We present here the development of a facility to generate high (multi megagauss) magnetic ﬁeld of 4 to 5 𝜇s rise time, using inertial magnets. The facility includes a low inductance, high current capacitor bank (280 kJ/40 kV) and an inertial magnet, which is a copper disk machined to have a keyhole in it. As the high current from the capacitor bank is discharged through the copper disk, a high magnetic ﬁeld is produced along its axis, before it is destroyed by the combined effect of the dynamic loading and skin effect. A maximum peak magnetic ﬁeld of 257 T is realized, when the magnet with 3·6 mm inner diameter, 35 mm outer diameter and 5 mm length, is powered by the capacitor bank charged to 28 kV (134 kJ). The transient magnetic ﬁeld is measured using a B dot probe with an error of ±25 T. The probe in most of high ﬁeld shots (> 200 T) got destroyed before recording the peak ﬁeld and the trailing edge of the magnetic ﬁeld. Experimental evidence of enhancement of the probe survival for longer time in copper disks using spatial non-uniform conductivity with 1 mm thick SS brazed to the inner wall of the inertial magnet is also reported.