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!
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.
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.
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).
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.
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.
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