I'd rather it were [url=https://en.wikipedia.org/wiki/Ordnance_QF_6-pounder#Molins_gun]a 6 pdr autloading gun[/url]...

Here's an 82mm version, [url=https://www.youtube.com/watch?v=X2UZgVbejns]it's actually a low-pressure mortar[/url].

Shields, er... any should capable of stopping serious firepower is going to be unwieldy.
-Real life ballistic-shields generally weigh as or twice as much as the rifle generating the threat.

At most would be something of a super-pershing's "let's take the front-plate of a Tiger II and bolt it to the front of a Pershing" story.
-And in that case, it's usually an all-or-nothing add-on, geared towards protecting the crew.
--Although it won't go into stats anytime soon, RP-wise Marlene is going to pile sandbags into her pit, because she knows what a sniper with an ATGM can do.