The form is confusing as it was set up to relate to rifles, hence the questions re amount of cartridges etc. and it's always better to make sure everythings correct otherwise delays are caused. With mine I just put "high capacity 12 bore shotgun" and purchased a Remington 11-87 that is set up for 5 shot, has an extension tube to make it an 8 shot, and also has an insert that turns it into a standard 3 shot.
Having had a nightmare getting my local force to agree to mine for mammals, and having researched well into the surrounding legislation, I can confirm that there is nothing in law (firear