Syntactic foam as a gun building material

A true mix and pour syntactic (hollow glass or ceramic spheres in a resin matrix) would almost certainly be too heavy. We never got the Polywood down to its specified specific gravity of 0.5 g/cc.

I think your best bet if you want to fill the barrel would be a two-part polyurethane foam in a 4 or 8 lb/cubic foot density. You could always drill into the ends of the foam and insert more weight if required.

The only other way that I can think of to estimate the volume of the barrel is to immerse the barrel (no muzzle or handle) in a small pipe full of water and measure the displacement by the change in water level. This is obviously a measure of the outside volume, but if the wall is relatively thin, then it would probably still provide a reasonable estimate. You could always "adjust" the estimate by measuring the inside and outside area at the end of the barrel, and scaling the total volume by the same ratio.

Jeff, remember we had good results getting the density down by using Polywood and foaming it by adding a little water to the mix. While not a perfect syntactic at that point, if plugs were still used in the ends of the barrel I think it would be fine.

I don't remember what specific gravity that mixture ended up. We don't have any numbers yet, but I bet that gun will require something around 0.3 or less to float without the shaft.

Dan - I assume that you don't need any reserve buoyancy to float with a reel as well?

Dan - Do you have any way to determine how long the rubber plugs at each end are? If so, you might be able to drill into the plug a little, and insert a lead weight, without compromising the watertight integrity.