From what I understand of sound transmissions through bone there are a huge amount of things that can effect it. Such as if the mouth is open, how tightly clenched the teeth are, etc.

The effect of having ear muffs seems to be minimal at best on how well the sounds transmits throuh bone.

Military testing has shown that the only way to maximise protection is a totally enclosed head and also wearing plugs.

In terms of earing protection at the volumes that shotguns can create the very best option is muffs and plugs! As neither will seal perfectly. However custom moulded plugs in the testing I have seen offer the same level of protection as a good ear muff.

So I think in some cases I will be looking to use both

Thank you Alastair I have no seen the Doyle site and they seem to have some very good products.