In article rs.com,
says...
According to William Wixon :
ever wonder what the inside of a dead blow hammer looks like? i have. now
i know.

http://www.frontiernet.net/~wwixon/deadblow.jpg

Hmm ... did you try removing the end caps from the cylinder? I
suspect that it is filled with lead shot.

Better luck with your next one.

It looks just like one of mine that recently failed the same way. If
William's lasted as long as mine, 25 years, I'd say his luck was pretty
good with *that* one. I'm pretty sure it's just a case of the urethane
getting brittle over time. Mine started chipping a couple years ago,
then started to split, then fell completely apart. I've been carrying it
around in my truck hoping to run across a Snap-On truck, but probably
won't have the nerve to actually ask for a replacement when the chance
comes.

A Proto of the same vintage (looks just like a Nupla) is good as new.

Ned Simmons