The Mkb42H ingame is modelled with a 10rnd magazine. A 15 round magazine never existed for the Mkb42 so the gun currently stores an extra 5 bullets from a pocket dimension.
The reload speed should be increased to compensate.
IRL discussion and proof of the 10rnd magazines
I think the damage should be nerfed and go to BR1 if has 10 rounds, is very few ammo in the magazine
Honestly I think even with 15 rounds it deserves to be BR2, so if they lower it even further it might even need to be dropped to BR1
If they changed the weapon from 15 to 10, then drop it to BR2.
Theres already a better option at BR2, regardless being event / random box reward.
with 10 rounds even BR1 seems quite high for it.
Agreed, I don’t even use it as it is, as you say, there are better options. 
Good way to make the weapon even worse than it currently is.
Use this as reason to add 25 rd scotti.
Yeah, with the Scotti I really don’t understand why the Dev’s gave us a good weapon, but felt the need to “once again” short change us with the small magazine size. It should have a 25 round magazine.
The MKb as currently is not a good weapon, and it would be a total pile of crap with a ten round magazine. Currently as it is, there is zero reason to even use the BR3 Mkb as an assaulter weapon with it’s current 15 rounds as you are just handicapping yourself when you can use an smg with twice the ammo. IMOP the Mkb should remain as it is, but moved so it can be used by a rifleman or engineer, otherwise it is not going to get much use. I will only use it in my own custom two player matches since it is just a handicap when using against other players.