I've read quite a number of posts where the shooter mic'ed the case to determine if the load was nearing max pressure. Can someone explain the process? I've got a good set of micrometers, etc. so doing the measuring isn't an issue but understanding what I'm looking for escapes me. I assumed (probably wrongly) that when fired, the brass will expand to fit the chamber and then shrink slightly due to elasticity. But how does the max load affect this?
I'm shooting 165 gn Zero JHPs over 11.8 gns of AA#7 to get 1300 fps out of my Witness Match but I'm worried I may be pushing a limit somewhere even though my primers don't show any flattening. Thanks for any insights here.
VA-Termite, Welcome to the forum! The EAA guns have very good chambers and Intercooler here on this forum has run some of the heaviest loads through his various Witness guns. As long as you are not seeing sever primer flattening, popped primers or "Smile" lines you should be fine. But if you measure the diameter of your chamber and your brass doesn't exceed that dimension you should be OK. The "SMILE usually shows at the feed ramp and it will exceed the chamber measurement.
I have loaded the 170gr bullets in the past with 12.0grains of AA#7 and they were shown as @ 1305 fps - 37,500 psi in the data. BTW, this 12.0 grain AA#7 load is the current one listed for the Sierra 165 grain JHP @ 1305 fps - 37,500 psi COAL of 1.2500".
Your loads are just fine and you shouldn't have any problems! Best regards. ;)
The Western Powders current Load data can be downloaded at http://www.accuratepowder.com/wp-content/uploads/2010/09/WesternLoadGuide1-2016_Web.pdf (http://www.accuratepowder.com/wp-content/uploads/2010/09/WesternLoadGuide1-2016_Web.pdf)
Thanks for the replies, folks. Much appreciated. Now if I can just figure out how to keep the brass within the same zip code, I'll be doing great :-).
How about a 200gr going that fast out of your same pistol. I still remember these!
Quote from: VA-Termite on February 04 2016 11:52:53 AM MST
I assumed (probably wrongly) that when fired, the brass will expand to fit the chamber and then shrink slightly due to elasticity.
You are mostly right. The hollow part of the brass, or case walls expand, and then contract back slightly. But we aren't measuring this expansion. We are measuring how much the compressive force on the solid case head caused a plastic deformation.
Metals have two deformation lines on the chart. The elastic deformation pressure, and the plastic deformation pressure.
When subjected to a pressure below the elastic line, they do not change shape, even while the pressure is being applied.
When subjected to a pressure above the elastic line, but below the plastic line, they change shape, then return to their original shape,
When subjected to a pressure above the plastic line, the change in shape is permanent.
In lab testing a blank of a specified shape is used, and then is tested to failure. Since the blanks are a very uniform shape, they fail consistently and the posted values for elastic and plastic deformation are a known for that alloy in that exact shape.
The reality of the world is that small changes in all ranges are constantly occurring as soon as you apply pressure. This is actually very useful to us. What it means is the the amount of plastic deformation increases relatively predictably with pressure, up to the point that the vessel fails.
As a result, in higher pressure loads the case head springs back less than lower pressure loads. Further, since we are measuring at the case head, where all the pressure was compressive on the base of the case, not expanding the case walls, there is much more consistent plastic deformation as a given pressure level. In effect, the case head has become a "pellet" in a copper crusher style test. (https://en.wikipedia.org/wiki/Copper_units_of_pressure)
As a result, you can buy ammo loaded to near max in starline brass, a known, consistent quantity, and fire it in your gun, then measure the case heads, right above the extractor groove. This amount of expansion would be considered a "max" pellet from crush style pressure testing. If you ever get a batch of "pellets", or case heads, that exceed that value, you have gone over max and should reduce your load.
Hope that helps expand your understanding a bit!
Thanks for the explanation, Sqlbullet. That is the info I was looking for. It clears up a lot of my confusion and I think I'll try out your process just to set my mind at rest. I assume from prior posts that people normally look at a group of 5 or so shots to see if any of the cases are approaching max rather than just looking at one case and calling it good.
When measuring case heads, I generally measure groups of 10. It is an interesting to read SAAMI methodology in testing. I generally try to apply their principles to my methods.
With a new load, I measure the first one. If it is within acceptable limits, I shoot four more, then measure again, looking for an "over". If I am good, I shoot five more for a total initial lot of ten rounds. If all fall at or below my "max" then the load is considered in spec.
Going forward, all brass at the range is sorted back in to the reload box it came from, grouped by the gun it was fired in. After the fact, a random 5% are measured, and if anything is ever noted then additional load validation occurs.
One thing lots of guys overlook is lot numbers on powder and primers. If they changed, you need to reduce and work back up, because burn rates vary by lot. This is why you should buy large lots of powder and primers. I like to buy 5-8 lbs of powder and at least 1k if not 5k primers at a time.