That 1 1/2 oz. is not traveling as fast as that 1 1/8 oz., load as a general rule. It is possible, but most loads travel faster for a lighter load and slower for a heavier load. That keeps the pressure more equal when you lack today's slower burning powders. That is the beauty of the proof system. Any gun under proof, is approved for any shell loaded to that proof service level. "Standard" CIP proof is 850 bar and has been that way since 1979. Standard proof working pressure, is up to 740 BAR (10,733 PSI). All, myself included, have been obsessing about here is what "standard CIP proof" has been for decades: standard proof "working pressure" is up to 740 BAR (10,733 PSI). No, you cannot shoot "right up to proof pressure", and is exactly why the CIP publishes "working pressure" load limits. We are making it just too hard. If your 12 gauge gun has 850 bar proof, at any shot load, which is no longer stamped on the gun anyways, then you working service pressure ought to be 740 bars, which when calculated should be about 10,733psi. If your load is under 10,733psi you are good, even better for the old stock if less, because old wood gets fragile.



Pressure can be calculated by a formula. P is pressure, F is force and A is area. Pressure equal the force divided by the area.

P=F/A

Pressure seems simple to understand as it is the result. Measure the force and calculate the area. Force is the effect of the gasses and shot acting against the barrel wall, the empty shell, the breach behind the shot and air in front of the shot, Area is the volume of space as the gases push the payload down the barrel. So as the shot goes down the barrel, the area increases and this is why we can shoot barrels not as thick as the chamber area, where the volume is smallest and the pressure can be the greatest.This is also why a slow burning powder can have lower pressure early but still achieve a good velocity. They also can have higher pressure down the barrel, compared to a faster burning powder which may peak early and drop off quicker. There are a ton of variables such as how fast a powder burn rate, how hot a primer to achieve quick ignition and full burn of powder, compress-ability of shot, steel compresses less than lead and the amount of compress-ability of the wad which will spread the pressure out a bit by absorbing energy as it collapses.

So what does this all mean. Well nothing and everything. The length of the shell is no more a certain predictor of pressure, than the length of the brass on the case. UK used long shells, pushing the same shot load used, more and better wadding. It is true there is more room for heavier load, the final limit is service pressure which being kept equal would tend to push heavier loads slower than light loads. Pigeon guns often had long chambers but the shot load was restricted by rules so the incentive was to get more and better wadding, not to cram in more shot. Speed does kill but better patterns kills as well and Black powder loads most likely were velocity limited by Black burn rate.

Now with modern powders, you can get higher velocity, with lower peak pressure because the burn rate is slower, so the area is increasing as the load goes down the barrel. That is the case until the burn rate is too slow and you get incomplete total burn of the powder and shells which vary in results form one to the other. That is why you struggle to get Longshot light loads to go much under 1250fps. If you drop the charge too much it gives you bloopers.