Pressure question

powdergun

powdergun

CGN Ultra frequent flyer
Rating - 100%
245   0   0
I was at the range shooting compressed loads in my .308. These loads were not above the reccomended loading parameters from the manual. I had 20 plus rounds through the gun with no problems in ejecting. Once at home I was looking at the brass and saw one of the brass with a flattened primer. Is this a sign of pressure being too high even though it was only one of twenty or is it something not to worry about ?

By the way it is proving to be a very accurate load in my rifle.

load info

308 win
47 gr IMR 4064
150 gr Nosler ballistic tip
Win large rifle primer
 
I have had the odd primer flatten out with reduced loads and compressed loads before as well. I did find when working up from min to max that a lower load will flatten primers while the next few up will not until the compressed load is getting near the "too hot to handle". I read somewhere that it is due to the air space in the case with some of the lower loads and as it gets to a compressed load the pressure comes back down again until a higher load spikes the pressure again. This is in .223 and Varget though, not sure if it would be any different in a .308 with your IMR powder.

For example:

23grn - Normal
23.5grn - normal
24grn - starting to flatten
24.5 - flattened
25 - normal
25.5 - normal
26 - normal
26.5 - starting to flatten (ABOVE RECOMMENDED MAX LOAD)
27 - Flattened (ABOVE RECOMMENDED MAX LOAD)

These may not be the exact loads I noticed but you get the picture.

When the primers flatten in a hot load all the primers will be in a state of flattening and the odd one is NOT flattened, my experience anyway.
 
Last edited:
Consider the ignition sequence. Firing pin strikes primer, drives case foreward until it is stopped by the shoulder, primer fires. Pressure inside primer pocket causes primer to back out until stopped by boltface. Pressure rises inside case, sealing breech, locking foreward part of case to chamber walls. If pressure is high enough, case stretches near head, case head moves back against boltface, primer cup is reseated, or, depending on the pressure, squashed a bit and flattened. This is probably why you are seeing flattened primers with midrange loads. Similar sequence with high pressure loads, but the entire casehead is being deformed slightly. Best way to determine if pressures are really too high is to measure casehead diameter before and after firing, with an accurate micrometer. If the solid casehead is being expanded, you are loading really hot.
 
A system for working up loads that keeps me out of dangerous territory follows, and all you need is a cheap chronograph to work up the maximum load in your specific rifle or handgun.

With the components of choice, make up 10 rounds starting well below the maximum shown in the loading manual, and increase the powder charge 1 gr with each load. Small cases would require half grain increments. It is useful to separate bullets and cases by weight to prevent unexpected velocity spikes. Beginning with the lightest load, shoot across the chronograph and you will observe dramatic increases in velocity as you proceed. As you get close to maximum something interesting happens - the velocity plateaus - and your velocity increase will drop to perhaps 25% of that observed from the previous load. Often the subsequent increment will result in a sticky bolt or sticky extraction from a handgun. Because the velocity increase was marginal when the velocity plateaus, it is prudent to drop back to the previous that to be the maximum load for that particular firearm. There seems little value to subject your firearm to the additional pressure and subject yourself to the additional recoil for less than 50 fps. Now you know the maximum load with those components in your rifle, you can work backwards to discover the load that produces the best accuracy. If the velocity plateaus at a lower level than you expect, your powder might be too fast, and if there is no plateau your powder might be too slow.
 
You must log in or register to reply here.
Back
Top Bottom