I did a FAL using a receiver wrench and open end wrench.
I have great patience when homesmithing (unlike the the rest of the time LOL) I find it therapeutic. I take great pride in my work and try to do the best possible build, with what I have available.
That said, I'll get to the point. I buggered up my barrel flats slightly, because the force required surpassed what an open end wrench was designed to handle.
An open end wrench is designed strictly for convenience. It only contacts two of the six sides of a normal fastener.
No matter what the quality of the wrench (and I was using as good as is available) at a certain point the "jaws" are going to flex and spread because they are only supported at one end. That's what happened to me.
With the bolt on the other end, the wrench above acts like a vise, clamping the flats and eliminating the possibility of the jaws opening up. Not to mention that it is much beefier than any open end wrench available.
I was lucky, I was able to clean the flats up to a point where the damage is all but undetectable.
Someone not using the utmost care, may not be so lucky.