I have actually fitted stocks to my 12 and 20 gauge 686's.
The first was an adjustable comb Beretta spare for the 682 Gold E purchased from Midwest Gun Warehouse. BTW, the image online of this stock was too dark to see the quality of the wood. When I actually received the stock it was a bit stunning EL quality at the least and very close to EELL quality. It was a bit of a bargain at 528.00 but it was old enough the checkering was hand cut and the comb hardware was Beretta's first design that proved a bit lacking. The hardware could be bumped off the set adjustment and has now been replaced with a Graco Kit.
The second stock was for an inletted 687 EELL stock also purchased from MGW, this time for just 238.00 because it was rather yellow and there aren't many 20 gauge EELL guns out there so it had been warehoused for many years and was deeply discounted. I took this stock, stripped it, filled the interior Field cavity to support a Graco Comb Kit and cut the stock for a Comb. The end result may be my prettiest stock. BTW, covered up the filler for the inletting with Burl Walnut veneer. Stain used was Red Mahagoney.
Anyhow concerning fitting, this is actually critical because wood will move depending on humidity and most stocks are constructed with the intention of it being fit to a specific gun because of that potential movement. In addition on the 686 the fit into the stock is basically a Wedge and if you don't get it right you can split the stock. On the 686 the recoil is taken up by the vertical face of the receiver where the stock bolt secures the stock. The radiused ends of the receiver should have just a tiny gap about the thickness of a piece of paper on top and bottom. You do NOT want recoil forces contacting the stock at these points because it will split the stock at the wrist. The transition between between the cheeks on the stock at the receiver should also be very slightly gapped, use the aluminum foil wrapper on a dove chocolate to check this area. After the stock is tightened that foil wrapper should slide out of that gap with a very slight drag but not enough to tear the foil. Yeah it is that fussy if you want it right. The wedge sides of the receiver should be a near perfect fit because you don't want the stock to "rattle" when shot. Ideally with the stock fully seated it should take a light Karate chop on the back of the stock to free it from the receiver. While you are working the stock into that fit you need to STOP as soon as you feel it starting to bind up, force it and you'll split the stock instantly. That stock should slide on easily until the very last bit. I didn't have any blacking when I did mine so I used a Sharpie pen to paint the sides of the receiver to indicate the high spots. Any sharpie rubbed onto the stock is an indicator where you need to remove material. BTW, the best tools to use are brand new small machinist files 4 to 6 inches long. I'll also note you will want at least one combination rounded/flat file, sometime the round side will allow you to remove material a bit faster on a stubborn high spot. Another tool I recommend is a half round woodworking gouge, it makes it much easier to shave wood out of the radius cuts if needed. As for the time budget, plan on at least 12 hours for your first stock. Because you will be doing many many tests runs to work in the sides of the wedge. BTW, getting the wedge right is FIRST for the fitting, you cannot check any of the other areas until you have that receiver fully seated. I'll also recommend you not tamper with the recoil face of the stock. Finally the wood will sit proud of the receiver by about 1/32 inch. Leave that alone, that extra wood is provided for future re-finishing of the stock.
BTW, I'm old enough to feel that life is too short to shoot with an ugly gun. Below is my 4 gun Skeet set, on the left is the 410/28 combo, the middle is the 20 and on the right is the 12 gauge.
