I hear bombs (not nukes) often referred to by weight, but what does that mean to people like me, who knows almost nothing about explosives? I get that the heavy the bomb, the more powerful it is, but I have no frame of reference to go by.
If I dropped a 500 pound bomb on SF City Hall, would it completely destroy it?
What if it was 400 lbs?
Disclaimer: Don’t need answer fast, & I have no plans to bomb SF City Hall. 
It’s a bomb that weighs 500 lbs.
I always though it was an explosive equivalency thing. As in, a bomb with the same explosive capability as 500 pounds of TNT or dynamite.
Usually when they talk about the weight of bombs, I believe they are comparing to that weight of TNT (dynamite), no that it gives much information to you or me.
To expound a little the Mk 82 is the general purpose bomb in the US armory. It’s nominal weight is 500lbs. The actual weight varies because it has various configurations that make it either dumb or smart. The guided configuration weighs a bit more with the sensor head and movable fins. I’ll have to look for the exact amount but less than half of the weight is explosives.
Not when you are talking about air delivered bombs. The actual weight is more important due to how it effects flight characteristics.
This site says that the Mk 82 500 lbs bomb contains 190 lbs of explosive.
Again 500 lbs is its nominal weight and the actual weight of the variants are all different but in the ballpark.
The Mk. 82 weighs 480-570 pounds, depending on configuration (i.e., what kinds of gadgets get screwed into the casing). It contains 192 pounds of high explosive; most of the weight is the steel casing, which makes for great fragmentation.
The Mk82 is a bomb that weighs 500 pounds, and has 192 pounds of tritonal. That amount of explosive is roughly equal to 230 pounds of TNT.
You can see a video of the boom here:
I hadn’t really thought of it before but explosives are measured depending on its delivery system.
Artillery: diameter of the shell
Air delivered bombs: nominal weight of the whole bomb
Ground demolitions: explosive weight
The last is important because when a combat engineer puts together a charge each one is different and he has to calculate the stand off distance by its weight and Relative Equivalence Factor. The danger close ranges for artillery and bombs are a known quantity.
That reminds me of something else: at some point, descriptions of large guns went from being based on the weight of the cannonball they fired (“32 Pounder”) to the diameter of the barrel of the gun (“14 Inch Gun”).
I would hazard a guess that coincides with cannon firing shells instead of balls.
That would totally make sense - except that it doesn’t appear to be true.
This “18 Pounder” gun for example fired shells:
I have no idea why some cannons are designated based on the weight of their shot, and others based on diameter. Maybe it is just arbitrary.
To add.
The Navy pretty much from modern times has described the guns (and shells) with not only the diameter of the barrel between the lands but also the length of the barrel in “calibers” where caliber is the diameter of the barrel interior between the lands. The guns main guns aboard the battleships Iowa, Wisconsin, Missouri, and New Jersey are described as 16"/50. The “50” is the length of the barrel in calibers [16" X 50 = 800"]. The secondary guns on the ships are 5"/38. Current cruisers/destroyers sport a 5"54 main gun.
For Army weapons, see wiki for M109 howitzer series. The original M109 howitzer was 155mm/23 calibers. The current US M109A6 has a 39 caliber barrel. Other variants out there have barrel lengths up to 52 calibers.
I guess the question I haven’t seen answered is why some weapons in some nations at least are described based on the weight of the ball or shell they fire, and others based on caliber. National difference? Change over time?
As has been mentioned, not necessarily. But to a limited degree in at least one service, the USN. The USN designated mainly round shot firing guns in nominal weight of round shot, eg. 32 pounder. It started designating guns which mainly fired, still spherical, powder filled shells in bore diameter, eg. the 8" shell guns of the 1840’s and the Dahlgren type shell guns of 1850’s through the Civil War, with the added twist that the latter were usually referred to by roman numeral, eg. IX-inch. The pounders could fire spherical shells and the later ‘shell guns’ could fire solid shot but there was that distinction based on type they were originally mainly intended to fire.
The first widely used USN guns which fired elongated shells Parrott rifles, were sometimes referred to in pounds and sometimes in inches. By the time of the ‘New Navy’ US naval rearmament from the 1880’s the USN used bore diameter from 3" up and ‘pounder’ to refer to smaller guns, which was a mix of contemporary foreign practice. Metric country navies often referred to the ubiquitous small anti-torpedo boat guns of late 19th-early 20th century as 37, 47, and 57mm, but sometimes as 1, 3 and 6 pdr as the US and British always did. Those calibers were standard in almost all navies at the time, and a lot of the guns from one designer, Hotchkiss.
The US Army designated the smaller cast/wrought iron composite Parrott rifles used on land in ACW as ‘pounders’ but switched to inch diameter in field guns when it resumed designing new steel rifled field guns in the 1880’s, then generally (not entirely) to mm (for bore size not all the engineering dimensions of the gun) in WWI from French influence. Most other armies but British likewise bore diameter (in mm or cm) took over in the era of steel rifled field guns. The British clung to ‘pounder’ for field guns through WWII, like the 25 pdr.
Back on bombs, the US Mk.80 series have a relatively thick case and low % of explosive weight compared to ‘general purpose’ bombs of the WWII era and some countries’ postwar bombs. The idea was to get usefulness against a range of light to relatively hard targets, and as was mentioned lots of fragmentation. The M65 500# bomb used by the USAAF in WWII had 262# of explosive.
So how big of a bomb IS needed to destroy SF city hall?
Here’s a video {YouTube 2’36"} of some 2,000# bombs going off … looks like they’d do the job on SF City Hall … maybe just pay the parking ticket and be done with the issue? …
There really isn’t any single bomb sized appropriately for a large building target. The largest conventional bombs will destroy only a portion of a building (making them larger will mostly just destroy that portion more thoroughly), while the smallest atomic bombs will destroy much more than one building.
