9mm Question about OAL and Pressures.

[Flashover16]

its pretty simple really!
a given projectile caliber is a certain diameter, if made from the same material then the added weight must go somewhere!
It goes to the length of the projectile as it can't increase in diameter.
you can only move so much to the taper or it becomes a chambering/stability issue.
the added length goes back into the case and reduces case capacity.
same with hollow points the larger the hollow point cavity the more weight that has to go somewhere, the larger the cavity the longer the projectile typically is.
Bearing surface also has a factor on pressure as it creates drag in the barrel/ due to more friction.
use a lighter material such as all copper (vs lead, lead & jacket) and you get a much longer projectile that extends even farther into the case.
this is usually reflected by a longer recommended OAL of those particular projectiles and reduced powder loads.

 

[Gun Nut]

Yes it is very simple you don't know what you are talking about. My main carry load for my 10mm is a 125gr Barnes TAC XP With 13 grains of LongShot giving a average velocity of 1700fps. Being an all copper projectile it is as long as a 180gr projectile with a longer bearing surface of the 180gr. I am 1/2 grain away from max charge for a 135gr Nosler JHP that is a lot shorter. After firing both loads the primers show no differences nor does the brass and yes I mic the brass before and after firing. Bearing surface does have some affect on pressure but not that much. Look at the Palma bullats built for long range precision shooting that have a longer bearing surface. Look at you Manual and the compressed loads, you can still put more powder into the case but you are compressing the charge and making it have a faster burn rate that gives it a faster MEAN pressure. Thats where the danger comes in. When compressing smokeless powder enough you are changing the burn rate of that powder. If bearing surface had that much to do with pressure there would be people making SWC with just enough bearing surface to stabilize the bullet so you could use N1 or Bulleye to fill the case up and get velocities unheard of out of any caliber.
Very simple to prove my point reload 2 rounds same bullet same powder and same powder charge, but seat one bullet at its normal OAL and the next seat the bullet as deep into the case as you can get it without deforming the brass that means compress the **** out of it crimp the rounds the same.I am talking about a good charge listed as max in your manual that fills the case up not leaving any room between the bullet and the powder. Mic the web of the case measure up .35 inches before firing and measure there measure the crimp on both rounds. Fire both rounds marking each with a permanent marker to be able to tell them apart use the same make of brass fired the same amount of times. Look at your primers and see which one pancakes the most. Mic the same as you did before you fired the rounds and also the length.