Reloading 101: Pressure Curves and Port Pressure

By Dave Dolbee published on in Reloading

In my reloading article for The Shooter’s Log, I gave a caution about respecting one of the differences between semi-auto and bolt-action rifles, and that was with respect to propellant burn rates. The summary reason for the warning is that different rate propellants will “peak” at different areas as the expanding gases and the bullet travel through the bore. Slower-burning propellants peak further, and that means more pressure is available at the gas port location in an AR-15, for instance, as the bullet passes it. If the system is oversupplied, then the system is overworked.

Medesha Adjustable Gas Block

Here’s an adjustable gas manifold from Scott Medesha. It’s correctly (in my belief) designed. Don’t get too greedy with one of these, but they help a ton. The reason I say “greedy” is because, yes, it’s possible to use such an appliance to counteract the effects of over-pressure (over-velocity) ammo, but that’s not the point.

Compared to ideal function when gas supply is delivered as engineered, mistimed peak pressures can result in the bolt unlocking too quickly and excessive bolt carrier velocity rearward. The system just gets hit too hard. The extractor tries to yank the case out of the chamber too soon, before the case is released from its grip on the chamber walls (from being expanded through firing). Spent-case condition shows a measurably more abused hull. Probably the worst popular example of these effects is the M1A. I’m doing an entire column or two on reloading for this beast. Essentially, a spent case from an M1A will show dimensions that don’t seem possible. These come from the bolt unlocking too quickly. AR-15s actually handle excessive pressure better than some other designs.

Always keep in mind that this is all happening in about 2 milliseconds. The average time a bullet spends in the barrel, for most modern centerfire rounds, is 0.002-second. Timing is everything.

Keeping in mind the behaviors of a pressure curve (which is like a wave cresting), are factors that influence the amount of gas-port pressure, using the same load, include barrel length, gas-port size, and gas-port location. When the bullet is sealing the bore, a longer barrel means more pressure is contained for a longer time. The smaller or larger the gas port size, the slower or faster the gas enters the system. The farther back or forward the port is located, the sooner or later. Bullet weight is a factor also: heavier bullets accelerate more slowly (This is also the reason heavy bullets erode the chamber throat faster than lighter bullets).

Sun Devil Adjustable Carrier Key

Here’s something new, and I like it: The ADIGS adjustable bolt-carrier key by David Beatty at Sun Devil. It regulates gas flow. What’s cool about this is it’s easier to retrofit than anything barrel-mounted, and, muahahahaha, it’s unnoticeably legal for NRA Service Rifle competition.

And, the amount of volume inside the bore has a huge influence on all this. That matters when we’re using another caliber than .224 in an AR-15 or .308 in a big-chassis AR (like an SR-25). For instance, in that rifle chambered for .243 Win., but retaining the gas system specifications (gas port size and location) of the .308 Win.—chambered rifle, there’s way more pressure only because there’s less space and less volume in the bore. The opposite is usually true when running an AR-15 with a larger caliber bullet.

Selecting a propellant with a suitable burn rate, which, again, is something in the vicinity of H4895, is really the only thing we can do on the loading bench to ensure that we’re not contributing to these symptoms. Beyond that, dealing with excessive pressure gets technical.

All my NRA Match Rifles, which usually have 26-inch barrels, get their gas ports moved forward one to two inches. These, of course, are custom-barreled. I also usually install an adjustable gas manifold.

Port Pressure Springs

Under-pressure is easy. The gun doesn’t work because there’s not enough gas. Observation usually makes this easier to spot. The technical solution for this in a stock rifle is cutting coils off the buffer spring. Handloading? Just make sure the load is what it should be, not “light.”

Moving the port forward effectively delays the wave of gas moving through the bore, kind of repositioning its peak with respect to its outlet; there is more space available for expanding gases. It also allows a little slower-burning propellant, which can take more advantage of the longer barrel. It’s common in a similarly constructed AR-10 to get a port moved as much as 5 inches forward to accommodate a .243 Win. or .260 Rem. chambering.

The adjustable manifold allows some tuning. There are essentially two forms these take. One way is to restrict or limit the through-flow; the other just bleeds it off. I like the first kind the best.

Also, I have searched far and wide for a consensus on gas-port sizes and came up empty.

All this changes with different chamberings and rifle configurations. Carbine-length barrels are particularly sensitive to port pressure because the port is located farther back.

Port Pressure Barrels

Here’s a modified big-chassis setup. Stock .308 Win. (left) and relocated on a custom .260 Rem. Moving the port really, really helps in this instance. On this topic: Be careful using factory ammo in your big-chassis gun. Reason is that, unless it’s mil-spec, it’s liable to be developed for bolt-guns for hunting use. That can mean the propellant burn rate may not be suitable for semi-autos. This is especially true in the .260 Rem. and .243 Win. This is hardly ever a factor in smaller cartridges, like .223 Rem.

