Go to the local hardware store or hobby shop and buy a piece of 1/2" diameter thinwall brass tubing. (K&L?). Stack the washer and snap ring over the BNC and push.
I struggled with the snap-ring pliers forever the first time I put it together. When I accidentally popped it off I was dreading a repeat. Tried the tubing, which I just happened to have in the scrap pile, and it worked like a champ. Literally took 30 seconds.