At National Parks & state parks that have very large cannon where visitors enjoy recreating this image. Most often a child or slim girl friend is inserted into the muzzle. It doesn't take long for the cuteness to wear off.
More often than not, the bore is rusty. It is like very coarse sandpaper. In the case of a child, sliding down further than intended life threatening.
Extraction can be a challenge. If the arms are not extended there is the vexed question of what do you grab hold of to pull them out with?
In one case (I am sure this story only got better in the telling.) the legend of the buxom young woman wearing a bikini is amusing. I will leave the details to your imagination.
The moral to the story is don't do this. Getting out can be… well… quite a bore.
Note: At living history events we search the bore administratively (not as part of the firing drill.) before beginning cannon firing demonstrations.
Visitors sometimes loose control of or forget their photographic props. Little ones just want to "…see what would happen if a toy truck was shot out of the cannon…" Fortunately, it is very easy to tip a cannon forward by lifting on the handspike. Have to admit, seeing a little firetruck come shooting out the muzzle was pretty funny.
Not for nothing my wife calls living history cannon volunteering cub scouts for adults.