Seven Palm Musket

[Queequeg]

I'm rereading The Religion, by Tim Willocks, about a German adventurer named Matthias Tannhauser at the Siege of Malta.

Twice the author mentions Turkish muskets:

p.221

In skirting the splayed and tangled slain who'd made their final stand by the tent, Tannhauser spotted a nine-palm musket on the ground. Its match still smouldered. It's stock was pinned beneath its owner's corpse. The blue-black hue of the damascened barrel, which seemed to glow from deep within its substance, and the arabesques of silver wire with which the ebony woodwork was inlaid, announced the hand of a master gunsmith.

p.298

Tannhauser had elected to avoid the rigors of the line by employing his marksmanship. Along with his wheel-lock rifle, he picked up a Turkish seven-palm musket from the stockpile of captured weapons...

Is a "palm" some kind measure of barrel length, with one "palm" being a semi-standard (4 to 5 inches) unit of measurement like the cubit (one forearm)? Or does it have some other meaning?

Google produces fairly little results.

Thanks in advance.