Glen Zediker of the Reloaders Corner

Reloaders Corner: Pressure Curves for Semi-Automatic and Bolt-Action Rifles

Facebooktwittergoogle_plusredditpinterestlinkedintumblrmail
Facebooktwittergoogle_pluspinterestyoutube

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

When you’re handloading for semi-automatic rifles and bolt-action rifles, 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 way more tolerant and flexible on what they’ll chamber than gas-guns. I’m thinking of cartridge dimensions and fuel.

For much of what he writes in this Midsouth Shooters Supply reloading column, author Glen Zediker will use the .223 Remington cartridge for the main example for the most part, and focus on how that round is reloaded for use in the popular AR-15 rifle and variations based on it. There will be other cartridges included, but the .223 is one of the more popular cartridges alive today. 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.
For much of what he writes in this Midsouth Shooters Supply reloading column, author Glen Zediker will use the .223 Remington cartridge for the main example for the most part, and focus on how that round is reloaded for use in the popular AR-15 rifle and variations based on it. There will be other cartridges included, but the .223 is one of the more popular cartridges alive today. 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 loads, but you must respect certain points necessary for success with a semi-auto. I believe factory-done 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. And, references like “long gone” aren’t literal. Everything happens in milliseconds.

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.
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 that. If all pressure subsided before gas reached the mechanism, then 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, 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.

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.
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.

More on this topic coming soon…

Facebooktwittergoogle_plusredditpinterestlinkedintumblrmail

43 thoughts on “Reloaders Corner: Pressure Curves for Semi-Automatic and Bolt-Action Rifles”

  1. roger on H4895. best groups for my M1A .308,22-250 and .243. Good for 300 win mag as well. uniform metering another advantage.

    1. I use adjustable blocks on all my Match Rifles. I also have the gas ports relocated (forward). There’s a new appliance from Sun Devil I’m happy about. It’s an adjustable carrier key… That’s cool because it can pretty much go into any AR15, unlike adjustable blocks. There are also adjustable gas tubes available. Any means to regulate gas pressure is worthwhile.

        1. 4064 may well be the very best propellant for .308 Win. I always used that when I shot .308 bolt guns, as did virtually everyone else… (I tend to use Varget now, for bolt guns.) In my estimation 4064 is right on the edge pressure-wise for the M1A, but that’s not the same as saying it’s not suitable. In my M1A experiments with that propellant I used WW cases and 1 grain less than my bolt gun load. At the same charge, function was excessive. Velocity was, as a result, better with 4895.

    1. When I load for my Garand and when I loaded for my State Issued M14, I used IMR 4895, like all the rest of the High Power Competitors. That as before Varget .Currently , the forums and groups talk about 4064 as well
      Fred Peterman
      Major US Army Retired
      NRA Life Member (2nd Generation)

    2. H4895… The M1A is REALLY sensitive to too much port pressure, even though it’s a piston gun. 4064 (or equivalent) is the slowest I’d ever suggest trying. Lake City Match (M852 and M118) is loaded with “a” 4895. They said it was a “specially selected for optimum pressure” and somehow they always got more of it into the case than “we” ever could… I always wanted about 100 pounds of whatever they used, but never found it!

      1. What is wrong with 748 ball powder? I have achieved minute of angle accuracy and never had any issues in a M1A? I have used in for over 35 yrs. in the same M1A and the op rod is still in great shape. It meters vary accurately too.

    3. For that rifle, 4895 is my first choice. The M1A doesn’t take kindly to slower propellants than that one. It’s the propellant that I, and everyone else I ever knew or heard tell of, used when we all shot that rifle in NRA Service Rifle competition. It was also “a” 4895 that was chosen for the M118 and M852 Lake City Match ammo. There are others that can work, but 4895 really works well. H-brand is my choice. Only other comment on the M1A is stay away from anything heavier than a 175 grain bullet.

    1. That’s good. True. It’s worse than rocket science because we’ve still never really seen what happens… In another way of looking at it, it may be more art than science. Most is learned through experience, first-hand. It’s just not possible to choose the best load for any one rifle based on any set of equations anywhere.

  2. Good start, but doesn’t get into pressure curve as advertised. Looking for ward to that.
    Please explain the last sentence: “I’ll go ahead and tell you now, my favorite propellant for all my .223 Rem. handloads for Hodgdon 4895.” Should the last “for” be “is” Or what?

