I'm not a technician so I cannot give alot of input - in general the difference is 'payload'. I for one pretty much only use 2 3/4", sometimes 3". I have never bothered with the 3 1/2's. I do not see the need for MY style of shooting and certainly the cost is inappropriate. Impact for steel loads, and correct me if I am wrong guys, is basically speed, since steel is so much less dense than lead. Therefore, I go for fast loads. The concept of the 3" and 3 1/2" is to offer more shot with that same level of high velocity.
Like what dukhtr describe, the 3" just give syou more capacity in shot and powder to either increase velocity or payload
One little bit of information to know. 3" and longer are considered magnum loads in 12ga and 3.5" in 10ga are considered magnums. To even add more confusion, you can get 2.75" loads in 12ga that are magnums
Like everyone said,more steel helps to make up for less density of steel shot.Steel has less energy at impact then lead so more steel combined with higher FPS rates is needed to do the same job.If you don't mind the extra cost you can use Bismuth shot shells,or maybe even tungsten-iron shot but again thats depending on your budget.
It's all about balancing speed and payload. In most cases if you want speed you have to sacrifice some payload or vice versa. Buy jumping to a 3" you can usually push the heaviest 2 3/4" payload a bit faster or exceed the heaviest 2 3/4" payload at the same speed. Under 35-40 yards you probably don't gain any significant advantage with more speed, over that the speed helps maintain the energy level but you may sacrifice pattern size or density. Personally I'm probably an average shot. I do better with a more open choke and larger payload to maintain pattern density and keep my shots under 40 yards. Some guys are very capable of consistently shooting birds with tight chokes, small payloads and more speed which gives them a bit more range. A majority of my shooting could be as easily done with 2 3/4" loads as with 3" but generally 3" are more available and only cost about a buck a box more so that is what I use.
I shoot 3 1/2" shells for just about everything with a 1 3/8 to 1 1/2 oz of shot at 1500-1550 fps. I probably don't need the big shells for the majority of my hunting as I usually hunt over decoys, but I am a glutton for punishment and I like the extra pattern density. I am not completely sure that it results in a significant increase in amount of birds bagged, but I have more confidence in the big loads, particularly on big Canada geese and wary late season birds that don't want to totally commit.
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