Powder PSI
 

I am wondering about PSI(pounds per square inch) on reloads regarding the powder. We see so many reloading charts about type of powder, grain of bullet, primers, crimp, etc. But when a bullet is seated while reloading and compresses the powder is there a measurement on the PSI pressure on the actual powder. And is it different in different loads? Is the actual PSI calculable or is it just "load by the chart".

:confused::confused:

I get what you are asking but I don't think most rounds actually compress the powder when seating the bullet. I will have to get the calipers out and do some figure'n but any specific load you are thinking about???

Am thinking about the .308. When seated, the compression of the powder. The bullet will not fire efficiently without compressing the powder, but is there a general PSI that the powder is compressed to to max it out while being safe?

I get ya... I will ask my FIL about this. He is a rifle guy and been loading longer than I have. You are the first person to ever ask this on a forum that I am aware of. You must be one of those thinkers or something:D

hehehehe...always need to know the specifics.

Most loads aren't compressed. Usually they will have an abbreviation CL or something to indicate it is compressed as compression can increase the PSI quickly and dangerously. Some types of powder may take up more space in the case but very few actually squeeze the bullet down far enough to compress the powder. When creating a compressed load you fill the powder according to the weight of the charge which will fill above where the bottom of the bullet would seat. Then the projectile being seated will compress the load when seated to the OAL.

So there can be airspace between the powder and the bullet?

Depending on the load yes. Sometes a lot,msometime a a little. I have some Trailboss that is a much fluffier powder to take up more space because 231 loaded in a 38-40 case woul only fill to about a third of the case. Then the powder sits on the bottom of the case and may not ignite correctly. Not enough to Squibb but maybe a lower velocity.

I loaded some .32 Magnum and could double charge the load without over flowing the case. That means single charge would take less than half of the case.

Yes many times there is a bit of space between the top of the powder and the bottom of the bullet. So a compressed load would be over filled with powder and squeezed or compressed down. VERY DANGEROUS with a powder that the burn rate is not appropriate for a compressed load.

For example, we load powder by weight. But 1 grain of W231 and 1 grain of AR Comp and 1grain of Trail Boss would all take up different volumes. Then compound the faster burn of the AR comp and you could have a high pressure spike than if you loaded the same volume of Trail Boss.

^this. Pressure in a firearm is measured in CUP (copper units of pressure). Some powders are meant to be compressed, most are not. Some powders are position sensitive (i.e. if they lay along the bottom of the case, ignition will suffer). Some powders characteristics are such that free space causes pressure spikes. In that case, OAL is reduced, or fillers are used. That is why loading manuals always list minimum and maximum overall length for each bullet and powder combination

I never thought about the powder laying along the bottom of the case. Now ya really got me going....some of this comes from a round, any caliber, being chambered over and over, lowering the OAL and then the round damaging the gun upon firing.