Pressure Curves for Semi-Auto and Bolt Actions

by Woodsbum

Who’d a thunk it? Cheaper than dirt has a blog that posts some really interesting articles. My father linked an article to me that I thought was quite well done. Not only did it address several things that many seasoned reloaders already know, but it really did a great job of putting the information into easy to understand language that even novice reloaders can follow.

Here is the article…..

Many times I link the article and then copy the post in its entirety to my site so that I can save it if changes are made on the original site. This time I just did a draft with the whole article copied (for my use) and am going to comment a bit on the original.

“Why?” you may ask. The answer is quite simple. I had someone that was trying to make money off their site request that I remove all links and material from one of my posts. Interestingly enough most of that content was taken from other sites, but instead of giving credit to the original authors she claimed it all as her own work.

Now I am just going to link things and save a draft copy for myself to reference and make comments. Sorry you might have to do some surfing, but hypocritical little bitches that complain when someone links and gives credit to her have made me change my mind on how I do things…..


The article brings up a few really good points about the gas operation of most semi automatic firearms. If you use too much or too little powder in the load, the firearm may not be able to self load another cartridge. There is a fine line where everything happens in literally milliseconds. The gases escaping from the ignition of powder in the cartridge are routed back into the action to actuate a process that ejects a spend casing and chambers another round. The timing on this process is critical. If it happens too quickly, there might not be enough time for the moving cases to be clear of each other. If it there isn’t enough gas the timing could be off where there isn’t enough power to eject the spent casing.

The other point that is brought up is the fact that the actual cases themselves must conform to a stricter sizing and length standard to be able to self reload. Fixed magazines in bolt action rifles are usually more forgiving than removable magazines. There is also a little more leeway given in the chambering process as well. If all else fails, the shooter has the opportunity to manually manipulate the cartridges or even modify their timing a bit to ensure that loading another round happens without incident.

All this can effect how you build your loads and the powder you use. This is where “slower burning” or “faster burning” powder variants come into the mix. There tends to be higher port pressure with the slower burning powders than faster burning variants. Be mindful that this distinction is based upon specific ranges that are suitable for your particular load. If some cartridges tend to be less accurate or cause failure to feed issues in your particular firearm, try to swap out for a different powder with a burn rate on the other side of the range.

Lastly, be aware that some powders are heat sensitive and change their burn rate depending on the weather. Some of the Reloader series are very susceptible to temperature variations.

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