Got Gas?

There are a number of things that tend to be the most critical in a reliable AR pattern rifle. Barrel quality, chamber dimensions, bolt and carrier quality, solid small parts and quality magazines are some of them. One of the areas that can be the most confounding is the balance between the amount of gas introduced to the operating system through port size, location, and barrel length past the port against reciprocating mass and spring force.

The dilemma faced by manufacturers is the variety of ammunition to be used by the customer and how that affects the overall system. Many manufacturers over the years have dealt with complaints that, "My rifle won't cycle with (insert underpowered .223 pressure level ammunition here)." As a response, some manufacturers that have a significant number of customers experiencing something like this accommodate by using larger than optimal gas port dimensions and a regular carbine (3 steel weights) buffer. While this solves the underpowered ammunition problem, it makes life sporty when 5.56 pressure ammunition is used--exhibiting an overgassed condition that can be just as detrimental to proper function, manifesting as bolt-over-base stoppages and other feed path complications. An additional recoil force is also added, when the too-light buffer bottoms out harshly at the back of the receiver extension. Overgassed guns, when they do function, are more difficult to keep on target.

Manufacturers of guns truly intended for duty use USUALLY gas and buffer their rifles appropriately for duty ammunition. While this my cause issues with underpowered .223 ammunition, it provides the correct bolt speed for duty use, allowing proper stack rise time from the magazine, unlocking forces are appropriate for best bolt life in a given gas system length, and buffer/spring combinations provide enough mass to slow bolt speeds and also provide enough "oomph" to push through sludge and debris as the rifle becomes dirty.

While the formulas for doing that correctly in a 14.5" or 16" carbine or even mid length gas system as well as 10.5" carbine systems are fairly established through either TDP or accepted practice, many new combinations of gas system and barrel length can introduce a confusing set of specifications that are hard to determine at first glance if they are "correct" and demand some trust in the rifle manufacturer's testing.

Sometimes an overgassed condition can be improved simply through the addition of buffer mass. Going from a standard carbine buffer to an H, H2, or even H3 may solve the issue and return the balance to a given system. Sometimes that's not enough, and gas port sizes that would probably be more at home on an AK confound all solutions short of an adjustable gas block--which, although getting better and better, are still best not used on duty rifles.

Although adding a little extra spring force, such as with a Springco Blue or even Red, may seem like a way to deal with an overgassed solution, it's not the proper way. Yes, having a bit of extra spring can help push through grime in a dirty rifle, so long as the system is properly balanced, changing weight of reciprocating mass is the correct way to deal with the correction. Add spring because you want more spring, not because you have too much gas. Overly powerful buffer springs also tend to add a bit of muzzle dip on closing, complicating keeping the rifle on target even further, so balance is important.

On competition rifles, the script is flipped, and lighter reciprocating mass and higher bolt speed is somewhat accepted to increase cycle speed and reduce primary and secondary recoil impulse. The sacrifice is that such guns prefer to run clean and well lubricated and don't have the extra reciprocating mass to push through as much sludge as a system that introduces more energy and weight into the operating system. We build carbine buffers with the steel weights replaced with aluminum weights and use low mass bolt carriers on competition rifles.

The bottom line is that the best way to build a quality rifle is to understand from the start the ammunition intended for use, the intended purpose, and gas and buffer it appropriately for that purpose. There are many resources online to help understand who is using what size gas port and what buffer in factory rifles, and these provide good starting points for custom builds. In any case, it's a wise buyer that researches the track record of the manufacturers from which they are selecting a rifle, and purchases appropriately for their intended use.