It’s not always possible to separate guns from loads, and there are some important things to know to get the most from your semi-auto. Here’s one! KEEP READING
I have spent the last couple of segments taking a big step back recollecting my own (early) experiences and education as a handloader. Hope you’re happily indulging me, and hope even more that there’s been some good ideas that have come from it.
I started reloading as a matter of economy, and because I wanted to shoot more. Said then and said again now: if the impetus for reloading is saving money, you really don’t save money! You just get to shoot more for the same cost. Hope that makes sense, and likely you already understand that. Clearly, there are other reasons or focuses that attract folks to handloading, and personalizing ammo performance, improving accuracy, are leading reasons.
I’ve been at least a tad amount (to a lot) biased all along in my department topics toward loading for semi-automatic rifles. That’s been done for a few reasons, and the primary one is that, no question at all, there are specific and important details, a lot of dos and don’ts, in recycling ammo for a self-loader.
This is the reason I’ve been careful to specifically point out the “semi-auto” aspect of any tooling or preparation step. I’d like some feedback from you all with respect to your motivations and applications in handloading. Why do you do it?
Another reason is that, and I know this from much input, as happened with me 45 years ago, my interest in learning to reload came with ownership of a semi-auto that I absolutely loved to shoot! Here of late, my plumber, for a good instance, proudly announced to me outside the local hardware store that he had just purchased his first AR15 and showed me the paper bag full of .223 Rem. cartridges he had just purchased there. A scant few weeks later: “Could you help me get together some tools and show me how to reload?” I did.
Back to the focus, finally (I know) of this topic: what are those differences comparing semi-autos to anything else?
There are a few points, but one of the first, and one of the most important, is component selection. Case, primer, propellant. Propellant first.
I’ll assume, pretty safely, that the semi-auto we’re loading up for is an AR15, or some take on that platform. If so, it will have a “direct impingement” gas system. That’s a pretty simple arrangement whereby the gas pressure needed to operate the system, which cycles the action, is bled off from the barrel bore via a port. From there it goes through a manifold and then into a tube, and then back into the bolt carrier via the bolt carrier key. Gas piston operation is more complex, but what’s said here applies there also respecting propellant selection.
So, it’s kind of a wave. The idea is to get the wave to peak at a point where there’s not excessive gas entering the system, but there is sufficient gas entering the system. Mil-spec. 20-inch AR15 calls for 12,500 psi, for what that’s worth. And “piston” guns are nowhere near immune from concerns about port pressure.
The burning rate of the propellant influences the level of gas pressure at the gas port, and this, easy to understand, is referred to as “port pressure.” The original AR15 rifle gas system component specs (20-inch barrel, port located at 12 inches down the barrel) were created to function just fine and dandy with 12,000 PSI port pressure. Much less than that and there might not be enough soon enough to reliably cycle the works. Much more than that and the operating cycle is accelerated.
Port pressure and chamber pressure are totally separate concerns and only related indirectly.
Rule: slower-burning propellants produce more port pressure than faster-burning propellants. As always, “faster” and “slower” are relative rankings within a variety of suitable choices. The answer to why slower-burning propellants produce higher pressure at the gas port comes with understanding a “pressure-time curve.” A PT curve is a way to chart consumption of propellant, which is producing gas, along with the bullet’s progress down the bore. It’s what pressure, at which point. I think of it as a wave that’s building, cresting, and then dissipating. Slower propellants peak farther down the bore, nearer the gas port. Heavier bullets, regardless of propellant used, also produce higher port pressures because they’re moving slower, allowing for a greater build-up about the time the port is passed.
To really get a handle on all this you have to picture what’s happening as a bullet goes through the barrel in a semi-auto, and keep (always) in mind just how quickly it’s all happening. Milliseconds, less than a few of them, define “too much” or “not enough.” As the bullet passes the gas port, there’s still pressure building behind it, and there’s more pressure building still with a slower propellant. After the bullet exits the muzzle, the pressure doesn’t just instantly go away. There’s pressure latent in the system (all contained in the gas tube and bolt carrier) that’s operating the action.
The symptoms of excessive port pressure come from the consequence of a harder hit delivered too soon, and what amounts to too much daggone gas getting into and through the “back,” the bolt carrier: the action starts to operate too quickly. The case is still a little bit expanded (under pressure) when the bolt starts to unlock and the extractor tugs on the case rim, plus, the increased rush of gas simply cycles the action too quickly. That creates extraction problems and essentially beats up cases. They’ll often show bent rims, excessively blown case shoulders, stretching, and so on.
