The bolt in the suomi kits needs to be modified from its closed bolt/fixed firing pin to a closed bolt/ hammer or striker design. I sort of followed the directions shown in the shotgun news article SGN-9. I started by cleaning the bolt and removing the felt wad (if any) in the recoil spring hole. Next before you do anything else REMOVE THE EXTRACTOR before it gets ruined in the following steps. Do this by taking a small screwdriver about the same size as the width of the extractor and put it in the extractor groove. Then slowly push the extractor away from the boltface. When you can see the pin that retains the extractor then lever the extractor forward out of its pocket and out. Then I drove out the firing pin retaining pin located about 3/4" behind the front of the bolt. I drive them out from the top as it appears they were driven in from the bottem.
The bolts as they come are too hard to machine with high speed steel cutting tools, so like in the sgn-9 article I annealed my bolt. I use the "fires of hell" brush torch from harbor freight - Harbor Freight Tools - Quality Tools at the Lowest Prices . I got mine on sale for about $25 and they have a cheaper version without an ignitor that goes for $20 on sale. Hang the bolt by a small wire and heat it until its a dull red color, then let it air cool or throw it in the grill set on high and slowly bring down the temp. Then I drill the firing pin channel from the rear with a lengthened 11/64" bit to a depth of just behind or to the firing pin retaining pin hole. Now you can drive out the firing pin from the back with a lengthened 1/8" drift. It should come out easily since when you drilled it you also releived some of the pressure holding it in.
In order to use the original bolt you also have to make it so that an unmodified bolt cannot fit inside your gun. This is usually accomplished on tube guns by reducing the bolts diameter or installing a blocking rail in an original diameter receiver and milling a matching slot in the bolt. On this build I reduced the diameter of the bolt by turning it between centers until it slid freely inside my receiver tube. If you go with the 1.5"x.188 wall tubing like I did the bolt will need to be turned down to 1.115"-1.118" as shown by the two middle bolts in the picture of the 4 bolts. IIRC the prexis bolts need to be 1.170"-1.175". If you decide to cut a slot in the bolt a great spot for the slot is where the rear sight groove is in the bolt already. If you choose this route then just mill the slot the rest of the way to the rear of the bolt. I slightly deepened the one in the photo just for good measure. I did this to the top bolt in the bolt pictures.
I will add more bolt info soon on how to slot the bolt and drill it for the firing pin.