Visiting a friend in Central Colorado. He is not a gun guy. Owns a 22 rifle and a very nice Marlin lever-action 30-30 that is at least 20 years old. Shots 5 rounds with the 30-30 annually. When I visit I clean his guns for him. This year he shot 2 rounds of 30-30 and noticed after ejecting the one of the spent casing had 2 splits about 1/4 inch. Asked me what happened? Told him defective ammo but thought perhaps a problem with the chamber. Think a gunsmith should check it out after trying some different ammo. If it's not ammo any thoughts on why the split casing?

Split where? Old ammo?

The 2 thin splits begin on the neck and extend to the tapered casing.

Ammo while not new isn't very old. Everything I've
read indicates ammunition lasts many years

These are not 30-30 cases but thought you might be able to use these as a reference if they reflect what you saw. You will need to clarify if what you saw is similar to the cases in this photo. Hope this is helpful.

The split is very thin. I begins where the taper begins and extends onto the neck.

Personally, I would need more than 1 case from another lot/manufacturer with a split to blame the weapon over the brass. But more than one would be indicative of a chamber issue. I’d sit down with a box of decent quality ammo and see if it’s a one trick pony but the only thing I’d do if it repeats is never reload cartridges out of that particular rifle.

Yes, first try different ammunition. I would bet a nickel that it is defective ammunition.

Something to be aware of is that if it is limited to that ammunition, no more of it should be fired in the rifle. Splits/defects in brass like that result in gas escaping and possibly eroding portions of the chamber. If it progresses far enough that can cause extraction difficulties.

Easy for me to say, but scrap the rifle if its common to more than one lot of ammo. Its even why many folks won't buy at a gun show. Also could be why the gun splits cases, but we're ahead of ourselves here. Isolate the problem to the gun or the ammo. Might need to get another guy with a .30-30 to come along and fire alongside.

One case? Old ammo?? Poor annealing on the cases and the brass has age hardened. It's not uncommon.

I have had 3 or 4 cases do that type of thing and at a similar location in my .243. My assumption is work hardening being the culprit. Is that brass Federal by chance?

As a 30-30 nut, I can say a few things… I’ve loaded thousands of 30-30. Splits like this tend to happen with old brass. You’ll see neck splits with 3-4x reloaded brass at the case mouth. But splits at the shoulder area are from older brass where the annealing has gotten hard. I had a batch of 280AI brass were all at the same time 6/10 pieces split along the neck. They were old. New brass, no problem. If they’re reloads, chances are that the die was set too deep and the shoulder was set back too far… couple that with age and there you go. Since 30-30 is such a low pressure cartridge I doubt you have any issue with the firearm. Try it with fresh virgin brass (Remington is soft) and see if you have any issues. I doubt you don’t have an issue with factory virgin brass, and the answer is old brass.