A325 structural bolts start at ½” diameter with the shortest length available being 1-1/4”. As far as the ASTM specification is concerned, there is not an official documented reason as to why they are not readily available; however when asked to provide 1” length, we have found that they do not appear to be easily obtained in the marketplace. That isn’t to say that they can’t be made, however it would be difficult to make a bolt in that short of a length. A bolt of this length would likely need to be done in a cold forging process, however cold forging is most commonly a process used in mass production. It may just come down to the demand of a 1” length A325 not being high enough for manufacturers specializing in mass production to justify running a batch in that size.