I never thought of this before, but there is a slight difference between a USGI castle nut and a commercial one that could be creating difficulty in getting the commercial one off. I can't remember who pointed it out, but the USGI nut has a distinctly larger flat on the surface that mates against the barrel than a commercial nut. I took my 19 year old castle nut off fairly easily recently. I was able to compare the nut to a commercial one I had on hand and sure enough it had the larger flat. It was on tight, but it was not difficult to remove with the pliers.