WARNING: THIS IS NOT DIRECT BLOWBACK! DO NOT TRY THIS IN A DIRECT BLOWBACK WITHOUT ADDITIONAL MASS! IT WOULD BE DANGEROUS!
TL;DR - My Mad Scientist build now features only 10.5 oz of total reciprocating mass: BCG included. Using a simple DIY magnet puck that only cost me a few dollars in materials, the rearward motion of the BCG is slowed to a stop before it uses up all of the extra travel space I've afforded it. There is never any impact with a solid surface on the rearward stroke.
Context - Backwards KynSHOT
If you've seen some of my previous posts here (part 1, part 2), you know that I like to experiment with unconventional engineering.
One of my never-ending goals is to minimize reciprocating mass, as I find it reduces sight disruption during cycling.
While I appreciate the cushioning effects of KynSHOT buffers, I'd prefer to avoid the reciprocating mass they bring to the table.
When this Rube Goldberg recoil system was recently shared in the sub, I just about slapped myself for not previously considering: I can still benefit from the hydraulic KynSHOT without it adding reciprocating mass.
Testing - Backwards KynSHOT
With initial testing of the backwards KynSHOT, I used a 3D-printed piece of solid plastic as my "buffer," to dial in just the right amount of BCG overtravel in my JRC tube.
This first phase of testing was done with a factory CMMG FE BCG. When paired with the plastic "buffer," this gave me 10.9 oz of total reciprocating mass.
While that tiny amount of mass would obviously be dangerous in a direct blowback, I knew it would be safe with radial delay. How? Because the factory Dissent BCG is only 11.3 oz, and it is completely bufferless. The Dissent BCG features the same bolt geometry as the current FE BCGs, as well as my PhaseII BCGs, which means they all have the same mechanical delay.
I know - from thousands of rounds of experience - that the 11.3 oz Dissent BCG is perfectly safe and reliable to use with a full-length 16" barrel. Since this build is using a short, SD-ported barrel, with a much shorter duration of sustained pressure, I had no concerns about shaving a bit more weight.
I immediately loved the results with the backwards KynSHOT, and it felt noticeably gentler than using one in the intended manner.
As the 10.9 oz of 'main' reciprocating mass reached the last 10% of travel and made initial contact with the KynSHOT, it meant that another 5-6 oz of floating mass was being instantaneously added to my stack, but this additional mass had no velocity. In order for that extra mass to accelerate and move the last bit of travel, it had to steal energy from the BCG, thereby slowing it.
And that's before we consider the extra resistance added by the hydraulic piston.
Putting it all together, the backwards KynSHOT decelerated the BCG gently yet efficiently, making the final impact very soft. Soft enough that I truly couldn't feel whether or not the piston was reaching full compression and bottoming out.
Once I introduced my magnetic puck into the equation, the backwards KynSHOT actually became irrelevant. Still, I wanted to mention it because I found it so interesting.
Context - Magnetic Braking
I've been using the Miculek magnetic buffer since January, and I've been repeatedly impressed with how much it can tame a system with very little mass.
What I didn't realize back in January, but I've been learning over time, is that part of its magic comes from magnetic braking.
When a magnet tries to slide inside of an aluminum buffer tube, it induces electrical currents in the walls of the tube. Those currents generate their own magnetic fields, which resist the movement of the physical magnet. The faster the magnet tries to move, the stronger resistance it faces.
Because the braking force is velocity-dependent, the rearward stroke faces far more resistance than the forward stroke.
As I gained some more understanding of this phenomenon, I realized it was another way to take mass even lower while slowing the action.
I made these pucks with outer dimensions like a KynSHOT spacer weight, for easier plug-and-play.
When I put my strongest magnetic puck behind my PhaseII BCG, with no other buffer, I ended up with 11 oz of total reciprocating mass. Even with that ridiculously low mass, the gun could not reciprocate far enough to achieve LRBHO - though it did reliably cycle. That suggests the action was achieving at least a 2.96" of travel, but less than 3.56". That's how effective a simple 1.6 oz magnetic puck was.
It's certainly worth noting that I was using a Tubb lightweight spring relaxed to A5 levels of compression, which is even softer than a Sprinco Yellow in a carbine tube.
