When writing up rules for unusual firearms in GURPS, I often find that it is really difficult to get a good estimate of how much the ammunition weighs. For some reason, although people love to post numbers related to guns on the internet, they very rarely include the weight of the cartridges they fire.
Obviously different loadings vary quite a bit; the bullet is often two-thirds of the weight of a round, and the heaviest bullets might weigh twice what the lightest ones do. GURPS simplifies that into a single WPS, which leaves a bit of 'wiggle room', which is fortunate because any simple formula is only going to give a crude approximation, given the massive number of variables.
Anyway, here's the math:
A = B + ( ( C ^ (5/6) ) / 100 )
Where A is the weight of a complete round of ammunition in lbs. (i.e. WPS in GURPS), B is the weight of the bullet, also in lbs. (divide weight in grains by 7,000, or weight in grams by 454), and C is the case capacity in cubic centimeters AKA milliliters (yes, this is a weird mix of units, divide grains of water by 15.4 to get cubic cm).
This generally seems to produce results within about 10-20% of the WPS figures in GURPS High-Tech for modern brass cartridges, which is close enough for me (given that different loads in the same casing can easily vary that much in the real world). Anything old enough to have been originally loaded in black powder will cause issues, and plastic cases, caseless ammunition, and other oddities will not work at all.