Tag Archives: AR15 loading

RELOADERS CORNER: Gas Port Pressure

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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

casing in air

Glen Zediker

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.

AR15 gas port
As .224-caliber bullets get heavier, there’s a tendency toward many using slower-burning propellants. Often, the slower-burning fuels produce lower chamber pressures, which means more velocity potential (that’s true with just about any rifle cartridge). But! Gas port pressure will increase with slower and slower burning propellants. Can’t have it all, and make sure “function” is first on the list. That’s safe and sane function, by the way, not “over-function!”

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.

RE15
I put the (very safe) cut-off at H4895 burning rate. I’ll go as slow as RE15, and have with safe success, but its influential differences are noticeable. I can tell you that a 4895 is well within the optimum range to deliver intended port pressure (“a” 4895, mil-contract variety, was actually the early original 5.56 propellant).

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.

m14 gas system
This doesn’t only apply to AR15s. The M1A is VERY sensitive to port pressure, which is also propellant burn rate. It’s a gas-piston gun. Same cut-off on burning rate is advised for these: H4895. I sho learned this the hard way by dang near wrecking my first M1A: bolt stuck back after firing a dose of H4350. That was before I met Sgt. Jim Norris and got the lecture I’me giving you. Thanks Sarge!

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

 

RELOADERS CORNER: Semi-Auto or Bolt-Action? Two Things To Remember

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There are essential differences in loading for these action-types. It might not matter if you know all about the one, but it is critically important to know about the other. Find out which is which… Keep reading!

casing in air
Any rifle with a gas operation system has to, well, have gas to operate! When it gets excessive is when the problems start. That’s another article, but the effects of the operating system is the basis for both the cautions in this article.

By Glen Zediker

Over the time I’ve been producing Reloaders Corner here at Midsouth, my focus has been exclusively on reloading for rifles, and, within that, primarily for semi-automatics. The reasons for that are based on two things, one is an assumption and the other is plain old fact. First, semi-autos are popular and represent the interest of a great number of new reloaders out there, and that’s my assumption. It doesn’t take long to come to the conclusion that high-capacity magazines and long days at the range combine to get expensive in a hurry! But the biggest reason I focus most of my material toward the needs of the semi-automatic rifle is because there are decidedly important differences in some decisions the handloader makes when tooling up for one. That’s the fact. Not knowing or respecting these differences can be disastrous.

I set out to be a sticker for clarity, but sometimes I overlook making more pointed references to these differences, when there are options associated with any one topic. I judge that based on the feedback I get from you all respecting tooling and component options. I want to start the New Year with this article, which I think contains some basic and important information to always (always) keep in mind. Hopefully it will also reduce questions, and I sure hope confusions. It also seemed to be, judging on feedback, the topic that created the most questions and comments.

Essential: When a round fires, the case expands, in all directions, as much as it can to fit the chamber. Since brass is elastic (can expand and contract) and plastic (can expand and retain that expansion) that last attribute, plasticity, results in a spent case that’s closer to rifle chamber dimensions than it was to its factory-new figures. Since many factory barrels have relatively generous chambers compared to most custom-done barrels, that’s either good or bad, depending on whether it’s a semi- or bolt-gun, and also depending (a lot) on what anyone buys into.

So, for reuse in a semi, that now overly-dimensioned case has to be brought back closer to nearer-to-new condition than it does for a bolt-gun. Has to be. Otherwise it might not chamber smoothly or fully.

full length sizing die
Due to the greater amount of case expasion, and also due to the need for smooth, easy feeding, any and every case used for a semi-auto should be full-length resized.

It’s important to understand that any semi-auto (at least any I’ve yet had experience with) has the cartridge case in a different condition right at the start of the extraction cycle. In a semi, the case is still holding pressure when the bolt starts to unlock. Bolt-gun, it’s all long gone by the time the knob gets lifted. That’s why a freshly spent case from a semi will raise a blister and one from a bolt-gun is cool to the touch. This pressure creates what amounts to greater case expansion in a semi-auto. Depending on the particular rifle and other factors that will get addressed in other articles, this varies from a little to a lot. The spent case measurements from one fired in a semi may not accurately reflect chamber dimensions, as they will with a bolt-gun.

The reason there’s still some pressure within the case when the bolt starts to unlock is because that’s how a gas-operation system functions. If all the pressure was gone the action wouldn’t even open.

neck only case sizing
A bolt-gun can be neck-only sized. I honestly don’t think this is a worthwhile practice, and I’ll talk more about that in another article, but as long as you’re willing to get a handle on case dimensions (so you know it’s still within specs to fit your chamber) it’s perfectly safe, and usually results in good group sizes.

Which brings us to the second essential difference in bolt- and semi-: Most semi-automatics, especially what is probably the most common (AR15 family) is very sensitive to gas port pressure. Gas port pressure is an actual measurement, but that’s not important to know, not really. What matters is understanding the effect of too much port pressure, and that is too much gas getting into the operating system, and getting in too quickly. That creates what most call an “over-function.” The action tries to operate, and the extraction cycle starts too early. There’s a lot of gas still binding the inflated case against the chamber walls. Many ills: excessive case expansion, excessive bolt carrier velocity, extraction failures (extractor either slips off or yanks the case rim, which can come off in a chunk).

.223 recommended components
Semi-autos are way on more sensitive about propellants, and, specifically, the propellant burning rate. Here is the set I use for my .223 Rem. competition loads (aside from a propellent that’s running in the range of the H4895, tough cases and thicker-skinned primers are part of the picture too).

From a reloading perspective, regulating gas port pressure is all in propellant selection. The burning rate range that’s suitable for semi-autos varies with the cartridge, but for both .308 Win. and .223 Rem. I cut it off at the Hodgdon Varget, Alliant RE-15 range: those are fine, but don’t go slower! Bolt guns don’t care about any of that.

RE15
Some will (certainly) disagree, but this is about the slowest-burning propellant I would suggest for .223 Rem. As a bonus, it’s also one of the highest-performing.

THE SHORT COURSE: Think “smaller” and “faster” when tooling up for sizing and choosing propellants for use (really, re-use) in a semi-auto. Smaller case sizing, faster-burning propellants.

This will all be hit on in upcoming articles in far greater detail but…

SEMI-AUTO: full-length case sizing, case shoulder set back at least 0.002 (from what a gage indicates as the fired case dimension), case neck “tension” at least 0.003 (difference between sized case neck outside dimension and loaded case neck outside dimension). Propellant selection: not too slow! Contrary to what logic might suggest, slower-burning propellants produce higher gas port pressures because they “peak” farther down the barrel.

BOLT-GUN: neck-only case sizing is (usually) okay (that means no case body sizing). Case shoulder set back: can be fine-tuned based on what’s necessary to easily close the bolt (ranges from none to “just a tad”). Propellant: doesn’t matter! As long, of course, as it’s suitable for use in that cartridge.

Check out some tools HERE at Midsouth

The information in this article is from Glen’s newest book, Top-Grade Ammo, available HERE at Midsouth. Also check HERE for more information about this and other publications from Zediker Publishing.