There are a few surefire things that will alert you when your rifle is exhibiting “over-function” symptoms, such as spent-case condition showing excessively blown (extended) case shoulders, extractor marks on the case rim, and a generally explosive sensation in functioning.

In a more extreme circumstance, an over-accelerated carrier can “bounce” back from its rearmost travel so quickly that a round can’t present itself in time to be picked up by the bolt, or the bolt stop can’t engage quickly enough to hold the bolt carrier.

Sometimes what appears to be a “light” load is actually not. I’ve seen excess pressure leave a spent case in the chamber because the extractor lost its grip, and I’ve seen chunks pulled right off case rims. That’s severe. That’s also another cause for the “short-stroke” appearance of over-function: the extractor issue has slowed the carrier.

Port Pressure Pins

Here’s a gas tube from one of my Match Rifles compared to standard. That little bit extra there really helps in a 26-inch barrel. Otherwise, the long-barreled guns get more sensitive than a standard rifle.

If you’re having any problems with “over-function,” solutions include retrofitting an adjustable manifold, increasing carrier mass, and installing a stouter buffer spring. I do all those things on my rifles. Keep in mind, I am primarily a Service Rifle shooter, and I am trying to push an 80-grain bullet as fast as reasonably possible from a 20-inch barrel that can’t get the modifications mentioned. I know a thing or three about delaying bolt unlocking—I’ll cover more on this topic if you all want to know.

This article was a specially-adapted excerpt from the forthcoming book,” Top-Grade Ammo,” by author Glen Zediker, owner of Zediker Publishing. Click here to order.

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Comments (9)

  • Tim

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    Most pistols are recoil operated rather than gas operated, thus manipulating spring rate or load power are the only options you have to speed up or slow down lock rate.

    Reply

  • Glen Zediker

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    That sort of thing has been done, and it does work. There was a “pigtail” gas tube for carbines that was a standard length tube curled around a few turns. Just more room, more available volume, for the gas to expand. Keep in mind, though, that sort of change will influence the function with lower pressure ammo. Adjustable systems are nice because they can be tuned, and re-tuned, for the load.

    Reply

  • Glen Zediker

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    I’m sho no pistolsmith, but usually we just tune the spring to deal with the load. Different weight springs.

    Reply

    • Tim

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      Most pistols are recoil operated rather than gas operated, thus manipulating spring rate or load power are the only options you have to speed up or slow down lock rate.

      Reply

  • Tim

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    Great timing!! I am having an issue with my new .300 Blackout upper failing to feed the follow up round after the first shot. The fired shot ejects and the bolt fails to pick up the next round. I don’t plan on suppressing this upper or even shooting subsonic loads. I built this for a moderate range hunting gun. So far I have only shot 120 grain Remington factory loads, although I do intend to handload for it. It is a 16 inch AAC barrel, gas block and gas tube. The lower is a standard carbine lower buffer and buffer tube assembly.
    Some forums have said that I need an adjustable gas block, but my buddy that builds a lot of AR platform guns says that I need to slow things down and suggested that I get a heavier H2 buffer.
    What do you think?
    Thanks for any help.

    Reply

    • Glen Zediker

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      Sorry for the delay in responding. Been swamped working on a new book… Honestly, I tend to do both for my Match Rifles: adjustable gas block and bulking up the carrier area. I’ve had really good success with David Tubb’s Carrier Weight System (CWS). I also like his flatwire buffer springs. Delaying bolt unlocking is the goal with those parts, and even a teeny bit (milliseconds are always teeny bits) makes a whopping lot of difference. Very noticeable in a carbine.

      Reply

    • Tim

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      Would I be better off running my .300 Blackout upper on one of my rifle lowers rather than my carbine lower?
      Thanks

      Reply

  • Bill

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    I was thinking of an expansion chamber mounted at the gas block would give a softer and slightly delayed recoil. This would allow a little added tolerance for heavy loads without the effecting the lighter loads. A slower rate of fire may increase accuracy as well.

    Reply

  • Mikial

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    Great article, especially for someone who doesn’t reload and is thinking about getting into it.

    How does competitors under-loading handgun rounds for USPSA and IDPA for faster follow-up shots fit into this equation. I know it’s a common practice. Not being a hand loader myself, and shooting Limited-10 Division in USPSA, I simply used Russian Wolf ammo in my G21 with excellent results, so I never got into the whole soft loads thing.

    So, my question is essentially, how do the guys who use under-powered loads get around the failure to function issues for weal ammo?

    Reply

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