Most SA-.410 guns will accept the Beretta/Benelli Mobil style choke tubes. However, Mossberg recently identified they had some issues when the barrels were threaded in Turkey. Some guns look like they will accept a Mobil choke but once you try to install one it wont go all the way in. This means the barrel is one of the barrels that was threaded with a tolerance issue. You cannot determine which one you have until you try a Mobil choke in the gun to see if it will wrok properly. Feel free to contact Carlson's if you need any further expalnation on this matter.
The Stevens 301 uses the Short Invector choke system. Correct TSS choke is item# 38030.
Carlson's does not specify the extended dimension, please contact them directly for further assistance.
Yes, this will work with steel shot.
No, you would need the TruChoke versions of those chokes, found here: https://www.opticsplanet.com/carlsons-choke-tubes-truchoke-12-gauge-sporting-clay.html
It can vary by manufacturer and usually depends on the level of constriction you are considering
These chokes are Compatible w/ Lead, Copperplated, Nickel, Hevi-Shot, Bismuth, Tungsten or Steel Shot