Reloading 101: Pressure Curves for Semi-Automatic and Bolt-Actions

By Glen Zediker published on in Reloading

Semi-automatic rifles and bolt-action rifles are related but far from the same. When handloading,  it’s helpful to realize they are not to be approached the same way.

A round for each may well be, and often is, constructed and reconstructed the same ultimately, but the tool settings (and the component choices) may be different. Let me be more defining: a round recipe (all things included) concocted specially for a semi-auto will work just fine, and usually dandy, in a bolt-action. However, it doesn’t always work the other way around. Bolt-guns are generally more tolerant and flexible on what they’ll chamber than gas-guns. I’m thinking of cartridge dimensions and fuel.

.223 cartridges in an AR-15 magazine

For much of what the author writes about reloading he uses the .223 Remington cartridge as the main example. This allows him to focus on how the round is reloaded for use in the popular AR-15 platform and variations based on it. Also, it’s representative of the vast majority of cartridges anyone might want to reload, and not functionally or materially different from .308 Winchester, .30-06, or .243 WSSM or other bottleneck cartridges. There are a few quirks involving magnum-class cartridges, and a few others that are plain quirky themselves, and these quirks will be addressed as they come up.

These differences result from the fact that the semi-auto feeds itself. Again, to be clear, it’s not so much about maintaining separate set-ups for bolt and semi-auto loads, but you must respect certain points necessary for success with a semi-auto. I believe factory-sponsored loading manuals are notoriously remiss in addressing these differences. Just looking through the tables of load data doesn’t tell you all that you need to know.

So, for this one, let’s talk about what I think is probably the first thing to know, and that’s fuel. Specifically, gas. I say “probably” because it’s really a little longer list, but because there can only be one “first thing,” that’s it for now.

Ever wonder why an extracted case is barely warm when it emerges from the chamber on a bolt-action, no matter how quickly the bolt is being worked to fire subsequent rounds? The reason is that all the gases are long gone by the time the bolt can be unlocked. This is also why an ejected case from a semi-auto will raise a blister on bare skin. And, references like “long gone” aren’t literal. Everything happens in milliseconds.

Gas port on an AR-15

The gas-pressure volume available at the gas port (arrow) has a huge influence on rifle function, and also spent-case condition, which we’ll talk more about next. Too much pressure is none too good. It’s one thing we can influence in handloading, simply by choosing a propellant with the right burning rate.

In a bolt-action, when it’s done, it’s done. The spent cartridge case sits in the chamber until it’s extracted by manual operation of the bolt. In a semi-auto, some propellant gases are bled off or redirected to operate either a piston or, in the case of an AR-15, sent directly into the bolt carrier key via the gas tube (also known as direct impingement). Piston-operated guns normally have a moving part that’s variously connected to the bolt mechanism (the “op-rod” on an M1A, for instance). Either way, the bolt is moved to the rear, unlocking in the process. Problem: The cartridge case is still under pressure, meaning expanded in the chamber, when this starts to happen. There’s really no way around it. If all pressure subsided before the gas reached the mechanism, there would be nothing to power the mechanism.

So, the volume of gas available to operate the system has a whopping lot to do with rifle function and spent case condition. What matters is port pressure. Chamber pressure has no direct correlation to port pressure. Port pressure is what exists at the gas-port location within the barrel bore. Port pressure can and has been measured, but I’ve yet to see it be a part of any loading manual because including that information is a bigger effort. In fact, though, the number doesn’t really matter because it’s going to be different for different port locations and barrel lengths, but what matters, mostly, is figuring out when there’s too much pressure.

If we plot out propellant gas-pressure levels against the progression of bullet movement through the bore, we get a “pressure-time curve.” Upon primer ignition, the propellant burns. The burning produces gas. The gas pressure pushes the bullet down the bore. The faster the propellant is consumed, the more pressure exists behind the bullet, closer to the bolt. Faster-burning propellants have a steeper peak (shorter time); slower propellants peak further down the tube. Bullet weight factors mightily in the pressure-curve shape as well, but that’s for another article, as does barrel length (the longer the barrel, the more pressure is contained within for a longer time), and also port size, port location, and other factors.

Reloader 15 smokeless powder

Reloder 15 is the slowest propellant I would suggest for .223 Rem. semi-auto use. It works well with heavier bullets, which near-universally work better with slower propellants. On a burn-rate chart, its neighbors include H. Varget and Viht. 140.

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 AR-15 barrels call for 12,500 psi, for what that’s worth. And piston guns are not immune from concerns about port pressure.

