I tend to consider bullet construction over cartridge and bore size when thinking about safety. There are places I'm more comfortable sending a 75Gr Sierra Varminter out of my .25-06 at 3700fps than I am sending a 40gr out of my .22lr at 1050fps. Why? Because the .25-06 bullet will in 99% of cases break up, the 40gr .22lr most certainly won't!Would you use a .50BMG where you would usually use a .22lr? Or shoot pigeons inside a barn with a .22lr instead of a 12ftlb air rifle?
Even Bisley (its a very famous series of ranges used by the military and civilians), have a small bore range that limits the calibre.
If you can not work out that certain back stops will be safe for certain calibres and not for others, maybe you should find another hobby.
Stick a 500ml coke bottle on a tree stump, place a cardboard box from your local white goods retailer a couple of meters behind it, and shoot it. See what results you get with the bullets you use. It's quite interesting. A powerful cartridge with a light frangible bullet is the best option I've encountered - velocity is king. That's why I always praise overkill when foxing. I like centrefire .17's, .22-250, .243, and recently .25-06. There's no substitute for a backstop but having a bullet that near enough vanishes into thin air when it hits something is a real bonus. The faster you can send it, the safer it seems to be.
