What is a Firearm?
"Firearm", within the definition of the Firearms Acts, means a lethal barrelled weapon of any description, from which any shot, bullet or other missile can be discharged. It includes any prohibited weapon, whether it is such a lethal weapon as aforesaid or not, and any component part of such a weapon, and any accessory to such weapon designed or adapted to diminish the noise or flash caused by the firing of the weapon.
Firearm, within the terms of what you are allowed to hold on a firearm certificate, would obviously not include any prohibited weapons. Neither would it include "shotguns", as they are held on a shotgun certificate. Except, that is, for Section 1 shotguns, which can only be held on a firearm certificate.
What is a Section 1 shotgun?
A Section 1 shotgun differs from a conventional shotgun, by virtue of the fact that it has a magazine capable of holding more than two cartridges. These are known as "pump-action" or "semi-automatic" shotguns, where cartridges from the magazine are loaded by hand "pumping" the action, or by the discharge of the previous round. These weapons are required to be held on a firearm certificate.
What is a prohibited weapon?
This is too large a subject to describe here. It has its own section, Prohibited Weapons [http://www.herts.police.uk/advice/firearms_licensing/faqs/prohibited_weapons.aspx], where greater detail is available.
So, what can be held on a firearm certificate?
Well, if you take away Section 2 shotguns, which are held on a shotgun certificate, and also eliminate prohibited weapons, you can have any other weapon you want. Provided you can supply a "good reason".
Most applicants require small or full bore rifles for target shooting, as a member of a Home Office approved club. Others request muzzle-loading hand guns for the same reason. Others still, use rifles for vermin control or deer stalking. Provided good reason is satisfied, all these weapons can be legally held on a firearm certificate.
...
What is a Prohibited Person?
A person who has been sentenced to preventive detention or to imprisonment or to corrective training for a term of three years or more; or who has been sentenced to be detained in a young offenders' institution in Scotland, shall not, at any time, have a firearm or ammunition in his possession.
This means for life and includes all firearms, even air weapons.
A person who has been sentenced to borstal training, to corrective training for less than three years, or to imprisonment for a term of three months or more, but less than three years; or who has been sentenced to be detained for such a term in a detention centre or in a young offenders' institution in Scotland, shall not at any time before the expiration of a period of five years, from the date of his release, have a firearm or ammunition in his possession.
...
In England and Wales rifles must be at least .240 inches in calibre. The bullet must be expanding/hollow nosed with a muzzle energy of at least 1,700 ft/lbs.
In Scotland, for Roe deer rifles must be at least .222 inches in calibre. The bullet must be expanding/hollow nosed with a muzzle energy of at least 1,000 ft/lbs and a muzzle velocity of at least 2,450 ft/sec.
For other species of deer the rifle must be at least .243 inches in calibre. The bullet must be expanding/hollow nosed with a muzzle energy of at least 1,750 ft/lbs and a muzzle velocity of at least 2,450 ft/sec.
...
The Firearms Act 1968 (as amended) prohibits the possession of expanding ammunition.
However, Section 5A(4) allows a condition to be added to a firearm certificate for possession of expanding ammunition for:
1. the lawful shooting of deer
2. the shooting of vermin or, in the case of carrying on activities in connection with the management of any estate, other wildlife
3. the humane killing of animals
4. the shooting of animals for the protection of other animals or humans
...
Section 57(1) Firearms Act 1968 defines a "firearm". (See "What is a firearm?")
It should be noted that this definition includes "any accessory to any such weapon, designed or adapted to diminish the noise or flash caused by firing the weapon".
This "accessory" is known as a moderator or, more commonly, a silencer. So it will be seen that to possess a moderator you will need to have the authority on your firearm certificate, the same as you would for any firearm.
As with any other requirement for a firearm you will need to satisfy good reason.
The most common reason for requiring a moderator is, in the destruction of vermin, to prevent additional quarry from being frightened off when the first round is fired.
Every application for a moderator, as with any firearm, will be treated on its own merit, but if you feel you need one, then it should be included on your application form, in addition to any weapons requested.
You cannot legally possess a moderator unless you have the authority on your firearm certificate.