You can probably get all the tires you want for free, and they'd last a long, long time. Tires are never in short supply, and will last a century.
If I were building one, I'd go with tires. Stagger them like bricks, and fill them with sand on each layer. Maybe put a vertical posts as anchors through sections every X feet to ensure they stay put.
I don't like RR ties. They leach chemicals and give off a really bad odor in the heat and sun. I mistakenly had some installed for a garden, paid $1 per foot. I will be removing them and giving them away soon.
They are filthy to work with, and cut, and they do rot and fall apart. If you've worked with them you see they rot out from the inside and get hollowed...
Tires are easy for a single person to move, and they are quite uniform, whereas RR ties are quite heavy and cutting them to size is a real PITA.
Edited to add: I have no experience weather tires will reflect or absorb bullets, but I've seen them on a lot of ranges so assume others have tested these.
Here's an example of tires used on a range. Ignore the gun review, but in the vid the user is on a live fire range where tires are used as backstops. Watch around the 2 minute mark:
https://www.youtube.com/watch?v=8XAYsYwZGxI