I'll leave the subject of wether a 357 is enough for bear to someone else and focus on velocity between the 2" and 4" gun.
Recently I've been doing a lot of shooting with the 44 Special in 3 guns, a 3" M24, 4" M29 and 6" M29 mostly with H110 and Lil'Gun but a couple loads with 2400 and Blue Dot.
With 10 different loads:
Between the 3" and 4" the average was 76 fps, largest 108 smallest 53.
Between the 4" and 6" the average was 45 fps, largest 82, smallest 17.
Between the 3" and 6" the average was 120 fps, largest 135, smallest 82.
So if you're shooting loads with a fairly slow powder I'd guess you might see 100-120 fps difference between a 2" and 4" gun, on average, if you use a faster powder you could see significantly less difference.
Recently I've been doing a lot of shooting with the 44 Special in 3 guns, a 3" M24, 4" M29 and 6" M29 mostly with H110 and Lil'Gun but a couple loads with 2400 and Blue Dot.
With 10 different loads:
Between the 3" and 4" the average was 76 fps, largest 108 smallest 53.
Between the 4" and 6" the average was 45 fps, largest 82, smallest 17.
Between the 3" and 6" the average was 120 fps, largest 135, smallest 82.
So if you're shooting loads with a fairly slow powder I'd guess you might see 100-120 fps difference between a 2" and 4" gun, on average, if you use a faster powder you could see significantly less difference.
