Working on consumer & industrial electronics for many years I would run into security screws.
Here is how to remove 2 types:
The oval head. One size of these was used on the Hamlin Cable Converter boxes.
I must have rebuilt 500 of these units in just one year.
The screw head was an oval type and a "special" screwdriver was needed to remove them.
Well, I found out that if they were not too tightly installed, I could take an old RCA plug socket and slightly deform it by removing the "guts". Worked great in most cases.
The other type was a Torx type with the pin in the center. At the time, the special removal bits were extremely hard to find and expensive.
Mostly used on Microwave Ovens and many imported Asian stereo's.
Looking at it and experimenting a bit I found they were made of a quite hard, but brittle metal.
I took my center punch and small hammer and whacked the little center pin, breaking it off. Viola! A regular Torx bit would remove them. One time I didn't have the correct size Torx bit with me, but had a set of Bristol Spline Wrenches. They worked well to remove the screw also.
The above methods worked a lot better than the hammer & cold chisel method.