The gun used in the film was most likley a Mk 101 or a Mk 103, both were 30mm's.
The 75mm anti-tank-gun, Pak 40, was loaded automaticly (unlike the low velocity 75mm in the B25H which was loaded manually afaik). The gun had a rate of fire of 40 rounds per minute. In the film 3 shells are fired at at about 1.5-2 seconds. So it should be from a Mk 101. (bout 180 rounds per minute compared to 250 or so with the Mk 103)
The 75mm Pak 40 anti-tank gun was modified for the HS129. To be possible to use in the air the recoil was much reduced. In its ground-based form it weighted about 1500kg (about 3,300lbs). it fired a 3.2kg (7 lbs) tungsten-carbide cored projectile at 933 m/s (3060 ft/second).
At ranges of 1000m (bout 3300 ft or so) it was capable of penetrating 133mm (5 1/4 inches) of armor meaning it could take out pretty much any tank from a direct HO attack. It could fire away about 3-4 rounds per pass.
The gun weighted so much that the pilot had an emergency release of it which severed the gun from the plane.
The empty weight of the plane was 3800kg in its normal form. 1500kg is somewhere around 40% of that.