The 250lb Depth Charge (As per MOTU Notes and as carried by the Shack)
Contained 180lbs of Torpex, and was designed to be dropped below 200ft/250kts. To release above 200ft a nose cap needed to be fitted - and they were in very short supply - and even then the max height only went up to 1000ft/250kts.
When dropped at 100ft and 160kts - standard pilot attack profile - the forward travel under water was approx 35 to 40ft, during which they would sink approx 30ft before making a big bnag!
The standard attack profile was a pilot aimed drop of 6 DCs over a 100yd length to straddle a moving periscope/snort with a line error of no more than 20yds. That way at least one DC (hopefully) would explode close enough to the hullso that the pressure wave caused by the explosion would cause major, if not fatal damage. The reason 6 were dropped was that the kill radius was very small - no more than 20-30yds at best.
Of note all the DCs I saw in the last years of the MR Shack were old WWII vintage, and quite rusty in places to boot. So much so that the first one I dropped you could push your finger through the casing! Even when we were disposing of the wartime reserves - again by dropping them in what were then active jettison areas - the latest date stamp I saw on them was 1945! Unfortunately I can't remember the mark of the weapon we carried, but I have a feeling the Navy weapons may be slightly different.
As YS alludes, they were not part of the Nimrod basic fit as the DC was not going to be as effective against modern submarines as the other weapons it could carry, and the attack profile was not suited to Nimrod ops. In addition, I don't think the 2 DC bomb carriers (Avro 144 and 169) were cleared for carriage in the Nimrod either due to size/compatibility problems, but that may be a red herring.