Online gun and vintage ammo sites often have 32 rimfire ammunition. Navy Arms manufactured large quantities of this ammo around 1990 and it can be found at these auction sites, plus most 1930s-1940s 32 rimfire ammo will still function just fine. The cost for 50 in a non-collector box is selling for $70+/= for 50 rounds. It is expensive, but I usually shoot my old tip-ups no more than once or twice a year, so even 50 rounds would last you a few years if shot modestly.

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Gary
SWCA 2515