Today I had this bug several times.
If you use a mounted MG for the first time it’s never a problem to shoot and let go. But if you use the MG several times in a row it’s pretty likely that the following bug happens:
You are on the MG and you can’t shoot and you can’t let go anymore. Only chance to go on playing is either to be shot or to commit suicide. Otherwise you can’t let of anymore.
Yes, I had this bug several times, last time today. It has been in the game for weeks.
I had this, it worked out good for me thought. I was on the tank mounted gun, I somehow ended up insde and it was all bugged. The tank kept moving and they couldn’t shoot me. I was crouched and I remember popping up and shooting a guy. Weird but helped me out in that situation
Since it lasts in the game during multiple patches now it would be great if they finally fix it in the near future. MG’s are pretty nice but getting stuck on them that often makes them unattractive.
I had this a few times too, but the cause is different for me.
If I throw a pack (ammo or med) just before I get onto the MG I get stuck on the MG and cant fire and cant unmount. Only solution is to suicide. Ive had it happen with arty and sparky.
Your are right, I’m able to reproduce this bug whenever I’m on the MG, I give Ammo and go back at the MG immediately after.
So would be great to get this fixed 
I have the same issue, with Arty.
I noticed that’s happen on chapel - 4 times -
But after that, other bugs appear : my merc was stuck on the same place, but along the wall at the end of the chapel street.
I finally push K key but the respawn was bugged too (see another player screen during one minute)
and finally the last time, I see this on my screen after the respawn bug :
link ot the screenshot
http://imgur.com/DcQTAnY
the merc have got strange twisted hands, I cant move and the merc turns very quickly when I move my mouse.
30 secondes after, I was able to move again.