Regarding gauges, the 12 gauge is getting more and more popular among hunters, as a matter of fact more pheasants are caught with a 12 gauge than with all the other gauges combined. The reason why hunters use this gauge is because it has heavier payloads and tremendous flexibility compared to other smaller gauges.
If you prefer being in the field with trim and quick handling guns you should use smaller gauges to preserve hunting. If you do want to use the 12 gauge the most suitable load would be a light 7/8 ounce or one-ounce that duplicates the performance of the 28 gauge (or the 20 gauge). The perfect combination for a regular double barrel gun would be a 1 1/8 ounce of 6s in the first barrel and 1 1/4 ounces of 4s/5s held in reserve for an eventual upcoming second shot. The all-around best pheasant load for shotguns might be 1 1/8 ounces of 5s exiting the muzzle at approximative 1200fps.
Moving on to the 20 gauge, some hunters prefer this one because some guns chambered for it feel and handle a lot better. Some recommended guns for the 20 gauge would be the Winchester 101 and the Browning Superposed Lightning.
A very tough combination to beat while hunting with an over/under is a 2 3/4 inch load with 1 1/8 ounce of 6s in the bottom barrel and a 3 inch with 1 1/4 ounces of 5s for the eventual upcoming second shot.
The 30 gauge is considered by many hunters not enough for pheasant hunting but can be efficient for shots out to 25-30 yards with the right load. Combining it with the Winchester Super-X or the Feather XS shotgun, the results are quite impressive.
The .410 Bore is growing in popularity among pheasant hunters although it still remains rarely used. The heaviest shot charge that can be hand loaded in the 3 inch shell is 3/4 ounce. This load was offered by Remington and Winchester. This charge is also the standard charge weight for the 28 gauge. It is recommended to use 3/4 ounce of no.6 nickel plated shot with the .410 Bore. In comparison with the Winchester and Remington hulls, the Federal 3 inch case is the best choice. The advisable thing to do (if you prefer using the .410 Bore) is to combine the heaviest pheasant loads you can find with the right chokes.