Home Forums Hunting & Shooting Ask The Gunwriters Shotgun reloading pressure versus velocity question for the experts.

I put this same question in the shotgun reloading and see what kind of answers I get but this might be a better place it does involve a bit of scientific level I guess.

I'll see if I can get this question out, in a nutshell does the pressure affect the cycling of a gas or inertia shotgun?
What brings this question I have one gas operated 20 gauge that is a bit ammo finicky on new factory ammo. Ammo of the same weight charge and velocity rating some cycles better than others. Is that because of possible pressure?
When hand loading you can pick a powder that has some incredibly high velocity in lower pressure or you can give some faster burning powder that gives you a lower velocity on higher pressures. That's where my question of does the pressure really affect the cycling of an auto loader.

I'm back to my favorites SxS..the 20 ga reserved for GS#1.

A gas operated semiauto needs a certain amount of gas volume along with adequate duration at the ports - too little and one will have operating problems while too much can cause operating problems as well as increase wear and tear on the gun. One can readily see this with Beretta 390 series guns as Rich Cole designed a set of gas exhaust springs to maximize reliability with loads ranging from very low volume/dwell to quite high.

Pressure is not really a part of the equation as that does actually affect the volume of gas nor the dwell time. That is more a characteristic of the powder used than anything else. That is why some loads will work well in a given shotgun while others don't it is also a reason low cost shells may work one time but not another as the components, especially powder, regularly change between lots. Higher end shells are made to a tighter standard often with less variation in components which is a reason they are more reliable.

Recoil and inertia actions are dependent on the amount of force (recoil) needed to push the bolt back rather than gas volume. Using light shot charges means higher velocities to properly operate the mechanism while heavier shot charges could get by with less velocity. As with gas operated autoloaders, too much recoil can also affect the operation of the shotgun so there is a balancing act here too. And that can change as springs wear and are replaced.

There really is no "do all" mechanism that doesn't make a compromise at one end or the other of the operating spectrum. The first Winchester SX-2 I owned and my current Browning Gold 3.5" guns cycle nearly everything I've put in them from 7/8 oz reloads to 2 1/4 oz turkey loads but I'd wager the heavy loads batter the mechanism pretty hard making me glad I only used a handful of them. Recoil/inertia guns can be even more fickle on the lower end as the two SBEs I've played with needed at least 1350 fps to operate properly with 7/8 oz loads and I've seen more than one inertia gun have problems at the range with 2 3/4 and even 3 dram target loads.

Some of the cheap ammo is also lower-pressure, which combined with variable wads can result in unreliable results.

Have also owned quite a few gas-operated autos that were supposedly designed to compensate for different pressures and cartridge lengths. One 12-gauge was supposedly able to handle everything from 2-3/4" to 3-1/2" hunting loads equally well. Found it did NOT handle 3-1/2" loads reliably at goose-hunting temperatures, so sent it back to the manufacturer. They adjusted it, and claimed it would now work fine. It did--with 3" and 3-1/2" ammo. But it would not cycle reliably with 2-3/4" ammo.

Was never fond of the 3-1/2" 12 anyway, finding the 3-1/2" 10-gauge patterned more consistently. But after trying a bunch of 12-gauge autos, eventually ended up with a Benelli Ethos, which due to its recoil-operated system combined with various advances in "kick reduction," works reliably and comfortably with any ammo I've tried so far, from cheap 2-3/4" to various 3" loads

ldholton,

I'm not sure I understand your last two questions, but pressure and velocity are related in shooting a shotgun shell, similar to with a rifle cartridge, but a little different.

The wad is like a piston traveling down a tube, pushed by expanding gases from the powder burning.

The velocity of the shot (and wad) is related to the acceleration. For a wad initially at rest,

Velocity (ft/sec) = Acceleration (ft/sec/sec) x time (sec)

That same acceleration is associated with the pressure:

Force = pressure (lb/ square inch) x area (square inches)
and
Force = mass (lbm) x acceleration (ft/sec/sec)

So,
Pressure = mass x acceleration / area

The above involves some simplification because none of the terms are constant (they vary with time) except the mass of the shot and wad and possibly the area of the base of the wad.

The tightness of the wad determines the amount of gas that blows by the wad instead of pushing on the base of the wad, which affects force on the base on the wad, which affects pressure and acceleration, and ultimately velocity.

Pressure also has a role in slightly deforming the wad to seal against the walls of the tube (barrel), which in turn means more of the gases are pushing on the base of the wad instead of blowing past it.

Edit to add:
Before anyone points it out, yes I did neglect some of the forces like friction and glossed over the Newton's 3rd law discussion for why the wad/shot mass acceleration is approximately equal to the force from the pressure of the gases on the rear of the wad, for the sake of simplifying for illustration purposes.