How to make locks bump proof

It is going to be very difficult if not impossible to easily modify a conventional pin tumbler lock so that it is "bump proof". The flaw is at the very heart of the design. Stiffer springs and custom pinning can make it resistant to various degrees but anytime you hear the phrase "XYZ proof" be real skeptical.

Cut and file a sliver of metal (eg from an old key) to stick in the bottom of the keyway long enough to touch the tailpiece and thick enough to sit under but not quite touch the last pin. This is dependent on the end cuts not being too deep.

Cut a piece out of the tail of the key so the key will pass over this obstruction when entering the cylinder. Be careful not to cut so much away that the tip of the key fails to lift the pins.

A bump key will not fully go in. Adding the extra cut to the bump key will undermine its strength. If this is done often enough, intruders may start making special bump keys to get round the obstruction.

Another method I saw (perhaps on this ng) is to make or alter a key so that one pin is a level below the normal deepest cut, and pin the cylinder accordingly. If this becomes common then bump key makers may make the bump key cuts deeper.

It looks that using some sort of high security cylinder is the only 'safe' option. There would not be any real need to replace all exterior door deadlocks. A Medeco or similar can be installed on the usual 'exit' door and the outside cylinders of all the other locks disabled by filling them with glue, epoxy etc.

The main problem with this one is that it's immediately apparent to anybody who tries to stick an unaltered blank in the lock that something is going on.

This one is the better choice because it's a lot less obvious.

That's one way. I prefer to stick a piece of drill steel or similar in one or two chambers. That way it can be easily undone at a later time if need be.

What if one of the top pins were removed so that if the lock was bumped the bottom pin would jump into the shear plane?

Try it. However you won't be able to use a spring in that chamber and you may run into problems because of that especially if the lock is installed with the keyway 'upside down', or if it's turned that way in operation. An upside down installed lock may also just result in the modification having no effect because the non spring loaded lower pin will fall into the upper pin chamber.

A method I used in a hurry when locks at our local church were compromised was to jam a sliver of wood into each cylinder and push it in with a key so the front pins dropped in front of it. Fortunately I had a spare cylinder with 12 or so keys to put on one door. Issuing temporary keys took longer as I had trouble tracking down a few premises hirers.

There's a dedicated tool called a 'keyblock' do this same thing for the various common keyways. Not to be confused with a 'keeblok' which is basicaly a boot for a doorknob. The 'keyblock' essentially uses a special two part key one part of which is designed to be pushed in by the a 'half key' insertion tool and stay in the lock. Far enough in that it would pose a challenge to remove by conventional means. It's then removed by another 'half key' tool designed to hook onto it and pull it back out. Sold by howardkeys.com.

Wouldn't that deter most intruders at least slightly? I mean, why spend 20 minutes filing the bump key when you can just open the neighbors door...

On a related but and more inquisitive note; can bumping be defended against by incorporating more tension in the springs above the upper pins? Maybe double stacking them? (and if that don't fit, perhaps cutting one in half to be stacked on top?)