Pluralizing abbreviations where the noun is not the last word

Virtually always, the s follows the full abbreviation even when a word other than the last one is the proper plural. Paul Brians in Common Errors in English Usage cites

POWs, RPMs, WMDs

This style is also recommended in the AP Style Guide, so it is what you will see in most newspapers. But not all.

The other option is to discard the s altogether, as in this news story from the Cleveland-area News-Herald:

Byrd had five home runs and 19 RBI in 34 games.

This is certainly not a typo. Many commentators prefer this, but it is simply that, a preference. As a former editor at ESPN, I had this discussion many times with writers and other editors, many insisting that there is only one right way to do it. Clearly, that is not true. If you prefer the s, use it, but only after the entire abbreviation. Nobody, but nobody, uses RsBI.