[mcguyver]

As you know, ICS guns do allow for the big stock screw to travle through what would be the buffer tube on a real gun and into the spring guide, as most companies do. This would not allow you split the recivers to swap uppers without removing said screw first. So what they do is make a removable "buffer" tube that is attached to the lower receiver with a big screw. This can be replaced as ICS sells this as a spare part. This piece is what any aftermarket stock will mount to.

On stock ICS guns, there MAY some fine-tuning of this "buffer" depending on the stock, with wire holes required for crane stocks. However, you can also buy a Marui-spec version of this part from ICS, so modifiactions should be relegated only to wiring holes. You will need to use a very short screw for the new stock tube, but the length of which will depend on the tube.