I was wondering what the best way to check the headstamp on some old .38 Super ammo. I have an old box of Remington ammo, and the headstamp says .38 Auto. No super, no +P nothing. I know that Super cannot be fired in a .38 auto, but how about the other way around?

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Vaya con Dios

.38 auto and .38 super are dimensionally identical. that was in "the old days" before lawyers got into the business and gun makers and ammo makers figured people were smart enough to both read and know what ammo their gun took. you can load .38 super loads in .38 auto cases safely as far as that goes. the trick is to make sure you remember which load is in it when you shoot it (presuming you have guns in both calibers).

Yes, .38 Auto ammo can be fired in a .38 Super pistol. The two cartridges are dimensionally identical. However the .38 Auto may not have enough pressure to properly cycle the slide.
As you noted, do not try to fire .38 Supers in a .38 Auto pistol as the Super has a much higher operating pressure.

I see Robert beat me to it. I was typing as he posted.

The rule of thumb is that if it has a nickle case it is a 38 Super. If the case is plain brass it should be a 38 Auto. Like most rules I'm sure that is has been broken more than once.

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OFT/NRA LIFE MEMBER

The last .38 super ammo I bought was Agilla?? and the cases are brass. I think the Remington cases I bought a couple of years ago were brass as well though both are headstamped ".38 Super".

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Big bang, much smash'em.

That rule of thumb was correct at one time, but is no longer. I do not know this from personal experience, but learned it from some very knowledgable people over on the ammo collector's forum. It stopped being true about WWII, when nickel shortages caused 38 Super to be in brass only cases. From then on the headstamp ruled, not the case makeup.