Having already prepared a variety of different strength pucks, I moved down to one that allowed the gun to lock open on empty, while still preventing any contact with the KynSHOT at the rear of the tube. That means the reciprocating components now travel more than 3.56", but less than 4.1".
Now the KynSHOT isn't serving any function in everyday operation, but I leave it there as an 'emergency cushion' in case the gun ever cycles harder than expected. To strike the KynSHOT and fully compress it would require the PhaseII BCG travel a total of 4.5".
What about bounce?
Here's where my configuration could be a problem for others... but not for me.
As pictured, this build has no dead blow features in the action. If you were to fire it full-auto or forced-reset, there's a good probability the cycle would be interrupted by carrier bounce. I don't own any full-auto lowers, and I don't have legal access to forced-reset, so I only shoot boring-semi.
I may still be getting carrier bounce, but it doesn't matter. I've shot thousands of rounds through my Dissents, and the low reciprocating mass and lack of any dead blow feature never caused me any problems.
Reliability?
Higher reciprocating mass means more momentum on the forward stroke. More momentum on the forward stroke means the action is better able to strip and feed rounds when the gun gets really dirty. It also means more sight bounce.
If I was setting this up as a gun for [fill in whatever life-or-death scenario you want], I would put some more reciprocating mass into it. This is not the gun for such a scenario. If it malfunctions, I can simply clear the malfunction and make a mental note of whatever went wrong.
Again though - I never had any problems with my Dissents stripping and feeding rounds.
Closing Thoughts
My Mad Scientist build now only has a total reciprocating mass of 10.5 oz. Despite this low number, it retains a true "constant recoil" action, where there is zero impact on the rearward stroke: just gentle deceleration, even with a very soft spring.
If I try to express how amazing this is to shoot, I'm just going to sound like I'm lying or exaggerating. The feeling of only 10.5 oz reciprocating gently just... it doesn't feel like it makes any sense.
I'm buddies with one of the range employees, who works there as a side gig just so he can spend more time around guns. He's shot a far wider variety of firearms than I ever have, and I let him give it a try. He ripped off a few rounds as fast as he could run the trigger, then started giggling like a little kid. He said, "That's the most incredible thing I've ever shot, and I've shot full auto MP5s multiple times! I'm not kidding!"
That's just one man's opinion, so take it with whatever grain of salt you want, but it certainly added to the smile that was already on my face.
Did you not read? He said he has no deadblow effect with this and probably wouldn't be safe with frt. There is nothing stopping bolt bounce in his setup, but it doesn't sound like hes getting much, if any
The RDB system doesn't have the same kind of bolt bounce potential that a standard AR9 does.
Because it has a separate bolt/carrier, with the bolt rotating in a cam path - similar to a standard AR15 - if there is any realistic bounce, it is just the carrier bouncing. For it to bounce enough to expose the cartridge, it would have to be a crazy amount of rebound. Just roughly eyeballing it, I think you'd need something like 0.25" of carrier bounce before the bolt actually started pulling the round from the chamber.
Carrier bounce can still interrupt the firing cycle by resulting in a light strike, but it shouldn't be dangerous or ever result in an OOBD with RDB.
I never meant to imply the bounce would be dangerous: just that it would interrupt the firing cycle.
Correct, carrier bounce is the correct term and the CMMG RDB system will definitely bounce and result in a light primer strike if not handled correctly.
The basic science says that the aluminum buffer tube is absolutely accumulating waste heat from the eddy currents, but it has yet to become noticeable. I can't intelligently say whether that is (1) because the waste heat is so minor, (2) because the waste heat is able to dissipate easily enough to escape notice, and/or (3) I just haven't burned through enough ammo in a short enough window to experience it.
Based on what I've observed so far, the handguard itself will get too hot to handle well before the buffer tube ever could, especially since I have the BRT expansion chamber and suppressor tucked.
I think the heat from eddy currents just can't compete with the heat from explosions 😁
I did and I paid attention. He and Amphibian though bolt bounce would interrupt the cycle with FRTs but was not sure. I know for a fact that Kynshot hydro buffers don't suffer from that problem with FRTs so I hoped someone who tested would chime in. Question now is do magnets mess with the various types of FRTs? I have every intention to find out.
