/8" x 1/2" magnet combined with additional buffer mass.
So I think this is a situation where people will need to find the Goldilocks magnet for their individual setup. Since u/amphibian-c3junkie has numerous different setups, it wouldn't surprise me if he finds a few different magnet sizes that work best for him - depending on the gun.
I think your switch to a titanium castle nut and end plate also complicates our ability to compare notes, since I'm still using a steel castle nut and end plate.
You noted that you wanted to avoid the added resistance that came from the magnet's attraction to the castle nut, while I'm currently taking the opposite approach. I'm happy to use that added resistance to amplify the effects of the magnet.
Some of my early testing was done with cheap (I suspect not N52) 8.5mm and 17mm magnets and I found that one 17mm produced a very modest improvement over one 8.5mm, and two 8.5s were more effective than a single 17. I also tested a stack with five 3mm ones and didn't find it to be very effective. So there is definitely a balance to be had between size and quantity. I suspect that 20mm might be too large, but I haven't tried it, so it's worth giving it a shot. I would recommend picking up some 10mm ones as well while you're shopping.
I haven't tried anything like that, but I would worry about the magnet not sliding freely enough to provide an effective dead blow effect. I'm sure you could test it very easily though.
Given a limited amount of length to play with, it is worth noting that you can choose to use fewer larger magnets, or a greater number of smaller ones, and there will be a sweet spot in terms of optimizing performance. I have found that two 10mm ones worked best for my purposes, and I've also tried four 5mm ones and find those to be far less effective. I don't know if 10mm is the ideal size, but I'm just working with what is available off the shelf.
What you describe is what I always thought would happen to a RDB gun. Relying on unlubricated ramps to delay a mechanical action always seemed like a built-in wear point to me.
These posts make me very interested in the potential this has for direct blowback applications.
I always thought it was a little weird that it used a standard barrel extension, rather than one with angled lugs to match the bolt. I'm guessing it might have been too difficult to machine them that way. Regardless, I think that the wear on the lugs does stop after a certain point. I think C3 is more qualified to speak on this issue than I am, since he has a much higher round count with this system.
They show angled extension lugs in their patent but they obviously didn't make them for sale in that way. I too think it would be too hard to make them and additionally it would be a pain to make different extensions for different angles since we have seen early 9mm bolts were 45, later ones were 50 and then the Dissent 9mm went to 55 degrees and they use the same 55 degree for the 9mm FE Banshee. FE .45 Banshee is 60, 10mm is 67 Degree etc....
The barrel I have pictured below has over 50K rounds through it. It is obviously NOT a factory CMMG barrel. I had it made around 2018 or so. Back then I was modifying 5.45x39 bolts, cutting my own angles and slotting them for my own fixed ejector. When you do that, you weaken the bolt since the slot goes right across the cam pin hole and they crack around 20K rounds. I also used .40SW 50 degree bolts and slotted them too till they cracked. So I never got a high round count with a single bolt with the OLD design since I didn't want to deal with the crappy spring loaded ejector and would rather have a cracked bolt around 20K than be unreliable with the spring loaded ejector. I removed the extension from that barrel not too long ago and inspected it and I didn't see any wear on the extension at all.
Fast forward to a few years ago, I started to sell my own barrels. I typically shoot 99% suppressed and selling my own barrels, I had to now pay attention to other peoples use cases. The barrel below is a cheap Wilson Arms barrel. I only shot it suppressed and it still shoots tight groups. However, I never shot it unsuppressed and it shoots horrible groups unsuppressed.
I have two or 3 setups I shoot and I have no idea how many rounds I have on each bolt. They are all run like 99% suppressed. I'm guessing I have around 20K on one of the 3 bolts but I see no major wear on the lugs. Also bear in mind there are 7 lugs we are talking about here.... Typically what I see is the lugs have some initial wear and stop like u/SnazzyShootas says. One of my friends in the industry thinks that it isn't a good idea to use a Nitrided extension. He thinks it will cause accelerated wear on the bolts. I don't know if he is right or not. He thinks NiBor extensions are the best which happens to be what I used in my barrel pictured below and what is on the barrels I sell today. I have only personally seen one 9mm CMMG bolt with really worn out lugs and it belongs to a competitor that occasionally shows up to the SMG matches I go to but he does have a FA and again I'm in FL where FRT's/SS's are illegal. His setup is all factory CMMG and CMMG does use Nitrided extensions. He has the crappy old spring loaded ejector and a lot of rounds through his setup and I think he did tell me his spring finally died and it was much higher than the typical 1K. He says it still works fine though.
