Thearos, I made a replica of a 5th C. javelin that's shown in the Museum we visited, and tested it for quite a long time.
I used to throw it 10 times a day, 2 days a week for 3 months, against a wooden board 2 centimeters thick placed 10 paces away.
The javelin head was very simple but it worked well; it was fixed on the handle by pressure only, no nails.
At the end of the first 2 weeks (which means after 40 throws of which 36 were successful and hit the target) the board was totally destroyed and I had to reinforce it with iron bars.
I've never tried it at a distance, though - not enough space.
By the way I'm here for any further question you might have, as is my javelin whenever you'd like to see it
By the way you can find an interesting information on javelin throwers used as auxiliaria by the Roman army in Silio Italico "Punic wars", book V, 208-219, where Silio describes a Picenian attack with javelins and say those javelins "bend" once they hit the target (where you can also find a description of which were the damages of both lead glans and javelins - check book I, 505-530).
Greetings,
Mauro.