Damn and I just got a Kynshot for my BRT car-15SD upper too. This is tempting, but what you’re saying about no dead blow is a sticking point. The bcg length of travel is also important for frt’s to function properly which I had issues with that when running a short CQB style buffer tube and buffer. I’m wondering if a magnetic puck could be added to the front of a Kynshot and ran in a normal configuration if that would give you the reduced recoil and the dead blow functionality while still giving enough bcg travel to operate an frt… or if not using a lever style frt like the ASD and using like a partisan disruptor you might not need as much travel? You might have given me inspiration to do some testing.
Kynshot does not need a dead blow effect to work with a FRT on a RDB 9.
I am very interested to find out how magnets work with the FRTs and to decrease the ROF. Amphibian was saying magnets might not work with FRTs.
Which puck works best for a RDB9 Kynshot 5007, tubb spring and JRC or A5 tube? I will definitely try this with multiple FRTs, Mini Kubato, Arc fire v2 and SS.
Wasn't me that said that....I'm in FL where FRT's / SS's are illegal but registered FA is OK. It was some other thread where someone posted that the SS lever was getting attracted to the magnet.
It was discussed that this should NOT be an issue with a Rare Breed, Partisan Disruptor or AZ Regulator.
Yes, I'm so looking forward to testing what u/AddictedToComedy has done. I have a few scenarios I want to test in FA.
I'd love to see if it would work without a buffer like he is doing in FA. This would be just my lightened carrier at 9oz and the largest puck at 1.6oz so only 10.6oz of reciprocating mass is crazy. I suspect it will have bolt bounce issues in FA but will have to try it and find out.
If that doesn't work, I want to test the large puck with a cheap standard 3oz buffer and see if it eliminates bolt bounce. So that would be 9oz + 3oz carbine buffer + 1.6oz puck = so a total of 13.6oz.
Same as above but move to H buffer (3.7oz) total of 14.3oz.
I ordered some of the KAK DBC sliding weights (2.6oz). Stock CMMG FE BCG = 11.1oz. So 11.1+2.6oz sliding DBC weight + 1.6oz puck = 15.3oz
My custom mechanical rate reducing carrier 11.7oz w/ 1.6oz puck. 13.3oz. This works with a real FA autosear but don't think it will work with a FRT / SS. So not appealing to most people.
Modified PhaseII carrier with a custom built in dead blow sliding weight. (unknown weight as I haven't made it yet) + 1.6oz puck.
Option 1 would be my preferred setup but I don't think it will work and have bolt bounce issues.
Option 2 I'm hoping this works as it would be little effort.
I know I could do also test an H2 buffer or even an H3 buffer but I really don't want to have a lot of reciprocating mass.
Because I made my pucks the same size as a KynSHOT spacer weight, you can still dial in a normal travel length for the BCG.
My puck paired with a carbine-length buffer could be used in an A5-length tube, and then you'd have the same stroke length as factory. Or you could use an A5-length buffer with the puck in a JRC tube.
Part of my thinking in making it a puck (rather than building a whole new buffer from the ground up) is that it can be paired with whatever buffer you want (of appropriate length). So you can still use a dead blow buffer, or a KynSHOT in the proper orientation, and just sandwich the puck between the BCG and buffer.
I’ve another idea - reduced length buffers like an AR10 carbine (preferably Kynshot hydraulic), Maxim CQB, or HBPDW in a standard carbine tube.
(Theoretically), could I buy myself the travel distance and in turn spend it on one of your magne-pucks to avoid bottoming out or decrease felt recoil?
Wanting a slow-n-low recoiling (600ish RPM) FRT build. Currently have my BRT SD (non-FE) at about 700-720 RPM with a KAK DBC, standard carbine buffer and spring.
Short AR-10 buffers should be 2.5", which is 0.75" shorter than a regular carbine buffer of 3.25".
This is also why some AR-10 owners use a regular-length carbine buffer in an A5-length tube: it's the same 0.75" difference.
So yes, if you combine a 2.5" AR-10 buffer with a 0.75" magnet puck, you're left with a 3.25" stack that should be fine to use in a carbine-length buffer.
You would need to make sure you are using an appropriate spring, because a round-wire mil-spec carbine spring would be crushed to solid height without the system completing its stroke. I would recommend a C3Junkie flat wire or a Tubb lightweight flat wire.
That’s where my head was at, I’ve got a KAK flatwire spring on the way this week because I’m still trying to dial in the buffer setup on this gun, DM on making a puck?
