I was asked to reload some 223 FC cases  for a friend.  He got these once fired case from a police range so I believe him when he stated these were once fired.  What has me puzzled is how much difference there is in the length of these cases and that most of them are much shorter than what I would expect them to be.  Most reloading manuals list case length trim to 1.750.  Now I'm finding lengths from 1.748  down to 1,732.  How far can a case be trimed before it's too short?    I have heard of loose primers in primer pockets with these FC cases.  For this reason I have not used this brand of cases in previous reloads for myself.  So I ask of you your openion on what is the most a case can be trimmed and is FC cases worth the effort ?  Thank you for your responces