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


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.

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.


11 thoughts on “RELOADERS CORNER: Semi-Auto or Bolt-Action? Two Things To Remember”

  1. Excellent read. I really enjoy your articles. Keep up the great work and I wish you and your family a Happy New Year.

  2. Neck size only for bolt action rifles. I found out the hard way. I was handloading for .303 British. Full length resizing and half the rounds would split and separate the case at the base. The chamber on my particular rifle, an Enfield 1918 SMLE MKIII*, is generous in dimensions. This I discovered was deliberate so that being used in the battlefield rounds would load easily even if the chamber got dirty or whatever during battle.
    Once fired brass that survived being fired in my rifle only needed neck sizing as the case was now stretched to the chamber dimensions.
    But no more shooting that rifle unless I can get original ammo, it fails a NO GO Gauge, but passes a FIELD Gauge.
    I do have another Enfield, a 1942 No.4 MKI* made by Savage and stamped U.S.PROPERTY.
    I haven’t fired that one yet. But it passes NO GO.

  3. Great info, thanks for your knowledge as always. So a question- I load 77SMK with VV N140 ( slowish powder) to something closer to .223 spec (2500fps muzzle velocity 16.5 in bbl) for an HK MR556 piston gun which generally provides .75MOA minus flyers.
    Based on your article, can I expect better a higher round count out of the different failable rifle parts (extractors, bolt head lugs, headspace, etc) since the port pressure is probably lower than a surplus NATO 556 round? If so, another added benefit to reloading. Thanks, JP

  4. Despite my protestations of “just as fast,” I only neck-size for any of my 3 lever action 30-30s. But fully compliant with Mr. Zediker, I must full-length size all my 8mm mauser brass, or end up with many of the wrong size in my favorite (coincidentally tightest) bolt-action (yes, all $65 surplus, remember them?), or keep them separated which is an accounting nightmare. : -)

  5. As Mr. Zediker mentioned about case expansion in semi-autos keep this in mind if you are picking up range brass. Many times you don’t know what make gun the brass was shot in. If I pick up range brass, especially that fired in 308/7.62. I use a Wilson case gage to see if the brass is anywhere close to SAAMI specifications. If it is not close, chuck the brass. It may be too hard to resize and not worth the effort. You can get processed Lake City brass or IMI new match brass which is more cost effective than processing range brass.

  6. I load .204 Ruger for both bolt and semi-auto. Both are tight chambered custom rifles. I am using a “small base” resizing die for reloading. I do have some brass in my pile that was fired in a generously sized chamber of another bolt gun and when these get fired in the semi-auto they tend to separate mid case. Using 28.5gr of CFE223 with 40gr Hornady V-Max.

  7. Three things I have found to do for successful reloading for semi-autos. Sort and or trim cases to recommended length. Resize with small base die. Use bullets with a cannelure and crimp with a collet die.

    1. Add to the above: use only mil spec primers, e.g. CCI #34. It also is imperative to ensure primers are seated below flush. A “slam fire” is bad enough; an out-of-battery slam fire can be deadly.

    2. I was taught 40 years ago to size then full length trim the case so as to provide more consistent trimmed lengths and crimps. I know this is not as easy but yields more accurate loads.

  8. What about cases that are too short? I’m talking once fired brass that, after sizing, are shorter than the “trim to” length.
    Some (30-06 cases) are as much as .010″ to .015″ short. Most are only .004″ to .006″ short.
    I just sort them by roughly the same range of length, but haven’t really loaded any of them yet. I do have some very accurate reduced loads for 30-06. I may just play around with those charges in the short cases.

  9. I have a Remington Rolling Block No. 5 in 7mm. The chamber is so far out that all gauges slip right in. I have fired about 20 new cases with lead bullets and no problems. The most I would do with these is to neck size them. I have heard horror stories of these guns coming apart and killing their owners if there is a case head separation.

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