Here is the deal with 870 barrels. Remington has used 2 different methods for keeping the magazine cap tight.
The first method uses a magazine cap detent that protrudes from the barrel guide ring and matches up wsith the serations on the bottom, outside of the magazine cap. As you tighten the cap, you hear a ratchett type sound. That is the serations going over the detent. This method of mag cap retention was/is used on older Remingtons.
The second method, normally found on Express and other newer models uses a system where the magazine spring retainer acts as the detent system that will match up with serations on the inside of the magazine cap in order to keep the magazine cap tight. There is no magazine cap detent on the barrel guide ring. You will see either a hole with no dent protruding on the barrel guide ring or possible no hole at all.
You may ask why should I care? The answer is that you must have something to keep the magazine cap tight, which keeps the barrel tight. You will damage either your barrel, but more likely the inside of the receiver if the barrel is loose.
You can put barrels with mag cap detents on the barrel guide ring on an 870 that uses the magazine spring retainer detent system with no problems. The barrel will not come loose. Plain English means there is usually no problem putting old 870 barrels on Express and other newer 870 models.
You can't put barrels that have no mag detent on the barrel guide ring on shotguns that do not use the magazine spring retainer detent system. There will not be any detent system there to keep the magazine cap from loosening. Plain English means there is usually a problem when putting new barrels on older Remingtons.
If you need barrels that use the older mag cap detent system that uses a detent on the barrel guide ring, but you can only find newer barrels that don't have the detent, you can purchase a detent and detent spring for less than $5.00. Drop the spring in the hole first, then the detent, then stake the detent in place. Problem solved.