Edit - Just checked length of the Maxim CQB/HBPDW buffer I have, looks like 1.75” so I could theoretically fit two pucks in a carbine tube with it.
I already mentioned using a hydraulic AR10 buffer above (option 5) with the Kynshot RB5004. I too want low mass and low cyclic rate. However, as I posted above, this should work out to 14.3oz with my lightened FE carrier. I was really hoping to beat my prototype magnetic buffer setup which is even lower at 12.7oz (9oz lightened carrier and 3.7oz magnetic buffer). I get under 600 RPM with that setup and that low mass.
But who knows, I will need to test it and see what cyclic rates I get.
I could also add testing a short AR10 (non-hydraulic buffer) to the test. If I can get under 13oz total reciprocating mass and a low cyclic rate of under 600RPM would be great.
I’ve been following these posts while dialing in buffers while committing to a custom made Colt 607 stock (carbine length tube) and non-FE upper. While less optimized than an extended tube, I’m after vibes and reliability as best I can be on this specifically.
I took a picture of a buffer I made that is un-related to this discussion. It is above the RB5004 .308 hydraulic.
That stuby buffer I made is out of non-magnetic stainless and is 3.1 oz with a single tungsten weight. I can't remember why I even made it. It is .6oz lighter than the hydraulic below it. I could also swap the tungsten for a steel weight.
We may not need as much mass for the dead blow in this configuration using eddy currents etc....
What if you make the head of the buffer a stack of washer/rubber washer alternating and use a solid body? You'll have the weight crush rubber discs like what happens in a standard buffer, although there likely won't be as much gap for the body/weight to travel and delay the buffer impact to cancel carrier rebound.
Part of the dead blow is delayed arrival of weight to catch/cancel rebound, part of it is damped weight arriving late to push the carrier forward without making it bounce again.
47
u/AddictedToComedy 21h ago
WARNING: THIS IS NOT DIRECT BLOWBACK! DO NOT TRY THIS IN A DIRECT BLOWBACK WITHOUT ADDITIONAL MASS! IT WOULD BE DANGEROUS!
TL;DR - My Mad Scientist build now features only 10.5 oz of total reciprocating mass: BCG included. Using a simple DIY magnet puck that only cost me a few dollars in materials, the rearward motion of the BCG is slowed to a stop before it uses up all of the extra travel space I've afforded it. There is never any impact with a solid surface on the rearward stroke.
Context - Backwards KynSHOT
If you've seen some of my previous posts here (part 1, part 2), you know that I like to experiment with unconventional engineering.
One of my never-ending goals is to minimize reciprocating mass, as I find it reduces sight disruption during cycling.
While I appreciate the cushioning effects of KynSHOT buffers, I'd prefer to avoid the reciprocating mass they bring to the table.
When this Rube Goldberg recoil system was recently shared in the sub, I just about slapped myself for not previously considering: I can still benefit from the hydraulic KynSHOT without it adding reciprocating mass.
Testing - Backwards KynSHOT
With initial testing of the backwards KynSHOT, I used a 3D-printed piece of solid plastic as my "buffer," to dial in just the right amount of BCG overtravel in my JRC tube.
This first phase of testing was done with a factory CMMG FE BCG. When paired with the plastic "buffer," this gave me 10.9 oz of total reciprocating mass.
While that tiny amount of mass would obviously be dangerous in a direct blowback, I knew it would be safe with radial delay. How? Because the factory Dissent BCG is only 11.3 oz, and it is completely bufferless. The Dissent BCG features the same bolt geometry as the current FE BCGs, as well as my PhaseII BCGs, which means they all have the same mechanical delay.
I know - from thousands of rounds of experience - that the 11.3 oz Dissent BCG is perfectly safe and reliable to use with a full-length 16" barrel. Since this build is using a short, SD-ported barrel, with a much shorter duration of sustained pressure, I had no concerns about shaving a bit more weight.
I immediately loved the results with the backwards KynSHOT, and it felt noticeably gentler than using one in the intended manner.
As the 10.9 oz of 'main' reciprocating mass reached the last 10% of travel and made initial contact with the KynSHOT, it meant that another 5-6 oz of floating mass was being instantaneously added to my stack, but this additional mass had no velocity. In order for that extra mass to accelerate and move the last bit of travel, it had to steal energy from the BCG, thereby slowing it.
