The one advantage other than noise reduction is that felt recoil is reduced significantly; disadvantages are the balance of the rifle is changed & obviously weight & overall length is increased.
As far as I'm aware all moderators are screwed to rifle barrels either directly to the muzzle or by what's known as a "two point mounting" using the thread at the muzzles & a bush at the rear of the can as in Ianís photo.
We can buy rifles off the shelf ready screwcut or you can have one screwcut by a gunsmith.
Once I'd tried a rifle with a moderator, as we tend to call them, there was no way I'd go back to an unmoderated rifle. As a general rule they knock about 30dB off the sound pressure level which is well worth having as a 308 produces about 165 - 170dB(C) which is going to do you damage real quickly. When hunting I would risk the occasional moderated shot without ear plugs/muffs, at the range I wear plugs and muffs and cause my neighbours a lot less pain than if I was to go unmoderated or, worse, fit a brake. You have to be aware, however, that your rifle will still be producing something like 140dB(C) so it is still very loud indeed. There is also a significant reduction in recoil which is worth having apart from the noise reduction.
My rifle, a Blaser, ships with a barrel threaded for a moderator so it is simply a matter of getting a mod with the right thread and screwing it on, assuming you can get permission from the licensing people. Unfortunately there are a significant number of different "standard" threads, certainly here in the UK. I think it is important to get the threading done right and I would say it is worth paying a gunsmith with a good reputation for such work a little extra to ensure it is a good job rather than getting a bodge. There are certainly riflesmiths here in the UK that will do after market threading at reasonable prices.
There is no question that the mod has an impact upon the balance of the rifle but I personally found that I quickly adapted and it wasn't a big deal, other people find they just can't cope. As stated there are two types, one screws on to the end of the barrel and the whole moderator extends beyond the muzzle and the other type, as shown in the photo, actually extend back over the barrel to improve balance and reduce length. I've seen some barrels pretty badly marked from the hot gas etc. with the over barrel type but don't know if this is a general problem. Mine extends beyond the end of the barrel as the Blaser rifle is already pretty short.
If you want photos of threads, thread protector, moderator etc. let me know and I will see if I can post some.
Unfortunately here in NSW, Australia, moderators/suppressors are illegal. Maybe our NSW government is more worried that us Australian's will shoot the neighbours dog rather than be used to help our hearing.
I predominately use a 30.06 when out chasing larger game, so as you can imagine the sound is fairly loud. Especially when letting off multiple shots from inside the car at a running mob of pigs,goats etc. However, tinitius hit my ears a while back after some big heavy seasons spearfishing, so my hearing can't get much worse (hopefully!!)
It would seem that the suppressor, although minimising the sound of the shot, does not completely reduce the sound to a small pop (like often seen in the movies). I understand that what you often hear is the crack of the bullet as it breaks the sound barrier. This correct?
Anyway, I'm going to be interested to see how they work when I'm over there in a couple months.
When I first used a 'Silencer' I thought it hadn't worked because it wasn't like the ones in the films.
The reason they are called moderators and not silencers is because it doesn't silence the report but it does moderate the noise
What it does is, instead of getting a bang at the muzzle that is integrated into the sonic bang made by the bullet so it sounds like one massive bang, it makes a 'whoosh' at the muzzle and due to slowing the gasses exiting the muzzle is quieter.
You will not stop the sonic bang of the bullet but because it is forward of the muzzle it is just a crack and, to the quarry, is not associated with the position of the firer.
That is a laymans way of describing it. It is also very good when firing at dusk or at night because it elliminates the muzzle flash.
I think I may be able to explain why animals have difficulty locating the source of a moderated shot.
A moderated rifle produces about 140dB(C) and the crack from a supersonic bullet weighs in around 120dB(C).
Our brain/ears use several ways of locating sounds but one method is "time of arrival" and one is "how loud something is." Both of these allow the brain to effectively select the original sound from reflections from a hard surface on the basis that the first sound to arrive is likely to be the original and the loudest sound to arrive is likely to be the original. In general the brain uses time of arrival in preference to loudness and only abandons this method when the later sounds are significantly louder than the first ones to arrive.
Generally speaking the bullets we fire are approaching 3 times the speed of sound and so the crack of the bullet as it passes close to the animal arrives some time before the report of the rifle.
I suspect when an unmoderated rifle is used the report of the rifle is, at many hunting ranges, sufficiently great in loudness to cause the brain to abandon the "time of arrival" approach in favour of the "loudest sound first" approach to position location. The brain therefore accurately determines the location of the rifle rather than of the sonic boom of the bullet as it passes.
When the rifle is moderated then I suspect that the report of the rifle is reduced such that the brain uses the time of arrival method of location of the noise and in the case of a group of deer that has just been fired at this leads to them selecting the position of the bullet as it passed them rather than the actual report from the rifle. This may also explain why different deer in a group may think the sound has come from different locations as it depends upon their position relative to the path of the bullet.
The problem with my theory is that, as far as I know, there are no studies to prove that the hearing of a deer works the same as the hearing of a human, though I think there is work on other animals that shows they tend to hear in a similar way to us. However, in my view it provides a reasonable explanation for what we see in the field when using a moderated rifle so I'm going to stick with it in the mean time.
Scroll down and dip in. There is a lot on this site about moderators how they were first made how they developed and to make them, latter could be illegal is OZ. I read somewhere that a plastic soft drinks bottle taped onto the barrel will work for a while if adapted properly, haven't tried that one myself as it might be illegal in the uk and possibly unsafe.
Scroll to the bottom of the first page and there is a lot on loading centrefire subsonic cartridges. The latter would be illegal for deer here but not for vermin, can't say about OZ but if legal could save the ears and so much recoil.
Heed the warnings though! very dangerous if you get it wrong or don't lubricate the bullets. The big consolation is that if you do get it wrong you probably won't know. The cat's sneezes are fun to play with and very cheap.
Subsonics, especially with bullet reversed should flatten goats ,although probably not pigs unless taking head shots. Lots of room to experiment, you could try 30 30 bullets (same diameter) in the 308 at a slower speed. They have a thinner jacket so they can open up at lower speeds than a normal .308 bullet, less recoil, less noise. You could just buy a 30 30 but possibly less fun. Like all reloading your do it only if you accept that nobody else is responsible for your mistakes and accidents.