I am not sure,but is there a difference on chamber presure between the two rounds? I think 15,000 is the limit???

12ga 2 3/4" and 3" -- MAP 11,500 psi
12ga 3 1/2" -- MAP 14,000 psi

Max. Aver. Pressure (MAP)

This may be a silly question, but why would the 3.5 have a higher presure range? Same Barrel and chamber(in a 3.5" gun),same brass and the plastic part is just a 1/2" longer

Not really sure why but I am guessing when they (don't know who but I think Federal ammo and Mossburg guns were first) decided to come up with the 12ga 3 1/2" shell they wanted to be able to increase payload and velocity to replicate 10ga loads in a 12ga shell. Increasing the MAP allowed them to do that, push heavy loads at high velocities. They also wanted to have good marketing hype to sell more guns and shells to hunters that already had perfectly adequate 2 3/4" or 3" guns!

"Same Barrel and chamber" -- I believe the chamber area is beefed up to handle the higher MAP.

"same brass and the plastic part" -- Brass and plastic have nothing to do with adding strength to the chamber.

"is just a 1/2" longer" -- Room for more powder, shot and BS.

Unless you shoot a lot of big geese at long range (50-70 yards), experiment with your load and chokes to make sure your combo is lethal at those ranges, and you can consistantly hit them at long ranges you will never realize any benefit from the 3 1/2" loads except lightening of your wallet and massaging of your shoulder!

Good luck.

Thanks! I(we) are working on some "Super Slug" Loads....cast880s at 1500+ and 1040s at 1200+!...These were shot at the Linebaugh Seminars and did tons of damage/penetration!That was is 3" shells,so due to my nature/quest for power,am thinking of more vel in the 3.5"

Long as I have been reading the two basic standards were "target loads" which might have any pressure, just not alot of kick. Usually in the 10K LUP range. Then there were "field loads" and Magnum field loads where the use of different powders with maximum powder and shot combinations got you more pay load toward target. (Yes, they KICKED!) High end pressure was in the 15K to 18K LUP (Lead units pressure. Copper crushers were not reliable.) And yes, some shotguns are "beefed up" a bit to handle these pressures but more likely to be heavier and help moderate the additional kick. Knew some law dogs that hated the 3 inch buck shot because of loss of control of the gun for additional shots.

If what you want is a slug gun... then by all means, more payload and appropriate powder can get it. Otherwise, with shot, alledgedly, higher pressure blows gas into the shot and explodes the pattern. The reason for the "bell" muzzles on the pilgrim guns. Luck.