I recently purchased the A400 explor action 20 gauge after shooting a 1187 20 for near 14 years. I highly recommend this gun and just shoot hevi metal. You will get the same outcome if you are in control of your gun at very respectable ranges with a lower price for the same shell count versus 3 1/2" shells in 12 or 10 gauge if you shoot the 20. Perception is not reality when it comes to the model of 2 3/4 through 3 1/2 inch shells.. So if you do know this it may sound elementary. Relative to speed there is not much difference in the 10, 12 or 20 gauge. In most cases you can match the speed of the shot but with lower shot weight and count through the gauges to maintain your swing whether you shoot you 10 one day then 20 the next and etc. Payload or weight of shot out of the muzzle is the only difference between all the gauges when shooting the same speed. If you are on target it does not matter, let me repeat this. If you are on target it does not matter. Whether I shoot an 1 once of 3" in 2s out of my 20, 1 3/8 once 3" in 2s out of my 12 or 1 3/4 once of 2s out of my 10 all shot loads are still traveling at roughly 1350 fps. So if I miss I either have roughly 160 hevi metal shot in the 20, 178 shot steel in the 12 or roughly 216 shot shot in the 10 missing the bird at the same speed. It only takes a few pellets in the kill zone. Other benefits, weight of gun and recoil. The one advantage of shooting the hevi metal mixture is the gain in foot pounds or energy of the hevi shot versus the steel shot in the same shell. The smaller shot of the hevi shot which has twice the amount of pellets is hitting the target at near 15% greater in foot pounds then the steel shot load within the same shell because of its greater density of around 30%. The hevi shot by itself lethally takes birds far beyond the range of the the steel shot within the same load. Birds are stone cold dead before hitting the water in most cases. Have never had a duck sail within 40 yards because it is still alive because of switching to hevi metal. It's momentum was the only thing that progressed it forward. Older gentlemen have told me for years lead was the best thing ever hunting waterfowl. Well guess what hevi shot has better knock down power, more energy, and shoots a better pattern in the same size load as lead. Kind of a no brained to me. I recommend a 20 gauge and switch to hevi metal. The only reason you will be disappointed is your not on target. And I can not help you there you are pulling the trigger.
Regards
Kristan