Recently I had to replace a Mares "Cyrano" piston, which I had damaged the nose of, and noticed that the white acetal plastic is rather tough stuff until it gets loaded up unevenly and then it can be crunched. Possibly sticking a drill in the piston front end might also crack it, but it seems to me that loosening the shaft tail fit in the piston is an easy way to provide a vacuum check after your gun is cocked and which has been supplied, or retrofitted with, a captive seal vacuum muzzle.
This topic came up recently on another forum when discussing the "Taimen" and "Pelengas" spearguns. The latter uses Mares pattern spears of local manufacture, all in spring stainless steel, hence when loaded the spear tail is jammed in the piston face and you have no way of knowing if the vacuum seal is still doing its job. On the other hand the "Taimen" will let you know because the spear can fall out, but not completely as for the spear to leave something has to replace it in the inner barrel. There is a weight limit that the vacuum can support, but most common gun sizes will not be a problem.
Enlarging the hole in the piston face would probably be safest using a sanding or grinding action with a support dowel that could be pushed into the piston face with abrasive paper wrapped around it while the piston was spun in a variable speed power drill. A careful incremental approach is necessary as you want to remove just enough for a loose, but not sloppy, fit of the spear tail.
This topic came up recently on another forum when discussing the "Taimen" and "Pelengas" spearguns. The latter uses Mares pattern spears of local manufacture, all in spring stainless steel, hence when loaded the spear tail is jammed in the piston face and you have no way of knowing if the vacuum seal is still doing its job. On the other hand the "Taimen" will let you know because the spear can fall out, but not completely as for the spear to leave something has to replace it in the inner barrel. There is a weight limit that the vacuum can support, but most common gun sizes will not be a problem.
Enlarging the hole in the piston face would probably be safest using a sanding or grinding action with a support dowel that could be pushed into the piston face with abrasive paper wrapped around it while the piston was spun in a variable speed power drill. A careful incremental approach is necessary as you want to remove just enough for a loose, but not sloppy, fit of the spear tail.