The Gentle Recoil System (GRS) for 9mm blowback has proven to be a success based on the feedback I’ve received. Can we make it smaller and still have good results?
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Please review the original Gentle Recoil Project page for the background on this work.
Feedback on the GRS has been very good. Most people who have tried it noticed a significant difference in felt recoil. A few didn’t notice a big difference, but there’s a number of factors involved, including how they perceive recoil, the ammo used, and barrel length.
The success of the GRS is based on the use of high buffer mass (11oz.) to slow the cycling, a hydraulic buffer to dampen buffer impact at the end of travel, and a flatwire spring to reduce resonance. To make this setup work with a Kynshot RB5007/RB5015HD PCC hydraulic buffer and two Kynshot 2.5oz spacer weights, we needed to use a Just Right Carbines (JRC) 8.5″ buffer tube which is 1.5″ longer than normal.
If we shrink it down to fit into a smaller tube, will we still get good results? There’s only one way to find out…
Enter the Kynshot RB5000HP and Kynshot RB5004 “Shorty” buffers:
RB5000HP:
The Kynshot RB5000HP is a carbine sized-buffer with higher dampening characteristics than the normal Kynshot AR buffer. It weighs 4.8oz. With 1 spacer weight between the buffer and the bolt, it will provide the correct bolt travel in a carbine tube with 7.3oz. of mass. With 2 spacer weights and a Vltor A5 buffer tube (7.75″) we’ll have the correct bolt travel and our mass will be 9.8oz. With 3 spacers we’ll need a JRC tube, but our mass would be a whopping 12.8oz.
The RB5004 “Shorty”:
The Kynshot RB5004 “shorty” is even smaller. This little pipsqueak of a buffer is only 2.5″ long compressed and weighs about 3.8oz. With 2 spacer weights between the buffer and the bolt we get the mass up to 8.8oz., and our bolt travel is correct for a 9mm bolt in a 7″ carbine or pistol buffer tube. An A5 tube lets us use 3 spacer weights (11.3oz. total), and the JRC tube lets us use 4 spacer weights (13.8oz. total).
But the 5004 presents us with a problem.
The 5004 is so short that we have only about 2.25″ of room for the compressed spring at the end of the buffer stroke, because the spring compresses around the buffer body. A standard carbine spring has about 37 coils, each one is about 0.072″ thick, so it needs 2.7″ or more space to work properly. With only 2.25″, a milspec carbine spring would compress to the point where all the coils are touching. This is called “coil bind” or “solid height” and it’s very bad. Coil bind damages springs and prevents the buffer from making a full stroke. Our best option is to use a spring with thinner coils.
Luckily a flatwire carbine spring, which also happens to be the quietest option and the original choice for the Gentle Recoil Project, is perfect for the job and I confirmed it won’t coil bind with the 5004.
[Note: I recently bought a Wilson Combat flatwire ($16) and so far it’s the best fit on Kynshot buffers, so these are going to be my choice moving forward until something better comes along.]
WAIT A MINUTE!!! The RB5004 “Shorty” page says “NOT FOR 9mm”!!!
It sure does!
I spoke to Kynshot and although they can’t officially condone it, they didn’t say it wouldn’t work, either. I believe the main reason for the warning is just because of the length of the buffer. Using this buffer by itself without the proper spacers WILL cause the bolt “gas key” to smash into the rear of the receiver and cause damage, and would cause coil bind with a normal carbine spring if the buffer could even get back that far. Adding the 2 spacer weights and a flatwire spring solves both those problems.
Since the hydraulic piston seems to have the same compression force as the RB5007, I expect it will be fine from that perspective.
Compression force:
I tested both the amount of force needed to fully compress all three hydraulic buffers.
The 9mm RB5007 and the RB5004 “Shorty” both need about 5 lbs. of pressure to fully compress the hydraulic piston. So from a dampening perspective they seem pretty similar. I can’t measure how they perform under a shock load.
The RB5000HP needed about 10 lbs. of pressure to compress. This really is a “high dampened” piston, and this became very important later in the testing.
Note too, the 5004 has shorter travel for the hydraulic piston. The “stalk” between the body and head is shorter than the others. This means there’s less room for it to absorb the impact at the end of the buffer’s travel, and this may affect the “feel” to the end user.
The Different Combinations:
Here are possible buffer mass/buffer tube configurations for these hydrauilic buffers that should all have the proper spacing for 9mm bolt travel. I could not test them all out, and to be honest they started to all blend together after a while and it became difficult to tell the difference, especially since I had to stop and reconfigure the buffer setups each time I switched.
Carbine 7″ tube:
- 6.0oz.: Carbine tube, flatwire spring, RB5007/RB5015HD, no spacer weights. [Not recommended – too light to reach 22oz. total bolt+buffer mass.]
- 7.3oz.: Carbine tube, flatwire spring, RB5000HP, one 2.5oz. spacer weight.
