Chamber pressure has nothing to do with recoil operation of a guns action other than with high pressure cartridges you need some delay to allow the pressure to drop before opening the action.
The 9mm has a SAAMI maximum working pressure of 35,000 psi for standard loads and 38,500 psi for +P loads. Shotguns chamber pressure generally operate in the 9,000 to 11,000 psi.
The recoil of shotguns in themselves is heavier than a 9mm handgun as recoil is a function of mass x velocity. Shotgun 438gr (1oz shot) x 1,200 fps = 525,000 vrs 9mm 115gr x 1,300 fps = 149,500, so a 1 oz light shotshell load is about 3.5 times the recoil of a hot 9mm. Good thing a shotgun usually out weighs a handgun.
You will find that for the most part all shotshells have the pretty much the same pressures and same velocities (1,200 to 1,350fps) be they bird or buck standard or magnum. The magnum loads have more recoil because they throw more shot, keep the velocity equal and double the projectiles weight and you double the recoil. The difference in this recoil is why a recoil operated shotgun like the A-5 must be adjusted for heavier loads when they are used while self metering gas guns can use a variety of loads without manual adjustment.
If you fire a Browing A-5 from the hip without the butt of the gun firm against your body you will get a failure to feed as the rocking of the gun will absorb enough recoil that the action will not work properly. Do the same with a Remington 1100 (gas operated) and it will cycle just fine as the action is mostly indepedant of recoil.
