I got a reply from one of my mates and as happens got this reply...looks like wise words ..hope it helps.
Pete
Place a strong but pliable plastic bag in there and pour in some resin.
This will enable the void to be filled, the hollow sound to be rectified and as importantly, enable the solid plug to be removed...That is assuming the hole is tapered in the correct direction, if not this would be a permanent solution.
Having weighted a gun for competition use, I can say without any doubt that he needs to be careful adding weight to a stock, it changes the entire handling characteristics of the gun and if a permanent solution is rendered, would result in major work if the outcome went pear shaped...!
Why not fill part of the stock with bags of expanded foam beads. (Wall insulation beads) then inserts a bag with plasticine in it, then more bags of expanded foam.
This method would allow the bags of beads to be adjusted so the weight of the plasticine could be moved forward or backwards.
It will achieve the desired result without the permanency....great when he comes to selling it.
Hope this helps
Tom
Those bead can be obtained from a sewing shop, they use them in cushions annd bean bags, I think when I do my Tikka stock I will do it this way, I always thought resin was the way to go but this sounds the safer option.