Well this one is simple suggestion.
There was a discussion about there being a 25 round magazine for Johnson LMG through some more research I found that there were longer magazines , 25 and 30 though none were issued to combat units only 20 rounds were issued.
Euthy mentioned that changing the model is out of the question
So where the 25 round that I am asking comes from you might ask?
Well a simple 30 second video from Ian will explain what I try to say better
My initial source was from a collectors guide book which in that book it mentions that you can add extra 5 rounds using a charger/ clip or by just inserting five rounds one by one.
Also another note Johnson LMG can be fully reloaded via clips so , you don’t have to remove the empty magazine just reload it via stripper clip from other side.
So if you couldn’t understand you spawn in with 25 round and when you spent it all , your soldier will change magazine but also insert a five round stripper clip to other side , obviously I would also suggest that the last part should be stoppable, if you are in a sticky situation you don’t want to spend extra seconds just to reload a gun.
Anyways that’s from me.
I use the BAR1918A2, the Browning1919A6 and Stinger, but have never incorporated the Johnson. 
Actually to be honest, I really only use the BAR A2, even in BR5 but wish they would add a 40 round mag for a level five BAR.