Review on Spanner Nut Wrench: Ultimate Tool for Locknut Removal, Tightening & Pipe Fitting by Jason Robinson

So as the name suggests the teeth flexed when I used it on my m16 but I don't think the teeth flex on other things. From what I've heard, M16 castle nuts are usually very tight. So don't expect this to work on your m16 if that's what you bought it for. I must add that the company refunded my money immediately with no questions asked. That's why I don't even get mad. Not what I expected when I bought something so cheap. I kind of knew that was going to happen. Either way, if you don't need to use all your body weight and strength to loosen a castle nut, this tool is probably for you.