I'd quibble. While I agree it wouldn't be vulnerable in practice, I don't think the longest sniper kill is relevant at all. A battle rifle caliber can definitely break 10,000 feet fired straight up. The issue would be whether it has enough energy left to penetrate the skin of the blimp, and whether it winds up anywhere near where you aimed at.

They're definitely not at risk to the odd potshot, but massed small arms/crew-served rifle fire could potentially be effective in the lower altitudes.