The 20 and 28 gauge actions are VERY similar and share many parts. I have test fit a 20 gauge stock onto a 28 gauge action and the fit is near perfect with one exception. The 28 gauge frame is 2mm narrower on the outside dimension than the 20 gauge frame so the wood is too wide but with enough work a 20 gauge stock can be fitted to a 28 gauge frame. What all this means is that a 20 gauge recoil pad or spacer will fit a 28 gauge stock. Following is the spacer you are looking for and yes they can be stacked.

Beretta Field Recoil Pad Spacer Kit

The Beretta Field Recoil Pad Spacer Kit fits on all the field and short stocks for the X-Trema guns. Learn more about the gun spacer kit here!

www.beretta.com

 

Half inch:
Beretta USA website. Field spacer

Beretta Field Recoil Pad Spacer Kit

The Beretta Field Recoil Pad Spacer Kit fits on all the field and short stocks for the X-Trema guns. Learn more about the gun spacer kit here!

www.beretta.com

One Inch:

Xtrema - A400 Stock Spacer 1" | Shotgun Accessories | Beretta

Check out the 1 inch Beretta Xtrema stock spacer for your A400 which fits on all Field stocks and short stock shotguns. Buy yours today.

www.beretta.com

Plus you can get recoil pads up to 1.1” All just screw on with no modification.

 

One thing to take note of is that with the 12 gauge 686 you have two different variants with two different stock dimensions for the stock pad length. The 686 Field uses a 5.03 length recoil pad and the 686 Sporting uses a 5.35 length recoil pad and spacer kits are available for both lengths.

Note the 686 Sporting has also occasionally been offered in 20 gauge and very rarely in 28 gauge so confirming that you have a Field Stock in these smaller bores is a good idea.

 

Just order some kick-eez grind to fit under spacers