Rifle moderators (never silencer, you cannot silence a supersonic projectile) in the UK are a bit of a contentious issue. Most forces will allow their use for stalking as it is accepted that people doing so don't wear ear defenders. Due to healthy and safety concerns, alot of forces are willing to grant moderators to target shooters on the grounds that they don't want people suing them in 20 years time for hearing loss issues. Conversely, some forces won't grant their use on the grounds that 'if we let one person have one, everyone will want one'... Another joy of the British Firearm licensing system!
The video on the left (courtesy of 6mmbr.com) shows a moderated Sako TRG42 in .338 Lapua Magnum being fired. It clearly shows that you cannot silence a centrefire rifle but you can tame the recoil and blast considerably.
The way a moderator works is to trap the expanding gases that emerge behind the bullet and to cool them slightly. This reduces the massive boom but cannot stop the supersonic crack that the bullet makes as it goes supersonic in the air. The closest way I can describe the sound of a moderated, large calibre rifle is that it is similar to opening a can of fizzy drink quite close to your ear. As a side effect, this slowing down of the gases behind the bullet can actually increase the accuracy of a rifle slightly. It also reduces recoil quite significantly. These two reasons are why moderators are becoming more and more popular among rifle shooters.
The diagram shows a Reflex T8 suppressor from BR of Finland. It is designed to sleeve over the barrel to avoid adding un-necessary weight to the front of the rifle. These moderators are extremely popular in the UK and a T4 version is available for smaller calibres such as .223 Rem.