here is the link to walter lewin video lecture please jump to 13:13

8.01x - Lect 2 - 1D Kinematics - Speed, Velocity, Acceleration

i thought that all meter ruler/ meter stick use ± 0.1 cm as uncertainty .how did he get ± 0.5 cm?

Whilst a ruler might be subdivided into millimetres you cannot necessarily assume that you are accurate to the utmost limit of your equipment (in fact you must not assume this.) And rulers over a metre long might not be marked with millimetres

 

He even says the situation is not all that stable (the ruler is flexing for a start), he doesn't know what will happen when the bullet will hit the wire, and that 1 in 300 is a very small error anyway

The important distinction here is between resolution and accuracy.

The resolution is the smallest difference you can measure with a measurement device.

The accuracy is the absolute error made.

 

The accuracy is usually worse than the resolution, for several possible reasons.