I have a few 38 Special cases trimmed to 1.143 per one of my recipes, and others at 1.140.

I did end up with some cases in the 1.138 area. Would these be safe to use or should I chuck them?

You're fine.

It's VERY RARE that a straight-wall pistol case needs to be trimmed.

6 minutes ago, njJoniGuy said:

You're fine.

It's VERY RARE that a straight-wall pistol case needs to be trimmed.

^^^^this

The question was about the shorter casings i have though.

If those shorter cases are good except for the length of 1.138", I'd use them. SAAMI spec on 38 SPL case length is 1.135" to 1.155".  Just check after firing that they haven't shrunk below the lower limit. Not sure what you're measuring with, but you're really splitting hairs here. I'd use a 1"-2" micrometer that had been set with gauges if I thought the length was that critical.

-Longranger

Straight walled brass cases get smaller as they are reused in reloading whereas rifle brass gets longer.

17 hours ago, Bagarocks said:

Straight walled brass cases get smaller as they are reused in reloading whereas rifle brass gets longer.

38 special is rimmed. I don't think it will shorten over time. It probably won't lengthen either.

 

17 hours ago, Bagarocks said:

Straight walled brass cases get smaller as they are reused in reloading whereas rifle brass gets longer.

I'd not heard that before.  What would cause a case to shorten?

1 hour ago, 10X said:

I'd not heard that before.  What would cause a case to shorten?

Its not well understood, but people have noticed it over time when kept track of... someone did a .45acp trial and notice them getting short.. something to do with the case blow back into the bolt face. 

Necked cartridges expand at the neck, not the body... just for reference 

Ive never seen any hand gun case lengthen like shouldered cases.

Here is a very informative article on it from mass reloading.

http://massreloading.com/loading45ACP.html