Pressure change due to seating depth change

[namvet44]

I have been reloading several years.  Because of the scarcity of w231, I bought some aa#2.  While reading the aa reloading chart, I see a caution note at the bottom of the page which states that REDUCING the seating depth may dramatically INCREASE pressure in some handgun cartridges.  Can anyone explain the technology which would make pressure higher when the bullet is shallower and therefore should leave the casing sooner?  I have a degree in phyics, and understand PVT but somehow this intuitively seems backwards to me.  I am assuming that REDUCING the seating depth means not pressing the bullet as far in the case.

[Silicon Wolverine]

the AA loading chart is talking about reducing the overall length by seating depth alterations.  short seating a bullet can drastically change powder burn characteristics and put pressures in the danger zone.  the only time seating a bullet out farther (longer OAL)  can cuase pressure problems is when you are using a large capacity case with a small powder charge.  IE  44 special with a 240 gr bullet calls for a staring load of four grains of AA#2.  A powder charge that small could concevably leave the primer flash hole exposed, should the bullet not be seated deep enough.  An exposed flash hole can cause all sorts of wacky pressure spikes and lows and in one case i know of destroyed a revolver when the powder burned from front to back in the casing.

SW

[namvet44]

Thanks.  That makes sense, that an exposed primer hole could cause a problem.  But in that case, unless the case is always full of powder, what keeps the same problem from occuring when pointing the firearm down?  Many of my reloads at recommended levels leave empty space in the case.  I have several reloading manuals, but don't remember reading any technical description of WHY pressures go higher or lower depending on bullet weight and/or powder charges.  I usually adhere to safety precautions for obvious reasons, I just wish I had a  better understanding of the physics.

[Silicon Wolverine]

if a firearm is pointed down, the flash hole is still presented to the back of the powder charge, allowing it to burn from back to front as it should.  when the powder sharge lies level below the flash hole is when you see pressure problems.   For the physics behind the presure you need to read some of the technical manuals on ammo.  i cant remember the reference number but there is a top notch military refernce manual on small arms ammo, The manual of modern cartridge reloading, lyman shotshell handbook and reloading basics all have good info in them.  its about reading, reading, reading when it comes to the "nuts and bolts" of how modern smokeless rounds function and the physics behind them.

SW