[Jmreiser]

The challenge is to secure the weight so it doesn't become a slide-hammer, amplifying the recoil.

The Graco weight, is that the one on their site listed for the Beretta 390? That looks perfect, they offer 4 choices of weight, and it threads on securely. And yes, I see the last part of the description says it also fits the A400.

 

[Jmreiser]

I've had no issues with any of the Graco weights (5, 7, and 10) in my MultiTarget. Fits nicely. I used the standard lock washer that was there originally before the weight went on. The shaft is an M8 thread, that is a pretty strong stud. I would recommend not trying to pack stuff in there, it will try to hammer the butt pad, amplifying the recoil.

 

[Jmreiser]

Graco's website says its weights are "designed for Beretta 390, 391, and A400 models".

So you might be able to use your 391 weight(s).