    1. Thanks for the encouragement. I am going to get farther into it… And YES is should be IS 4895. Don’t know what happened there. A little hitch in my getalong I guess.

  3. While explaining why bolt vs gas operated guns are different, please also discuss how closely the same load might be to “good” for both.

  4. Mr. Zediker,

    Thank you for the article and thank you for being willing to share your knowledge on this subject.

    I’m new to auto-loaders, since most of my experience in competitive shooting as been in F-Class, with bolt-guns. I purchased a civilian version of an M4, a couple years ago, and then added a 300 Blackout Upper with pistol length gas system, a few months back. How do you feel about shooting supersonic (Hornady 110 V-Max) ammo in a pistol length gas system? Being new, I have no idea if I’m doing something bad for my gun or not. I feel you have considerable experience and would like to hear your thoughts. Thank you again for being such a sharing person.

    Shane

  5. Was anyone going to show curves in this article on pressure curves? I was rather expecting to see some data.

  6. I will go farther with that also. Essentially, any reload for a semi-auto needs a little more case sizing, a little more bullet retention, and a propellant that fits in the parameters outlined in the last article. So, that means a bolt-gun load CAN be different, but doesn’t have to be to still get very good performance.

    1. My Saiga 223(AK-47 clone) ate everything but, faster Reloader 7 is what my Mini-14 wanted. I read somewhere the pressure curve needed to happen sooner…

  7. Where are the pressure curves? Where are some typical load specs for different barrel lengths? Interesting read but not much beef.

    1. I’ve never really seen a pressure curve routinely plotted out to accompany loading data. It’s more of a description of propellant burning progress, in the way I refer to it, than it is something tangible. Point is that slower propellants can produce excessive gas port pressure. There are other factors also like bullet weight, barrel length, port location, port diameter. I’m going a little farther with this whole thing next article, which is next week.

  8. Thank you for this article. While I shoot bolt action rifles for the most part I am having problems with factory ammo (like federal) in my VZ58. It will shoot fine in my bolt guns but will not cycle the action on the VZ58. I am guessing that a higher pressure, faster burning powder may help this. I have reloaded a lot for my 9mm and 45ACp handguns but I am just starting in on reloading for my rifles and that will include me .223 VZ58 and .223 weatherby bolt action rifle. I am hoping this article will help direct me to what will work best on each rifle and then also a load that will work well for both of them. 3 different loads.

    Note: the VZ58 is new, but has had about 300 rounds through it. I am assuming the recoil spring is in good shape and of the right tension.

  9. Thank you for this article. While I shoot bolt action rifles for the most part I am having problems with factory ammo (like federal) in my VZ58. It will shoot fine in my bolt guns but will not cycle the action on the VZ58. I am guessing that a higher pressure, faster burning powder may help this. I have reloaded a lot for my 9mm and 45ACp handguns but I am just starting in on reloading for my rifles and that will include my .223 VZ58 and .223 weatherby bolt action rifle. I am hoping this article will help direct me to what will work best on each rifle and then also a load that will work well for both of them. 3 different loads.

    Note: the VZ58 is new, but has had about 300 rounds through it. I am assuming the recoil spring is in good shape and of the right tension.

  10. Great article, never considered port pressure being different. I’ve been using 3031 with no problems and ejected brass all going in the same small pile. An old Gunny claims that the military equal to 3031 was the original powder that came out with the Stoner. I do have to resize brass to the most that the dies will allow, using the Lee 3 piece set and a Delton M4 5.56 upper.

    1. He’s right. 3031 is an outstanding propellant for .308 semi loads. 4895 can usually produce a little more velocity, but system operation manners are outstanding with 3031.

  11. Very informative article and I am looking forward to the next. You are 100% correct about loading manuals leaving out things about powder burn rate, gases, etc.

  12. H4895 is a great powder but does not metter all that well. 223’are often loaded in bulk and a powder like Tac that flows well throug a meter is very nice.

    1. The biggest issue with TAC is its a very temp sensitive powder. Being in the Dakotas, a 120fps variation from 1 week to the next isn’t acceptable due to weather. But I wish I could run TAC. Good velocity etc.

  13. I have to say, you have got my attention! Back when ammo and reloading supplies were hard to find, I picked up an 8 pound jug of H4895. I searched for loads for my AR in .223 and found ones that worked in all bullet weights (55,62,68,69,79) and in all my barrel twists. BLC-2, and A2230 were my go to powders before. My rifles do like H4895 ! Can’t wait to see your load data . looking forward to compare !