The upshot is that there’s going to be higher port pressure with slower-burning propellants and lower with faster-burning propellants. Here’s one connection to chamber pressure, and that is that slower-burning propellants tend to show lower chamber pressures in most semi-auto cartridges, so it’s easy to make a serious mistake in propellant selection, with the best of intentions. And keep in mind that “faster” and “slower” refer to propellants within a range of choices that are suitable for a particular cartridge, not to extremes.

Making those choices is fairly easy, however, because most loading manuals have a propellant burn rate table. Find the grouping that includes Alliant RE-15, Hodgdon Varget, Viht. 140/540, and don’t go slower. That’s safe. I’ll go ahead and tell you now, my favorite propellant for all my .223 Rem. handloads for Hodgdon 4895.

Do you reload? Are you interested in reloading? Share your reloading experiences in the comment section.

The following is a specially-adapted excerpt from the forthcoming book, “Top-Grade Ammo,” by author Glen Zediker, owner of Zediker Publishing.

Tags:

Trackback from your site.

The mission of Cheaper Than Dirt!'s blog, "The Shooter's Log," is to provide information-not opinions-to our customers and the shooting community. We want you, our readers, to be able to make informed decisions. The information provided here does not represent the views of Cheaper Than Dirt!

Comments (7)

  • Arc Angel

    |

    Very good article about an often poorly understood and potentially dangerous characteristic of modern guns and gunpowder.

    (“Cold Dead Fingers”, wake up! If you haven’t got something genuinely useful to offer then think to remember, “Silence is golden!”)

    Reply

  • Rob C

    |

    The “gastube” article was interesting. However, most articles cover mostly the AR type frames in semi-auto.
    I have a 308 L1A1 inch model.
    Do you have any thoughts on that particular gas system. I’m always trying to balance the ejection with recoil. As they have an ajustable system, then balancing the bullet weight with powder type. I currently use an IMR 4198 with a 175 gn. Barnes match grade bullet. It seems to work well, but I’m always looking for a “better” match up..

    Reply

  • Bill V

    |

    While Glen has brought up some very important considerations while working up your hand loads, there is another option to controlling the gas pressure in your DI platform auto. If you have already worked up an accurate / reliable round, consider an adjustable gas block for you DI to control the pressures in the gas system. This alternative will decrease the amount of gas to the BCG (just enough to operate the bolt), result in less felt recoil, and keep that scalding hot brass out of your shooting partners shirt collar.

    Reply

  • Jerry Marshall

    |

    I have been experimenting with a variety of powders, velocities, bullet weights, etc. in .223 AR platform (16″ barrel), and in .222 Remington 722 (mfg 1955), since they’re both using the same projectiles.
    However, since I have to work for a living, and I’m not funded by anyone but myself, this process is taking awhile.
    Common ‘wisdom’ for a .223/5.56×45 with a 1:9 twist rate has been to only have satisfactory accuracy with lighter projectile weights, not to exceed 62 grains, but I’ve been trying to see what 69 grain and 75 grain bullets will do with different velocities and different powders.
    Currently I’m experimenting with IMR 4895, Hodgdon BLC (2), and Winchester 748.
    But I’m doing this with all of the lighter weight bullets, too. From 50gr. Hornady V-max, 55gr Hornady FMJ, Remington 55gr. spire points, 60gr. Hornady V-max, 62gr. Bulk FMJ (M855?), and 69gr. (can’t recall right now, and I’m not home to verify).
    So far, I’ve run tests with the 50, both 55gr, and lower powered 75gr. (BLC (2)), and have had satisfactory results with them, but noticed my point of impact with the 75gr. was about 1/2″ lower at 100 yards, when sighted for 55gr.
    I’m shooting 50 rounds of each loading, and for each caliber and bullet weight, I’m loading toward the higher and lower range specified for each caliber/bullet weight/powder.
    Has anyone gone through this process of testing?
    I would be very interested to find out what anyone else has found.
    For me, it’s a labor of love, and I just wish I was home more often, so that I could test more, and reload more, to satisfy my curiosity.

    Reply

  • Cold Dead Fingers

    |

    Yàaaaaaaaaaaaaaaaaaaaawwwwwwwwwwwwnnnnnnnnnnnnn……. Must,a been a slow day at the old whatever Dolbee calls this garble gobble…

    Reply

    • Dave Dolbee

      |

      Not into reloading or don’t like the article? ~Dave Dolbee

      Reply

Leave a comment

Your discussions, feedback and comments are welcome here as long as they are relevant and insightful. Please be respectful of others. We reserve the right to edit as appropriate, delete profane, harassing, abusive and spam comments or posts, and block repeat offenders. All comments are held for moderation and will appear after approval.

Time limit is exhausted. Please reload the CAPTCHA.

%d bloggers like this: