FLT is right.

But there is more to it than that. Depending on the load, bullet weight, and powder used, the subsonic .300 Blackout generated between 35,000 - 40,000 PSI of pressure.

Ain't no barrel going to stand that outside a ballistics testing lab where the barrel is 8" in diameter with a .308 hole through the center of it.


So . . . time in the bore, and gas expansion rate also comes into play. In the case of ALL the things we think of as modern firearms a great deal of science is put into the speed of gas expansion versus barrel length.

You can see this in action in any gun that shoots a ball of fire out the end when discharged. THAT is like smoking your tires at the drag strip; it looks cool, but it is actually not what you want because it is non, or even counter productive. That ball of fire is powder being burned AFTER the bullet has left the barrel.

Anyway, back on track . . . the problem with "below minimum loads" can be that the pressure spikes too soon. Before the container (the space behind the bullet to the back of the cartridge case) is large enough to accommodate it without catastrophic failure.

Remember that pressure/volume relationships are inversely proportional, and that resistance to flow is what causes pressure. The farther down the barrel the bullet gets the more pressure there can be without 'blowing up' the person or the gun.

It gets pretty scientific, right up until you 'go to far' then it simply gets really painful . . . or not.