  14. For my at-15s I get sub 1/2 groups and perfect function with 55gr bullets w/20.7grH4198,and I can throw charges with the short granules.this is both with 16&18″ barrels and carbine and midlenth gas tubes

  15. Gun Writer,s version of making mountains out of mole hills? ANY medium burn rate powder loaded within factory specs will operate a gas gun if metered properly. What really matters is bullet seating depth, a proper crimp and how dirty your selected powder burns. Burnt power residue is a gas gun,s worst enemy. You should be talking about that.

  16. Thanks for your reloading books.
    They have helped clear up so much clutter and fables. It’s refreshing to see someone talk about experience instead of hearsay.
    M-

  17. After 45 years of reloading (yes, I am a dinosaur) and a good part of that devoted to the .223 Remington, it has been my experience that Winchester 748 ( W-748) will give consistently the best performance in both bolt guns and semi-automatics. I know it is the slowest burning but you cannot argue with predictable outcome.
    It will meter like dry sand and you can get very even levels of case volume. Consult your loading manuals for charge amounts but I say W-748 is my go to for that caliber.

    1. Yes, I agree with W748. I have reloaded with it for over 35 yrs. This love with 4895 is a result of the fact that shooters think that it is the holy grail because it is used by the Government. Frankly, that is why I do not use it. They go low bidder….. not the best powder. And I agree. It does meter very very accurately. Also having worked at an Army ammunition command… the government buyers are not fussy about minute-of-angle accuracy when selecting powders. Low bidder wins…..!

      1. I’m about tired of hearing people say “low bidders win” all the time. It shows you know nothing about government contracts. I’m a Tool & Die Machinist that has dealt with government contracts, off and on, for over three decades. The lowest bidder does NOT always win. In fact, if your bid is too low, the government will question your bid.
        Please do some homework about the subject, and not just re-state the falsehoods of the internet.
        FYI, the government has Mil-Specs for EVERYTHING they want manufactured.The final product HAS to meet this specifications or they are rejected. So, if the person making the product IS the lowest bidder, you get the same, EXACT product as you would from someone charging more money to produce it.

      2. Comments on both the 748 users. I used to be one of you… I have been a big fan of 748 and agree with all the good things you mention. For a few years, it was my standard 69-grain Sierra Matchking fuel. But, when we started using heavier bullets it didn’t work as well. It’s also a tad amount temperature sensitive, and mostly to cooler temps. That’s when I moved to 4895. It’s not wise to use different propellants for the different events (throws zero off for a few rounds when you switch).

        1. I will yield some ground on 748. The majority of the loads I have cooked up were for 50 or 55 grain projectiles; barrel twists from 9 to 12. But Man ! Those are some accurate loads. Four shots in a quarter inch hole at a hundred yards kind of accurate.

  18. Have two of your books, they were extremely useful for several prairie dog AR builds. We built slow twist space guns uppers with fairly normal butstock lowers and spent our money on barrels and triggers. Worked great-thanks.

    Quick Book Review for the uninitiated. Zediker pretty much covers the waterfront with his build books. If you want a simple recipe to build a known style that works, it’s there, along with a description of the easiest way to reload for it. If you are puzzling about the whys and wherefors across the range of available components to tweak a build for a specific game or to emphasize a particular characteristic, then the reasoning on how to make either gross or subtle differences is available. The downside is that you may have to bookmark your book as to topics, as the snippets are scattered across multiple example builds. He is a High Power shooter, so 60 % of his stuff relates to Hi Power. That being said, I’ve learned way more useful things, both Bolt and AR, from High Power shooters than any other shooting bunch.
    Here is my question

    How does one tell if a gun is over or under gassing?

    I was unable to tell on one gun. I built it with a very short throat so as to jam the lands with mag length 40 gr bullets. At first it had cycle issues, I thought it was under gassing. The build was completed just before a PD hunt, I substituted a softer recoil spring, it started functioning and off we went to Wyoming. Later on I learned from my chronograph (3800 fps) by losing about 800 pieces of brass to loose primer pockets that it was way over pressure due to the jam (shot like a house afire though, just was absolute death on PDs). I wonder if the powder (a max load of TAC) was spiking early and failing to have enough pressure at the port. Or conversly if it was over gassing. But the softer spring cured it, so I’m bamboozled. Per usual.
    Are there any observable symptoms of over and under gassing?
    Thanks,
    One of Your Fans
    Gary Glick

Comments are closed.