Understanding Pressure

thats a wondeful writeup - hopefully you get lots of google hits with that.


i remember reading on another forum where a fella said that blown primers were just fine, as long as there were only "a few here and there". IIRC he got offended when i asked him where he shot so that i'd avoid that range so i could never end up on the bench next to where he was shooting.
 
Great write up and it really illustrated clearly signs I should be looking for as a relatively new reloader. I hear things and have a hard time determining exactly what that looks like on a case and now I know. I have stayed in the lower charge weights and still seen ejector print on cases and had to ask an older more experienced reloader what that meant. Thanks for the great info
 
Lets say one shut off the gas system so bolt wont move for an AR.
One ejects cases manually by pulling the BCG back.. One has no swipe marks and brass looks fine.. But when gas system is running it leaves bad markings. Is it considered over pressure or just the timing at which can not be slowed down enough to function as a semi auto? There would be more time for the brass to cool down by manually eject with gas off than if being ripped out so fast before has a chance to cool and shrink.. Over pressure or not?? Then what would the signs be? The same as a bolt rifle then?? Thanks. Dan
 
Originally Posted By: dan158Lets say one shut off the gas system so bolt wont move for an AR.
One ejects cases manually by pulling the BCG back.. One has no swipe marks and brass looks fine.. But when gas system is running it leaves bad markings. Is it considered over pressure or just the timing at which can not be slowed down enough to function as a semi auto? There would be more time for the brass to cool down by manually eject with gas off than if being ripped out so fast before has a chance to cool and shrink.. Over pressure or not?? Then what would the signs be? The same as a bolt rifle then?? Thanks. Dan I'm sorry but I have not done adequate testing to confidently answer your question, and I specialize in work with bolt action precision rifles.

My speculation would be that the gas system is part of the "expansion chamber" in a semi-auto, as is necessary for proper function in a gas-driven system. In order to read pressure signs effectively, the entire system must be functioning as designed. It is no surprise that bolt action rifles are considered to be much easier to work with, as this variable does not exist. There is no cyclic "timing" to worry about with bolt rifles. While I'm certain that the pressure shown in your hypothetical should be viewed as "real," I have not done adequate testing to tell you why it appears in semi-auto mode and not when the bolt is remaining locked. Perhaps someone with more experience with semi-auto's can comment.
 
You would be looking at case head expansion and primers on the home front along with chronograph numbers on the manual; operated rifle just exactly like a bolt action rifle. The gas system is opening up when there is still pressure on the cartridge effectively turning it into a rocket hitting the bolt face ripping on rims and pounding the brass down into the ejector hole. The brass is hot and very elastic at that point. Hence the fact that you can fire ten rounds in the same AR chamber and get three or four different shoulder height readings. The earlier it opens, as in over gassed, the more violence to your case heads will be observed. The adjustable block or a heavier spring/bolt combination will mitigate some of these issues of observed over pressure indicators. Also be aware that a large firing pin tunnel in concert with a small firing pin will give false signs of pressure as this will induce cratering. Modern Remington BA rifles show this all the time with KNOWN lower pressure loadings.

Greg
 
Good read. At what level of magnification were the pictures taken? Can you see most or all of the danger signs with a 4x magnifying glass, or is stronger magnification needed??
 
The pics were taken with a special "macro" lens on my DSLR. Here is the lens: http://amzn.to/29KFZH8

You can usually see pressure signs with the naked eye. The images were high resolution so that the effects could be easily seen by anyone reading. Any magnification will help however. The illuminated loupe I mention in the article will work quite well. Here it is: http://amzn.to/29KGAs4
 
Wow...written in a way that "average folks" can understand. The pics were essential in getting a grasp on what you were trying to convey. Thank you!!
 
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