Difficult to do the main portion of the stock without taking it apart and I haven't found a way to take it apart without potentially doing damage.
Copied from Rimfire Central thread Henry US Survival AR-7 Review
Part 3.
Making it Float.
The original ArmaLite Ar-7 had Styrofoam in the stock to ensure flotation, even if the stock itself is not water tight, which was part of their advertising at the time.
Henry does not advertise the rifle as having the ability to float. In spite of this, many youtube and internet peeps still think the current Henry AR-7 is designed to float, but it's not.
Henry's redesign of the stock eliminated the Styrofoam, and thus its only buoyancy comes from whatever air is trapped in the stock.
The stock is made from two half shells that are glued together over the orange interior piece which is molded to fit the rifles components. These seams are not 100% sealed. The biggest leak though is the plastic butt cap. The fit is just not tight enough and there is not a formed gasket to help keep the water out.
My goal was two fold. (1) Try to make the rifle float longer than it does in issue form, which is only a few minutes based on several youtube vids of folks that ain't skeered to throw their new rifle in the pond.
And (2) Try to make it buoyant to the point that it would still float even if it leaks some, so at least you have more than a few minutes before it goes under.
The first thing I did was to determine where it leaked by filling the stock with water and seeing where it leaked out. These points were noted.
Then, I examined the stock to see where the empty spaces were inside. I found that the pistol grip is completely hollow and due to the through bolt, cannot be sealed.
I purchased a product called Flex-Seal, which is available in clear and applied it with a syringe with the tip cut off the needle to the seams where leakage was noted, and to the gap between the orange inner piece and the outer shell. This is a liquid that easily wicks into the cracks, then dries to a rubbery consistency.
For the pistol grip, I removed the through bolt and taped over the hole at the base of the grip. I then filled the space in the grip slowly, from the bottom up with Great Stuff aerosol expanding foam. You must fill very slowly and stop before any foam comes out at the top. The foam will continue to expand filling the entire hollow of the pistol grip. Then you have to stand there with a knife and remove the extrusion so it does not get onto the stock. The solvent for clean-up of the foam is acetone, but acetone will melt the ABS stock, and I'm sure it won't do the camo dip finish any good either.
The foam does two things, it keeps the water out, and it provides buoyancy to aid flotation.
The last thing to tackle was improving the seal at the butt cap. I used ordinary RTV clear silicone for this.
Float test.
I didn't want to get the rifle and magazines wet and have to dry and oil them, so I weighed the parts and filled the stock with an equal weight of cast bullets. The bullets all roll the the back of the stock and are probably why the butt is so low in the water. It will likely float a little more even with the actual rifle inside.
I stated previously that the goal was for 30 minutes being ideal and 15 minutes OK. I was easily able to get 30 minutes, and the small amount of water that did get in means that it would likely make an hour without sinking as well.
