Originally Posted by Tench
AA in their wisdom (?) don't use a gland seal on the firing valve stem, this causes air to leak down the valve stem when the transfer port is under pressure during the firing cycle. I think this hole also allows air to escape during the forward travel of the hammer. If this is the case blocking the hole will slow the hammer and reduce the power. Blocking the hole will also cause the air escaping down the valve stem to blow the hammer back causing a double (or more) tap on the valve, this is most likely what people hear when they say the sound of the gun changes. Basically it all comes down to a design that cuts corners, unfortunately loads of different makes of rifles are made without stem seals.
With the exception of BSA I cant think of another maker that uses valve stem seals on all their designs?
What do you think of the AA solution of fitting a fairly low density foam block over the hole? Keep it on or take it off?