Experimenting with maximizing BCG Stroke Length – M16 ‘Ultimax’

The inspiration for this tinkering was to try to get a constant force push like the Ultimax without making drastic irreversible changes to the host firearm.  While this testing was focused on full auto, it also shines for suppressed usage with very little gas to the face.

The desire was also not to go crazy with reciprocating mass as that can make the gun bouncy.  The goal was a very smooth cycling configuration with a very controllable cyclic rate.

If you don’t know what the Ultimax is, I would recommend that you watch the following video with an interview of Jim Sullivan who helped to downsize the AR10 to the smaller AR15/M16: https://www.youtube.com/watch?v=gOUKXIrDE0I

Jim is also the designer of the Surefire OBC in which he tried to get as close as possible to the Ultimax without making a lot of changes like I am doing.



Pictured below is the baseline MK18 configuration with the only variable being changed is the increased stroke length.

If you look at this link: Vltor A5 – My Personal Opinion and Testing you will see that the final configuration at the bottom of the page was 625 RPM with the MK18.  Now with the JRC tube, I can use my favorite RB5007 buffer and get an even lower cyclic rate than before and still be very smooth but the carrier still bottoms out.

Now see how the LWRC integral carrier performed.  Almost identical cyclic rate as the Surefire OBC.  Note that the Surefire has less stoke distance but also has the modified cam pin path geometry as well a spring loaded carrier weight designed to slow down the cyclic rate and increase reliability.


Taking the MK18 rail apart is tedious so I use a 10.5″ for tinkering.  Configuration below is using the PRI Fat boy gas tube.  Super slow cyclic rate and still very smooth.  The PRI Fat boy gas tube combined with the appropriate port sizing was the key to getting the carrier not to bottom out.

Video of configuration above:

I tried the LWRC carrier and again obtained almost identical results…only 11 RPM faster than the Surefire OBC setup.

Video of setup above:


For suppressed usage, very large port sizes were not required.  In both suppressed configurations below, you can see they are very smooth and very little gas to the face.


BTW, I did clock this suppressed full auto configuration below and it was 561 RPM


In conclusion, you have two options for getting a setup where the carrier doesn’t bottom out.  In both scenarios they require:

  1. Kynshot RB5007, Botach has the lowest price but good luck ordering from them.
  2. PRI Fat boy gas tube: https://www.mountsplus.com/pri-fat-boy-gas-tube-AR-15-Gas-Tube.html
  3. 556 Tubb flat spring, Wilson combat also has a flat spring.  Tubb and Wilson combat get the springs from the same source and they both fit nicely over the RB5007.  Do not get Strike Industries flat springs as they are too tight.
  4. Rifle length buffer tube with spacers or the JRC buffer tube (no spacers for LWRC or POF Ultimate BCG) or the equivalent of 4 quarters if using a Surefire OBC.
    Link to JRC buffer tube: https://justrightcarbines.com/product/long-buffer-tube/?v=7516fd43adaa
    I was also told that Vltor makes an 8.5″ tube to run the A5 buffers in 308’s so that is another possibility.

If you go the Surefire OBC route, it will be all drop in parts.

If you go the LWRC / POF Ultimate BCG route, you will need to get the charging handle milled out to prevent the key from slamming into it.

If you want to go with a dedicated suppressed setup and need a .052″ port, I would recommend getting the YHM suppressor gas block.  Picture below is of one I purchased and it was already at .052″ so a drop in solution for this configuration.  If you run weaker ammo, you can simply drill it out to be a larger port size.
Note also that the port is also countersunk .093″ to allow for some misalignment.

Again, my testing was with a 556 Specwar.  If you are using a .30 cal can or something with lower back pressure, you will need to adjust accordingly.


Update:  I was doing all my testing above with Colt / Milspec firecontrol parts.  I tried to move to a post sample lower using a Geissele SSF and you can see that the different hammer geometry of the SSF, that now the hammer is in front of the carrier while this same carrier, buffer and tube configuration worked fine before.  So keep this in mind if you are not running Colt / Milspec firecontrol parts.

However, as pictured below the configuration of using 4 quarters with the JRC, A5 worked with the Colt / Milspec firecontrol, works perfectly with the Surefire OBC as well.



Coming soon: I plan on replicating some of this testing with a 12.5 Mid gas and 16″ Rifle gas.  The MK18 / 10.5 was used since it is a good baseline.

Pictured below is some testing I did with a 11.5 mid gas barrel and a custom large volume gas tube that I TIG welded together.  I suspect that it isn’t bottoming out but didn’t do any high speed video footage of it yet.

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