March 5, 2012, 08:28 PM
Hello, I have a question, I have a 9 mm ammunition with a projectile of 124 g HAP for a total length of 1.100
I have three different kinds of bullet measuring
124 gr .585 R.N
124gr H.A.P. .5.70
124gr .555 F.P
My question, no matter the length of the bullet, do I have to put the importance on the final length of 1.100 or I have to compensate for different length of the bullet to have the same pressure in the case?
I don't know if i explain the right way.
March 5, 2012, 08:33 PM
OAL is determined by the shape of the bullet.
Not by the weight.
Take the barrel out of the gun and chamber check your loads as you seat them a little deeper.
When they drop in the chamber without hitting the rifling, that is the correct OAL for that specific brand & weight of bullet.
Change brands in the same weight?
Or change weights in the same brand?
Start all over again.
If factory loaded 124 HAP is loaded to 1.100". and you have the same bullet to reload?
Seat them the same OAL as the factory loaded 124 HAP.
If you change to a 124 something else, do the chamber check thing all over again to find the correct OAL for the new design.
March 5, 2012, 08:46 PM
the 3 differents bullets is the lenght and not the weight. I just wanted to know if different lenght of the bullets could change pressure in the case with the same oal of 1.100
Sorry for my bad explaination....
March 5, 2012, 08:46 PM
You are correct in your basic assumptions. Chamber pressure is the focus of all our reloading efforts. And the main 2 contributors to chamber pressure are 1) the amount of powder, and 2) the volume inside the case (which in pistol cartridges is measured from the outside by what we know as OAL).
If you have a bullet/powder combination that already works well, then by knowing the 2 naked bullet lengths and using some simple math, you can alter the OAL to have a new bullet's base seated to the same depth. At least in theory the new bullet should shoot with very nearly the same chamber pressure.
What's wrong with your thinking is that if the new bullet uses a much longer OAL, then the barrel may not allow it to chamber. It is therefore much better to measure the maximum OAL the bullet-barrel combination will allow. (And this measurement will be different for each bullet brand and weight, and each gun manufacturer.)
But 'yes', (if I understand your question correctly) you clearly understand the basics of chamber pressure.
March 5, 2012, 09:06 PM
Don't forget to include magazine fit in the criteria.
March 5, 2012, 09:34 PM
rfwobbly - very nicely explained! nice graphic too
rcmodel - as usual you have a good explanation as well.
March 5, 2012, 09:57 PM
Knowing OAL alone tells you nothing about case volume. You have to know the bullet length so you can calculate the critical value which is seating depth. But bullet length is a well kept secret so you have to have the bullet in your hand and measure it. Which doesn't help much when trying to decide which bullet to buy.
I just posted this in another thread:
If I have a seating depth, I can more comfortably apply load date for a 125gr bullet that is listed in a loading manual to the 125gr bullet that I have in hand. If all I have is OAL and not bullet length for both bullets, I have no way of knowing if the seating depth will be the same. And slight difference in seating depth can make a big difference in pressure.
March 5, 2012, 10:54 PM
Thank you rfwobbly, very very good schema, very clear, you resume very well.