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

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.

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.

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.

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.