- 8.8oz.: Carbine tube, flatwire spring, RB5004 Shorty, two 2.5oz. spacer weights.
Vltor A5 7.75″ tube:
- 8.5oz.: Vltor A5 tube, flatwire spring, RB5007/RB5015HD, one 2.5oz. spacer weight.
- 9.8oz.: Vltor A5 tube, flatwire spring, RB5000HP, two 2.5oz. spacer weights.
- 11.3oz.: Vltor A5 tube, flatwire spring, RB5004 Shorty, three 2.5oz spacer weights.
Just Right Carbines 8.5″ tube:
- 11.0oz.: JRC tube, flatwire spring, RB5007/RB5015HD, two 2.5oz. spacer weights. (Original Gentle Recoil Setup)
- 12.3oz.: JRC tube, flatwire spring, RB5000HP, three 2.5oz. spacer weights.
- 13.8oz.: JRC tube, flatwire spring, RB5004 Shorty, four 2.5oz. spacer weights.
Testing:
So now that I’ve done all the math, all I needed to do next was test them. For testing I brought all three buffers and all my Kynshot weights to the range with the different buffer tubes. I tested a variety of combinations with 2.5″, 5.5″, and 16″ barrel uppers using a KVP 14.7oz. bolt and 115gr. range ammo.
Results:
As the weight went up, the felt recoil improved. As barrel length went down, felt recoil improved. The biggest improvement was seen with heavy buffer setups and short barrels.
However, even at the lighter weights the felt recoil was better with the hydraulic buffers in side-by-side comparison with a heavy deadblow buffer.
In all the categories, in all the tubes, the 5000HP felt like it performed better for reducing felt recoil than the 5007 or the 5004. The higher dampening characteristics of this buffer made a noticable difference to me.
The 5004 Shorty seemed to perform a little less well than the other two. I don’t know if it’s because of the shorter “throw” of the piston or something else. Otherwise, it worked fine and still felt better (with the proper mass) than the deadblow buffers.
My choices:
MAXIMUM IMPROVEMENT: For the biggest improvment in felt recoil, the heaviest options seem to be the way to go. Using a JRC buffer tube, the “Standard Gentle Recoil System” using a 5007 or 5015HD and 2 spacer weights for 11oz. total is a great setup and really knocks down the felt recoil. An alternate for the JRC tube could be the 5000HP + 3 spacer weights for 12.3oz. I tried it and it is very nice. It’s a least as good as the original setup, if not a little better.
COMPROMISE OF SIZE AND MASS: For the mid-size A5 buffer tube, the 5000HP + 2 spacer weights (9.8oz.) is my first choice. An alternate for the A5 tube could be the 5007 or 5015HD + 1 spacer weight (8.5oz.)
SMALLEST OPTIONS: For the carbine buffer tube (the one you’ve all been waiting for) the 5000HP + 1 spacer weight (7.3oz.) OR the RB5004 Shorty +2 Kynshot spacer weights (8.8oz.). Both seemed to feel basically the same to me but I slightly preferred the 5000HP. Note that a flatwire spring is absolutely required when using the 5004.
I was really expecting the RB5004 Shorty +2 Kynshot spacer weights (8.8oz.) to come out on top for the carbine buffer tube, but in side-by-side comparisons the recoil impulse was practically identical to the 5000HP +1 spacer weight. The higher dampening characteristics of the 5000HP seems to make a significant difference in felt recoil. Both felt better than the 11oz deadblow buffer, but not as good as the full-weight Gentle Recoil System.
My concerns:
With the Aero 16″ barrel, all of my buffers bounced. I tested the 5000HP, 5007, and 5004 for bolt bounce during different filming sessions and I did not record any bolt bounce from them using different uppers. I suspect that the Aero barrel’s chamber is a little too long, causing the bolt to bounce off the chamber face (believe it or not, in a 9mm blowback the bolt face and chamber face should not touch when a cartridge is loaded).
For the 5004, my concern is durability, and that shorter piston travel. Will this little tiny buffer hold up to the pounding all the extra weight from those spacer weights, especially in the heavier configurations? It’s made for .308 rifles, so it should be fine but I have no way of telling. That shorter piston stroke may be having in effect on it’s dampening, and perhaps that’s why it didn’t seem to perform as well.
Next steps:
Since the 5000HP has such heavy dampening characteristics, I think a standard 5000 may be a good option for future testing and would allow a more apples-to-apples comparison with the 5007 and 5004.
In the end…
I guess you could say I’m now a believer in hydraulic buffers. They still suffer from insufficient mass by themselves, but with some clever mixing-and-matching of different parts we can make up for that without any issues.
Another testimonal!
This time from a reader using the 9.8oz., A5 buffer tube Gentle Recoil System setup:
“So the gentle recoil system (A5 Length Tube, Kynshot 5000HP, 2 spacer weights AR Stoner flat wire spring) on the direct blowback felt better than my CMMG RDB with just a standard spring and buffer.”
Blowback9.com 