I haven't read all of the responses, so forgive me if my response is redundant.
As for handling your in-laws, I would definitely talk to them. Don't just ignore them by throwing the guns away. If you can talk to them, fine, but you may need your husband to talk to them, if these are his parents. You need to ask them to respect you & your husband's desires. They are undermining your authority as his parents by giving him things that you don't want him to have. So, I would definitely address the issue with them.
As for your question about "kids playing with toy guns", here are my thoughts:
When my son was little, we also chose not to allow toys guns, but he "made" toy guns out of legos, a banana, a piece of sandwich bread, tree branches, whatever. He wasn't obsessed with them, he just enjoyed making them and playing with them (oh, and making the "sound" that goes along with it). It almost seemed instinctive!
When he was about four, we attended our church's family camp in Colorado. We had a speaker by the name of John Eldridge who has written several books including "The Sacred Romance" and "Wild at Heart". (both great books, btw). He spoke of how God has created a desire in men to be the protectors, hunters, to be adventurous, to even live dangerously. He also spoke some about how denying our sons swords, guns, etc, might even deny them the chance of playing the good guy, the man in white, the prince who rescues the princess, or the defender of the castle. I had never looked at it that way.
On our way home, at a souvenir shop, my husband bought my son his first "pop gun", the kind with the plug at the end. We taught him to never point it at other people, or animals - real or not. We also taught him to never look down the barrel of a gun - again, real or not. As he got a bit older, we taught him more about gun safety so that when he was at someone else's house, he would know what to do if he saw a gun. Once while at a neighbor's house, the little brother was playing with a broken bb gun. It scared my son, and he came home and told me about it. Once the mom was informed, the gun was either put away or thrown away and it was no longer an issue. As for the pop gun back when he was four, it broke in the first 10 minutes! lol
I say everything in moderation. If you deny it completely, he may only desire it more. If you allow it - within reason, age appropriateness, and with many lessons in gun safety, then it may be fine. If you find that he is too obsessed with it, then you may have to take them away. Only you know what your son can and can't handle.
But you and your husband, as his parents, should be the one to decide, not your in-laws. :)
As for the book "Wild at Heart", I bought it for my hubby when he turned 40 (I liked the title). I think I may have enjoyed it more than he did. It really helped me understand both my husband and my son. I highly recommend it!