by Gwion » 20 Jan 2016, 8:42 am
As there are often questions about load data for various chamberings; often accompanied by warnings of 'approach MAX loads with caution'; i thought a thread on the all important signs that your action/chamber/brass are experiencing excessive stress through high pressure spikes was in order.
Now, i'm absolutely no expert on this subject, so i'll just start it off with my understanding of a few things that can cause changes in pressure and some pics of primers from gradually increasing loads. Please feel free to elaborate, correct or add to anything i write here. I'm wanting this to be a place for questions to be asked and experience to be shared.
Excessive pressures occur because there are either too many gasses produced by powder ignition and/or they can't escape the chamber in time to avoid placing cartridge, chamber or action (or all three) under undue stress. In extreme cases, this can cause something to go POP that isn't the bullet exiting the muzzle. This is never good for anyone but there are signs that all hand loaders should watch during load development to avoid excessive pressure spikes.
That pretty much exhausts my knowledge of the What and the Where. Now for the Why.
The most basic event causing excessive pressures is overloading the cartridge; that is, putting too much gunpowder in the case. This can be put down to exceeding MAX loads FOR YOUR FIREARM. The max load for your firearm may well be either above or BELOW the max loads listed in hand loading manuals.
However, sometimes using too little powder can cause spikes through what is termed FLASHOVER. This occurs when there is so little powder in the case that it lays flat in the case (for want of better terminology), the primer flash then ignites the top layer of powder momentarily, causing pressure in the chamber to rise. Then the rest of the powder ignites and pressures spike way beyond that which even a full (but evenly ignited) charge would create.
Excessive freebore, through seating bullets too deep, may also cause a similar effect but my understanding of this is limited so i wont go indepth. Basically i understand that pressures build, causing the bullet to leave the case but then as it engages the rifling, slowing down, pressure back up as the burn reaches it's peak.
What can be overlooked is changes in temperature. This can occur environmentally or circumstantially. One i have been guilty of is not accounting for a change in seasons. I developed a load during a crisp but clear Tassie winter week. I settled on a load about 0.4gn below the book max and everything was sweet at 1/2MOA accuracy and no real pressure signs. Come summer and some uncommonly hot days (for Tassie), rounds sitting out in the sun while i'm shooting and things behaved very differently, resulting in some very flat and cratered primers!
Similarly, if you have a hot chamber and insert your hottish but usually safe load, then take longer than usual to fire the round, the powder in the case may well have got pretty hot and changed the dynamics of the ensuing burn.
So, how do we avoid the pressure issue? Be careful, be cautious, be precise and be pedantic! Check, check, record and check.
Start by cross referencing load data from a number of manuals and study the spent brass while developing loads. Start your load development low in the numbers presented in your manuals and work up gradually. If you change a component or specification of your load; such as a new primer, new cases, new brand of same style bullet; decrease your charge and work it back up, just to be on the safe side! Also go gradually with changes in your seating depth, watching not just the groups for accuracy but also the cases for signs of a change in pressure.
So, what are pressure signs? Well, i'm pretty new to it all as well, I've only been reloading a couple of years. What i keep an eye on are the primers in my spent rounds. Look for gradual flattening of the primer and signs of "cratering" in your firing pin mark. If the round edges of the primer have been forced very flat to the edges of the primer pocket, or if the outer edge of your striker mark is raised and not smooth; your pressures are getting up pretty high and you should be VERY conscious that small changes in load specs or environmental/atmospheric/situational circumstances may well cause a pressure failure.
I will edit this post later with some pics of primers and maybe some more experienced reloading gurus can discuss what is and isn't a safe looking spent primer.
Can someone else discuss other signs of pressure?
Again, please feel free to correct any error or misunderstanding i have displayed above; and please; expand, elaborate or add to anything that needs mentioning.
If you're a newbie, ask questions so the experienced people can help you out.
Now i'm off to do some hand loading and testing.
Good luck, stay safe and have fun!