What type of steel is the barrel on the Remington 870 Express Super Magnum made of & what is the tensile strength ?
Traditionally, gun manufacturers (except Ruger who was forthcoming), have not been willing to revel what steel their guns are made of. Ruger has been the exception, stating that their primary gun steel was 4140 and 4130 as I remember...41XX anyway (When Bill was alive).
Do different manufacturers use different thicknesses / types of metal on their barrels in order for them to be able shoot a 3-1/2" shells ?
Metallurgy is an continuously evolving science. Early 2-3/4 12 gauge guns were not likely to have the exact same steel as late model, modern 3-1/2 inch guns. In any event, you can rest assured that the steel used in modern shotguns is adequate for the task.
Since we like to accessorize / personalize our shot guns how important is to take into consideration weight & length of barrels / mag extensions / etc? When it comes to handling characteristics ?
It is relative to your particular purpose. A trap shooter usually chooses a gun with characteristics that will improve his score. For instance in the instance of some Browning BT-99 Trap Guns, Weighted butt stock to decrease felt recoil, long barrel for smooth swing, porting to hold the muzzle down, thick recoil pad to reduce felt recoil, etc. His "accessories" will all be for the purpose of the game of trap, little for any "personalization".
With lots of practice with your weapon of choice does it really matter ?
Although with much practice, a person can adapt to the gun even though it does not fit him will (only one size M1 Garand, for lefties, short people that the stock had too long a pull for, etc.), he will adapt to its characteristics. Nevertheless, the characteristics do matter if a person is serious about obtaining the very best results. That is, if a person got to be a
good trap shooter with his Mossberg 500, he would all most certainly could be a
better trap shooter with a dedicated trap gun...no national trap champion is going to show up with a Mossberg.
Everything is about compromises. A long barreled trap gun, is a better choice for trap shooting than a short-barreled, extended mag, bayonet mounted, house defense gun. As a house defense gun, you will not be too concerned about the handling characteristics.