I don't think yabu op. I've had this problem recently.
Ds1 is 3.5. DP and I both didn't want him to have guns (nor ds2, 20mo). However about 6 months ago ds1 found a space gun at our friends house and of a sudden became obsessive. They have never bothered either way about guns, their ds (same age) has them around but not really fussed.
It was one of those occasions where I felt a bit cringey. My (best) friend didn't say anything but a wry smile as my ds would not put this bloody gun down all day. This is when the gun obsession started.
When we got home he talked about this gun all the time. Last thing he would say before going to sleep was 'if there's a monster will you run to (friend's) house get that gun and shoot it?'
It was all he wanted to play at nursery (who don't allow gun play). It was all he made out of duplo, the only thing that he would draw a picture of (well scribble but when you asked what it was he said 'a gun'). At this stage a group of boys at nursery were all obsessed so that did loads of stuff around superheroes who don't need guns
I then got him a bubble gun to try and take the edge off - within 5 mins he pointed it at his little brother and said 'share or I will kill you' - nice!
So it was taken away (and given back later).
Basically that didn't stop it so we got water pistols. That didn't either.
All his friends at nursery has moved
On to other stuff but my ds was still obsessed. They have asked me about it loads of times - I reckon they think we let him play Call of Duty on the xbox or something 
Anyway, sorry to ramble on. In the end I caved. I bought him two nerf guns with a hundred bullets and told him to knock his socks off.
I realised that I was turning into a far bigger issue (for me and him) by not letting him - it was like some secret excitement. He's played with the need guns constantly for about a week and it's calmed down loads. Still builds them out of duplo but does build other stuff as well.
It's turned everything on it's head a bit for me. We are in the countryside, most family members have shot guns. I'm not quite sure why I dislike them so much but I do but ds1 (even ds2 has joined in now) loves them and they are just toys. I will try and steer away from big replica ones but he's still only little yet so that might change - I'm not as bothered as I was.
Good luck op I hope it goes better for you but it just wasn't worth the battle for us.
On this basis I'm going buy him a packet of fags and a crate of beer for his 4th birthday
.
Writing all this down (actually before that) I do think I've been massively pfb about it all.
rolls eyes as 20mo is shooting his bother with a sticklebrick