It dosnt matter which gun/rifle ammo I pickup but it adds itself to my .44 as per screenshot below. I havnt collected any .44 ammo in a long while and I am not going to drop the ammo to see what happens but just doing the right thing and reporting it 
That’s the same bug that I got, where my AG5 started firing grenade rounds. It’s related to the vanishing ammo bug.
And you with all that .44 and still frightened of Tanks, ye big Jessie!!
Hahahahaha quite true Bootie
Please tell me now how to reproduce it 
That happened to me a patch or 2 ago. 5.56 ammo bugged into my Carl Gustaf and I had over 2k dual sploders. I killed 90% of my tanks that way at the marshes. But didn’t get much loot back than so it evened it self out.
Whats a Carl Gustav? apart from being the king of Sweden
It’s actually the name of the Grenade Launcher, Carl Gustaf M/48.
Dropped some of my .44 to make room for other ammo and my .44 now reads as it should. I tried picking it up to reproduce the bug but no, just whats in my arsenal is showing now, nice while it lasted though.
My .50 cal did that last night where I had an eternal 3 rounds available (well for about 10 minutes anyway)
Gave me enough to take a tank down from a distance
Your have been lucky !