Ok, would you say “a 500lbs bomb” or “a 500lb bomb”? The bomb is singular but the pounds are not (there happened to be five hundred of them in fact). So is the abbreviation for pounds modifying the “500” or the “bomb”?
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A three-year contract. A 500lb. bomb.

A period after the abbreviation is standard.

Best wishes, Clive
Wrong. Units are not normal abbreviations. There is no period after units. Units are the same for singular and plural. There is one space between the number and its unit. Check any technical style guide.

So your original question is moot.

It would be "a 500 lb bomb."

regards, Greg
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Hi Greg,
Thanks for the comment.

There is a difference between writing a magazine article and writing an engineering report. You sir, are wrong. You should not assume all writing is technical.
Units of measure are technical by nature, even if written in a sloppy non-technical news paper for example. There is never a period after a unit of measure, regardless the context or media. It is always Lb and not Lbs and there is a single space between the unit and the unit of measure.
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I agree, except there is only one unit of measure which it is correct to add a period and that is inches: it should be "in.". There should be no "s" at the end of "lb". I am a mechanical engineer.
You would say "a 500 lb bomb" in either case: not only is it proper to say "I am a 200 pound man" (rather than "I am a 200 pounds man"), but the convention of adding an "s" to "lb" to make it plural is actually incorrect anyway. The term "lb" is Latin for "libra". The Latin term for more than one pound is "librae"; note the lack of "s" being added. Hence, while it's far more common than not to see "lbs" indicating a weight of more than one pound, it should actually be "lb" regardless of how many pounds it is.
While linguistically we add an "s" to pound, if there is more than one, or -1 < x < 1, when you write it out the abreviation doesn't change. it is always lb 1 lb or 500 lb. Think about kilograms. Would you write 500 kgs? Of course using lbs is such a common mistake, I doubt anyone would ding you for it.
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