The MP18 is just common sense,it’s really just an MP34 with a larger looking magazine.
The StuG 3A however,might need convincing to lower.there are multiple points I will touch on to justify this
The armour-
on paper the armour may seem great,but in reality it’s not,as it has some fairly large weak points and has similar armour values of other tanks at BR1 and weaker then BR2.if we compare the values,the panzer 3n has the same armour values as well as 20mms extra on the upper front plate as well,while the normal panzer 3J has the same armour profile but not such a large weak spot.the T70 has only 35mms,but due to the high angle,it is almost always 63mms,not to mention the T50 at BR2.any allied or soviet tank gun can easily pen this,even the “thick” sections can be easily penned from normal guns.
The gun-
While the gun may seem out of place for being too powerful for BR1,there are limiting factors that could justify this.First off,a large point is the obvious casemate design,which makes any worse,due to having to turn the entire veichle to fight,making urban fights near impossible.Another very large reason is the very limited ammunition,as it has only HALF of other short 75mm wielding tanks,and another half of that is spent on AP shells,so you won’t be able to continually lob shells forever like in other tanks.unless it has better armour to match it (in this case,it doesn’t),then the point of the TD being the same BR of tanks with the same gun has fallen through.
Lack of MG
This one is really simple,MGs are really important when there are always tons of infantry around,and this lacks it,whereas almost all other veichles have one.
Summary
While the gun would be really good for the BR,this veichle is limited in just about every other way,it’s slow,the armour is subpar,casemate,no MG,and heavily limited ammunition.for these reasons,I think it would be fine at BR1.
. I switched the squad ammo pouch to being able to carry a second weapon, armed them with ZK 383’s and use them in level 3 with the M18 as the (I hope I am not desperate enough) to have to use it backup gun. 
Other then that, that squad had been collecting dust for over two years as I hated the MP18.