Getting gas port pressure under control makes for improved function, better spent case condition, and less wear and stress on the gun hisseff.
There’s a huge amount more to talk about on this whole topic, and a good number of ways to get everything working as it should. But. For this, the most a handloader can do, and it’s honestly just about the most influential help, is to stay on the faster side of suitable propellants. Without any doubt at all, there will be rampant disagreement with my advice: no slower than Hodgdon 4895. Most all published data lists propellants from faster to slower, so find H4895 and don’t go below it. That’s conservative, and there are a lot of very high scores shot in NRA High Power Rifle with VARGET and RE-15, but those are edgy, in my experience, and define the very upper (slowness) limit.
That alone doesn’t mean all AR15 architectures will be tamed (carbine-length systems are particularly over-zealous), but it does mean that port pressure will stay lower, an important step.
A caution always about factory ammo: some is loaded for use in bolt-actions (especially hunting ammo(, and might bea very bad choice for your .308 Win. semi-auto. AR15s are actually fairly more flexible in showing clear symptoms, some no doubt due to the buffered operating system and overall mild nature of the .223 Rem. cartridge.
This article is adapted from Glen’s books, Handloading For Competition and Top-Grade Ammo, available at Midsouth HERE. For more information about other books by Glen, visit ZedikerPublishing.com
9 thoughts on “RELOADERS CORNER: Gas Port Pressure”
Glen, you just barely scratched the surface. To fully understand how to load for a gas gun, you need to know about not only overall burn rate, but how much gas volume a powder generates, the size of the gas port, using an adjustable gas block, whether its a carbine, rifle, or mid-gas system and so on. The guys I hang with do a LOT of wildcat work in the AR-15, in calibers that cover everything from .224 to .458. So, we are all pretty checked out on gas systems. Once you get the hang of it, its pretty easy.
I agree. Burning rate is only the first step. Hard to cover it all in the space I have. I guess that’s why I do books… Haha.
Like John said Mike, this is a wide open subject with many different answers. One of which that I was wondering is can you use a gas key on the bolt to regulate pressures? The positive side is sharing a tunable BCG for as many different uppers that that bolt will work with and you don’t have to have an adjustable gas block for every upper that you have! The down side (I think) is that the gases vent into the upper receiver and that would lead to having to have to clean the upper more frequently and over heating of the upper frame as well! I have never used a BCG with a gas key to regulate gas pressures or an adjustable gas block so any information on the different adjustable systems I would really appreciate!
There is an adjustable gas key that seems to work pretty well. I have used one. It’s from Sun Devil Mfg., same who make the billet receivers.
GAS operated rifles are a complicated syst yet very simple. there is hardly any comparison between bolt rifles and gas operated rifles.1st you have to understand how the gas syst works. the time delay between the time the bullet leaves the cartridge mouth till the gas starts working on the bolt. One must understand the burn rate of the powder your using and not all rifle powders CAN be used. adjusting the gas port diameter will widen your choices of powders somewhat. And finally how heavy the bullet is.
As noted, this article, while good only scratches the surface. The thing to do as noted from posters above is for each reloader to understand the nuances of the cartridge of their choice and how it works with their gas system.
I would make one correction to the article. “” Heavier bullets make a given powder burn faster. “Heavier bullets, regardless of propellant used, also produce higher port pressures because they’re moving slower, allowing for a greater build-up about the time the port is passed. Which means they also burn that volume of powder at a higher initial pressure. So, for a gas gun using heavier bullets you should use a slower powder, but it has to match max chamber and then max port pressure. Chamber and port pressures are related directly through the burn/pressure drop curve. What each can be still needs to be within operating limits.
The article was very interesting, but you barely mentioned the effect of bullet weight, and didn’t touch on the effect the case can have. For example, while Win748 works beautifully with Winchester brass, in my .30-06 Garand, I experienced 100% feed failures using the same series of test loads in military brass.
The heavier bullet just goes slower, and that’s its influence on pressure inside the barrel. And you are certainly right: big differences in cases related to their capacity. I generally prefer those with larger volume, like the Winchester brass you mention.