Although I've only used three different brands of canned/aerosol insulating foam, I've never had problem one, with the foam reacting/expanding inside a stock - since there's plenty of air/access in there.
I just shake the can according to directions, attach the little tube to the spray nozzle, and start foaming as deep inside as the tube will reach.
The can's pressure insures that the foam gets squirted much deeper than the tip will reach, and if only a little is done at a time (as in: short bursts/expand/burst/expand/etc) it's fairly easy to avoid a void (cute, huh ?).
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