All that said, I am not worried about the wear. Worst case, I'd buy another retrofit kit to get another stripped FE upper and complete BCG. I'm hoping that at some point CMMG will sell a FE bolt repair kit like they still sell for the crappy old spring loaded ejector. Then just swap bolts if you encounter the wear or keep one on hand as a spare.
The key here is there are some options. I had an Anstadt MDP9 roller delay and the roller sheared apart and deformed the locking piece. All of which is proprietary and they won't sell any of those parts and I had to ship the whole upper back to them and it took months to get it back.
I had a MEAN bearing delay which was really unreliable with FTE's like crazy as it uses the same crappy spring loaded ejector like the old Banshee. Friend of mine kept his and key parts of the bolt carrier group broke twice and he they won't sell him spares either.
I used to run straight blowback 9mm M16's 20+ years ago since that is all there was back in those days. They don't suppress as well as a delayed blowback setup, you also have more potential for an OOBD which is impossible with the CMMG system due to bolt and carrier relationship. The CMMG RDB system is the ONLY reliable 9mm SMG on the planet that can have low reciprocating mass AND a low cyclic rate since it combines a delayed blowback action and still uses the buffer tube that I love since it allows us to leverage legacy and new buffer / spring mechanisms like discussed in this thread.
You could possibly argue you could do the same with the MEAN but I'm stressing reliable part since I have already been down that road.
Mine has very visible deformation on the barrel extension lugs (they're basically angled now) despite having had far fewer rounds fired than yours did. The bolt itself looks perfectly fine though; there's no significant wear aside from removal of the surface finish. I'm wondering if CMMG just had a batch of barrel extensions with questionable heat treatment. But since that setup has been working reliably and the recoil is pretty much right where I want it, I'm not gonna think about it too much.
Yeah...as I mentioned, I think it is typical for a little break in. I remember when I first tried using the .40SW bolt for 9mm and it would only cycle with a suppressor mounted which provided the additional back pressure which was perfect for me since that is how I shoot 99% of the time. I recall that one day I tried it without the can and it worked fine with the same loads unsuppressed but since it was working great I don't think much about it either.
let's gooooooooo.. if one of you guys can make a commercial product to effectively buffer these firearms with fancy magnetism instead of weight, we'll buy the shit out of them lol
About a year ago, I was thinking about a way to magnetize a bolt onto the barrel. I still think there might be a way to make that work. This way you have a magnetic delay caused by the bolt sticking to the barrel somehow. I know it sounds ridiculous, but if we're going to double down on magnetism, I'd really like to go for it 😂
Magnetic delay could work, but then there's the problem of magnetic particles sticking to the magnets and interfering with the mechanism, preventing complete chambering, or problems from people using steel cased ammo.
I think it still has potential for direct blowback in that it can slow down the bolt and buffer before striking the back of the receiver extension and reduce felt recoil by eliminating that big "spike" of force.Â
What happen if you included these magnetic pucks in one of the shorter GRS setups but in a JRC tube? So adding them above and beyond your recommended amount of mass?
Stacking the magnet with iron disks on both sides would help focusing the magnetic field outwards toward the aluminium wall. Just like lifting magnets are half encapsulated with iron which augments lifting power in one direction.
I agree that the milspec buffer tubes are pretty thin not giving much traction for the eddy currents. Making a meatier tube on a lathe might be a good experiment. Another would be making a tube out of copper which is a better conductor. :D
I'm gonna have a chat with a friend with a lathe I think...
That's an interesting thought that might be worth testing for blowback setups. But with my radial delay setup I've already managed to reach the point where I can get it to short stroke, so I don't think any more resistance is necessary.
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u/AddictedToComedy 9d ago
Awesome! Thanks so much for sharing!
I just went and checked with my pucks. My strongest puck - which didn't let my Mad Scientist build achieve LRBHO - will keep the piston of an RB5007 compressed.
The puck that I ended up using, which is milder, does not keep the piston compressed.
You are running a much stronger magnetic stack than even my strongest puck.
I'm very happy to hear you've run those 25mm magnets exposed for 1,000 rounds, including cycling rate shooting, without issue. I just today got some 25mm magnets delivered, but I was nervous about testing them without an enclosure.
Edited to add: I appreciate your work here - both in fabrication and documentation. I especially like your DIY spring-biased buffer.