Pounding from the muzzle does no good if the bolt is locked.
Use the technique known as "mortaring":
1. If it has a collapsible buttstock, put it in the collapsed position.
2. Remove the magazine and put the selector on "safe".
3. Starting with the buttplate 6" above the ground, slam the buttplate on the ground while simultaneously pulling the charging handle.
4. It may take several tries to get the bolt carrier to retract enough for the bolt to unlock.