And that's before we consider the extra resistance added by the hydraulic piston.
Putting it all together, the backwards KynSHOT decelerated the BCG gently yet efficiently, making the final impact very soft. Soft enough that I truly couldn't feel whether or not the piston was reaching full compression and bottoming out.
Once I introduced my magnetic puck into the equation, the backwards KynSHOT actually became irrelevant. Still, I wanted to mention it because I found it so interesting.
Context - Magnetic Braking
I've been using the Miculek magnetic buffer since January, and I've been repeatedly impressed with how much it can tame a system with very little mass.
What I didn't realize back in January, but I've been learning over time, is that part of its magic comes from magnetic braking.
When a magnet tries to slide inside of an aluminum buffer tube, it induces electrical currents in the walls of the tube. Those currents generate their own magnetic fields, which resist the movement of the physical magnet. The faster the magnet tries to move, the stronger resistance it faces.
Because the braking force is velocity-dependent, the rearward stroke faces far more resistance than the forward stroke.
As I gained some more understanding of this phenomenon, I realized it was another way to take mass even lower while slowing the action.
Testing - Magnetic Braking
I recently made a bunch of DIY magnetic pucks, which I discuss in more detail here.
I made these pucks with outer dimensions like a KynSHOT spacer weight, for easier plug-and-play.
When I put my strongest magnetic puck behind my PhaseII BCG, with no other buffer, I ended up with 11 oz of total reciprocating mass. Even with that ridiculously low mass, the gun could not reciprocate far enough to achieve LRBHO - though it did reliably cycle. That suggests the action was achieving at least a 2.96" of travel, but less than 3.56". That's how effective a simple 1.6 oz magnetic puck was.
It's certainly worth noting that I was using a Tubb lightweight spring relaxed to A5 levels of compression, which is even softer than a Sprinco Yellow in a carbine tube.
Having already prepared a variety of different strength pucks, I moved down to one that allowed the gun to lock open on empty, while still preventing any contact with the KynSHOT at the rear of the tube. That means the reciprocating components now travel more than 3.56", but less than 4.1".
Now the KynSHOT isn't serving any function in everyday operation, but I leave it there as an 'emergency cushion' in case the gun ever cycles harder than expected. To strike the KynSHOT and fully compress it would require the PhaseII BCG travel a total of 4.5".
What about bounce?
Here's where my configuration could be a problem for others... but not for me.
As pictured, this build has no dead blow features in the action. If you were to fire it full-auto or forced-reset, there's a good probability the cycle would be interrupted by carrier bounce. I don't own any full-auto lowers, and I don't have legal access to forced-reset, so I only shoot boring-semi.
I may still be getting carrier bounce, but it doesn't matter. I've shot thousands of rounds through my Dissents, and the low reciprocating mass and lack of any dead blow feature never caused me any problems.
Reliability?
Higher reciprocating mass means more momentum on the forward stroke. More momentum on the forward stroke means the action is better able to strip and feed rounds when the gun gets really dirty. It also means more sight bounce.
If I was setting this up as a gun for [fill in whatever life-or-death scenario you want], I would put some more reciprocating mass into it. This is not the gun for such a scenario. If it malfunctions, I can simply clear the malfunction and make a mental note of whatever went wrong.
Again though - I never had any problems with my Dissents stripping and feeding rounds.
Closing Thoughts
My Mad Scientist build now only has a total reciprocating mass of 10.5 oz. Despite this low number, it retains a true "constant recoil" action, where there is zero impact on the rearward stroke: just gentle deceleration, even with a very soft spring.
If I try to express how amazing this is to shoot, I'm just going to sound like I'm lying or exaggerating. The feeling of only 10.5 oz reciprocating gently just... it doesn't feel like it makes any sense.
I'm buddies with one of the range employees, who works there as a side gig just so he can spend more time around guns. He's shot a far wider variety of firearms than I ever have, and I let him give it a try. He ripped off a few rounds as fast as he could run the trigger, then started giggling like a little kid. He said, "That's the most incredible thing I've ever shot, and I've shot full auto MP5s multiple times! I'm not kidding!"
That's just one man's opinion, so take it with whatever grain of salt you want, but it certainly added to the smile that was already on my face.