Sample 1


Up one level
Sample 1
Sample 2
Sample 3
Sample 4
Sample 5
Sample 6
Sample 7
Sample 8
Sample 9


Quarterbore's 300 Whisper Project

As described though out my 300 Whisper pages, the 300 Whisper is cartridge that has unique character as a multipurpose round ideally suited for a number of pistols and rifles. In this project I am looking at having an SSK AR-15 upper built that will accomplish a few goals that are unique for any AR-15 upper much less an upper built in 300 Whisper.

To begin, I assume that most people that would read this knows that a USGI M7 or M9 bayonet will correctly mount on a 20-inch and 14.5-inch M-16 upper without problems. This is because the full length and short-carbine grips are the correct length for the front sight with the bayonet lug to be mounted where they need to be for a mounted bayonet's ring to sit on the flash suppresser in the area of the barrel's threads. Following is a photo of a pair of Pre-Ban Bushmaster XM-15s that show a pair of Buck M9 bayonets mounted. Note the top rifle has a 20-inch HBAR upper and the bottom rifle has an 14.5-inch M4A2 upper.


Bushmaster AR-15s with M9 Bayonets

Note, the length of barrel from the bayonet lug to the
     the flash suppressor is the correct length for the
bayonet ring to sit at the correct location.       


My project took a lucky turn when I purchased a unique Medium Short FF RAS forend tube manufactured by Knights Armament (KAC P/N: 991687). Knights labels this part as an "M4 (FF) Rail Assembly System" but please be advised that this forend tube is longer than a standard length carbine grip. This Medium Short FF RAS measures 7 5/8-inches overall length making it the proper length to allow me to use it with a 16-inch barrel that will mount a bayonet. The Knights FF RAS Assemblies are available in four distinct lengths that I describe in considerable detail in my Knights FF RAS Assemblies page.

I make the statement that this Medium Short FF RAS is the correct length to mount a bayonet because a Full length FF RAS (KAC P/N: 99266) is 12-inches while my Medium-Short FF RAS (KAC P/N: 991687) is 7 5/8-inches. If I use the FF tube length as an indicator of barrel length and subtract the 7 5/8 (my mid-length FF RAS) from 12-inches (my full-length FF RAS) I get a difference of 4 3/8-inches. If I then take my 20-inch barrel and subtract the 4 3/8-inch length difference I come to the conclusion that my Medium-Short FF RAS will allow me to correctly mount a bayonet if I have a barrel that is 15 6/8-inch in length.

Given the above measurements, and the facts that I am doing this project in 300 Whisper means that I will need to have a custom barrel made. This also works in my favor as with a custom barrel I can have it profiled to allow me to mount the gas block wherever I need. In this way, the barrel can be the properly sized to allow me to mount the gas block so that the bayonet lug is the proper distance from the barrel threads. Given that the FF RAS tube does not have any forward attachment to the barrel or front sight, I am then able to move the front sight base forward of the FF RAS the 1/4-inch that I get from a theoretical measurement above. This will allow me to have a 16-inch AR-15 upper with the following advantages:


  1. Minimal 16-inch non-NFA length with removable flash suppresser.
  2. Ability to correctly mount a bayonet.
  3. Longer forend for more comfortable shooting hold and more Mil-Std-1913-rails for mounting accessories.
  4. Longer sight radius for better sight performance.
  5. Silencer addition will not make the gun look/feel nose heavy.
  6. Disassembly of upper, swapping flash suppressers, adding a screw-on silencer possible because of #1 above.


The next major task in this project is figuring out how to incorporate an adjustable gas tube. As I am sure most visitors here know, the AR-15 and M-16 rifle is gas operated meaning that propellant gas is used in a manor that cycles the action. When a cartridge is fired, gasses trapped behind the bullet enter the gas port in the barrel and gas tube, which conveys the propellant to the bolt carrier group. See Chapter 2 of FM23-9v1, a training manual for the M16A1 and M16A2 for more details.

In the case of the 300 Whisper, the wide variety of pressures experienced with 120-gr to 240-grain bullets in subsonic and supersonic velocities creates a tremendous challenge. The challenge is due to the wide variety of pressure curves experienced based on the bullet, powder, and charge combinations used. I have received many E-Mails from visitors of this page over the past few years and it is clear to me that to get the most out of this very versatile cartridge, it is necessary to be able to control this pressure via an adjustable gas system. This in turn improves accuracy and operation while reducing some of the wear and tear on the weapon itself. In addition, the ability to turn off the gas system will turn an AR-15/M-16 upper with a silencer into a charging handle operated bolt action that would be the ultimate quiet upper that retains the ability to be run full auto with an M-16 lower when the gas system is simply turned back to it's proper location!

I had found a full length FULTON ARMORY Adjustable gas tube that I had considered using for this project. Following is an image of the adjustment head for this gas tube...


I purchased the full length adjustable gas tube (AGT) because of the special mid-length nature of my project and the fact that a carbine AGT would have been too short. From looking at the AGT it looks like the adjustment head of the AGT may well line up with one of the holes through the top of the FF RAS. The AGT will need to be adjusted as the tube has the proper profile needed to work with a 20-inch upper. In addition the AGT is much longer than I need and I will need to carefully determine the correct length that I require as well as confirming that the Adjustment head will continue to line up with the opening in the FF RAS. Then I can bend the tube to the profile that I require, cut the tube to the correct length, and reshape the end of the gas tube that fits inside of the AR-15 carrier's key.

Well, this is where my project took a turn as I called JD Jones at SSK Industries to see if he is willing to build me a 1:8 twist barrel to the specific profile and length that I require. The 1:8 twist barrel is needed with the 300 Whisper to stabilize the heavy bullets at subsonic speed. I have also heard from many 300 Whisper users that have advised that the lighter bullets are also quite accurate when fired from this fast twist rate. When I talked with JD Jones about the barrel, we also discussed the AGT.

JD Jones said that they had tried using these types of gas tubes in their 300 Whisper uppers in the past and that they had problems. JD accordingly recommended against using the AGT for this project with a strong recommendation for his own AGT system. Accordingly, I have decided not to modify my Fulton Armory AGT as I will end up using it on an AR-15 223 upper project which will not require the adjustability of a 300 Whisper upper.

My next step for this project is to come up with some money and send all my parts including a new Bushmaster Flattop upper, bolt - carrier, free float RAS, GG&G flip up front sight base with bayonet lug, and any of the remaining small parts to SSK and have them make me a special barrel with a special gas tube that will bring this project to completion!

Obviously, I am just beginning this project but as I would hope you could tell, I have given careful consideration to how this project will be completed. I hope to share this project as it comes along, as I will be updating this page by adding text, images, and links as I work through the issues associated with this project. I welcome anybody with suggestions, comments or questions to E-Mail me and I will gladly give credit where credit is due!



More 300 Whisper Weapons

Sample 1 • Sample 2 • Sample 3 • Sample 4 • Sample 5 • Sample 6 • Sample 7 • Sample 8 • Sample 9

The Rest of the 300 Whisper Pages

SSK's Whispers® • 300 Whisper Brass • Sub-Sonic Loads • Super-Sonic Loads • Sample Weapons • 300 Whisper Links • Whisper on EBAY

Return to


Please Note: I am looking for additional images and descriptions
of 300 Whisper or 300/221 Fireball rifles. Please Contact Us if you
would like to see your rifle or pistol on this website!

Go to

This site was last updated 03/26/04