Two things.
1. I would think that with this 'long stroke' design after the last round is fired, the BCG now has more momentum to slam into the BHO than a standard configuration and I guess could break it...this is what we see happening on 9mm blowback uppers..people making these long buffers to help prevent that.
2. I personally have too many other configurations that rely on the standard stroke length and personally want to minimize how many times I'm swapping buffer/spring combos...
On that note, I've posted in other threads about my tweaking with the new 9mm CMMG Guard which is a delayed blowback operating system that uses a similar bolt and carrier configuration to a standard AR setup. So the 'geometries' are the same.
Through discussions on these boards, I've done testing with what I think is best setup so for this system. (at least for me running full auto and desiring a slow RoF and reliability)
This is documented here:
http://c3junkie.com/?page_id=538
This combination is: A5 buffer tube, 556 Tubb flat spring and '9mm' Blitzkrieg/Kynshot hydraulic buffer.
The '9mm' Kynshot buffer is the same compressed length as a standard A5 buffer...below is HeavyBuffers.com's A5 buffer.
Now this is the '9mm' Blitzkrieg/Kynshot hydraulic buffer - compressed.
And now this is it extended:
I believe this hydraulic buffer is absorbing the initial recoil impulse and probably does addition absorption on the rearward stroke as well.
Combine that with the rifle length spring that isn't working as hard as a carbine spring and has a more consistent load, results in a smooth and slower running setup.
David Tubb has a great video explaining and proving the consist load of his springs here:
https://www.youtube.com/watch?v=8fMVZbrnvu8&t=8s
A byproduct of this testing is that the same combination is working great for me in other configurations like my shorty 5.56 suppressed and unsuppressed configurations, 9mm Belt Fed FM9 and 5.56 Shrike (However with the Shrike I have to use the 308 Tubb spring).
The blow back FM9 was shooting fast and giving me trigger slap.
With this configuration, It smoothed it out and I now get: Rem 115 Gr Green box 518RPM
With my Shrike it also shot so fast I couldn't get singles and now I get the ROF's below and easily pulling singles (using 308 Tubb spring).
Short barrel M855 630 RPM
Long Barrel M855 693 RPM
I can pull singles easily with those RoF's.
My go to setup is a 12.5 suppressed upper and I can now pull singles with that. I can't find my notes on RoF right now...I'll have to redo that...but I'm guessing I'm in the 600's.
Below is a picture of the old 'Counterpoise' system:
'D' is the weight that goes in the carrier. Years ago, Biggerhammer.net (the original hosting site for subguns.com) had a review on their site years ago and it is no longer there...they had a positive review on the counterpoise and then later revised it saying that the sliding weight cracked and then a chunk of it fell into the lower...obviously causing issues of course.
I would be interested in trying the Ferfrans but would be leery the weight breaking off.
I kinda like the one below.
https://www.rainierarms.com/master-of-arms-ar-15-titanium-bolt-carrier-group-w-deadblow-kit/
They have 3 different ball weights...aluminum, brass, or tungsten but the carrier is Ti...don't think I want the LW carrier. Would rather have a steel carrier with the ability to have that weight system and wave springs.
I think I have everything dialed in pretty nicely now...but I would like to try a carrier with a moving weight at some point in conjunction with the buffer system I have now...but not interested in increasing the stroke due to the two reasons I mentioned above and the fact that I'm getting some nice RoF's now.