BlackDragonSpeerAre you using pure tin or the bismuth/tin/antimony alloy rotometals has put out for lead free casting? In the latter event, bismuth expands when cooled. The alloy in question is said to mitigate bismuth’s natural expansion some. Pure tin should shrink considerably, but still less than lead, hence it’s added to lead for better mold fill-out.
If there aren’t any sizing dies for for your needs, Lee and other manufacturers will make them by request. You don’t have to worry about gas cutting in air rifles, so it should be more lenient. Still might get some cerrosafe and slug the bore. Measure the diameter and I’d personally size pellets only .001″ max, but I’ve not done much with air guns for a while. Always just used what’s on the shelf. If it chambers, it should at least be safe, though I presume your concern is more with accuracy.