I picked up some cheap Federal aluminium case ammo in .40 S&W because I can target shoot with it, it is better than nothing in the Zombie apocalypse (zombies don't seem to care what you shoot them with) and it's cheap. It has that warning on the side -- "DO NOT USE IN FIREARMS WITH PORTED BARRELS". Forgive my ignorance, but why not? I don't have a ported barrel, BTW, just curious.

I don't know for sure, but I was curious so I Googled it, and quite a few results came up. It seems to come down to one of two, or maybe both, possibilities. The information I found related to Federal Champion ammo. The first reason given was that they use thinly-plated bullets, and apparently it's possible for a piece/strip of the plating to come off the bullet as it passes the port and get flung back at the shooter. Some people suggested that this occurrence was rare, but Federal put it on their boxes for liability reasons. The other was that the ammo was low-powered, and with a ported/compensated barrel it could negatively affect reliability.

I have no idea how accurate this is, but it's what I was able to find. If you really want to know, it might be worth the trouble to contact Federal and ask them.

Hope that helps.

Ported barrels just hurt the ears of anyone or dog (hunting) near the shooter and end up making a harder job of cleaning your weapon vs a barrel without all those holes drilled in it.

Not my cup of tea, thank you.

ContinentalOP hit the nail on the head.

When I got my PC Shield 1.0 as it first came out i happened to notice a box of ammo I had was labeled as not for ported use. I did some checking and several brands were the same. I contacted each manu at the time and the common thread was potential flaking of the thin plating that could come off and exit ports. I assume even if true that liability concerns wrote those labels.
And each manu had a line that was ok to use. So for range ammo maybe I couldnt use Fed Champ but the Am Eagle was ok. Couldnt use Blazer Brass but Speer Lawman was ok. All based on thickness of plating. I always recommend an owner contact the ammo manus directly to find the most current data.
I had no problem following the guidelines but am sure others dont worry about it. Heck these days I bet new gun owners will buy whatever they can get regardless of what a package states.

ContinentalOP and Ranger17, that makes a lot of sense and also answers another curious question I had. I was shooting some white box range ammo the other day and noticed some small tears in the paper. Upon close inspection, I found a fine shaving of copper with a slight amount of lead. I am now certain that was because of the thin jacket. Interesting and good to know. Thanks to both of you!

A shooting buddy of mine had problems with a ported barrel leading up using cast bullets. Had a heck of a time removing the lead from all the holes. So, beware of shooting lead through ported barrels.

My 4" M500 came with 2 factory removable comps. One for jacketed and one for unjacketed. As to why, I don't remember.
ETA:
I currently have the blank (no ports) aftermarket comp on it and have no intention of taking it off.