This tutorial will explain step by step how modify your stock SKS bolt and allow you to insert and remove a detachable magazine without having the gun out of battery (bolt and carrier in the rear position). This modification will not have any negative effects on your rifle except diminishing the value of a matching numbers gun. If your numbers match or you want this done professionally just buy a Tapco replacement bolt without the steps and be sure to have it head-spaced before you shoot. The SKS D/M models were manufactured with these modified bolts. If your unsure on how much to take off on the side of the bolt with the chamfer, I posted a pic down below.
First step is to break down your gun to remove the bolt. Take the top cover off, remove spring and pull the bolt carrier backwards and up. With the gun pointing downward the bolt should be hanging under the carrier. Now that you have your bolt removed go ahead and remove the firing pin by punching out the retaining pin, then remove the extractor. I left my extractor spring in because it was stuck but if you can remove yours then go ahead. Removing these parts off the bolt will save you money if you happen to slip and hit them. Now's a good time to clean your firing pin and it's home, the hole it resides in. Cleaning this will prevent slam fires. There's really no need to buy a spring loaded firing pin if you keep yours clean.
Now here's where you have several options as to how you remove the lips or ledges on the bolt. You can use either a angle grinder secured with clamps to your work table or a dremel with the bolt secured. If you use the angle grinder, have the side handle installed so you can rest your hands on it while removing the material. Also I would buy a new ľin. wide fine and course grinding wheel or stone so you have a nice flat edge on it. If you use a dremel, set up something to rest your hands on so you can be comfortable and not shaky. If your going with a grinder use the 1/4in edge of the wheel.
Start with the course wheel or stone and remove the ledges at the bolt face end first. Make sure you keep an eye on the angle of the material left over. You'll be surprised how easy it is to get a bad angle going on your cut. Also don't get the bolt too hot when grinding. This will change the properties of the metal and possibly weaken it. If it gets hot then just take a break and come back to it in about 5 minutes. Work your way back leaving about 1/32in. of material for you to finish with the fine wheel. A easy way to get a straight line when grinding is to move the bolt left to right or the dremel and keep going back and forth instead of grinding one area at a time. When you get to the rear of the bolt where the lip disappears you'll want to make sure you remove the ledge entirely but don't cut to far into the area without a lip. If you don't remove that lip entirely at the back your mags are going to catch on it.
Switch over to a fine wheel or stone when your done removing the majority of the lips. Once you have a nice flat smooth surface that's flush with the unaltered area you'll need to bevel the bottom edge with the fine wheel or file at a 45 degree angle just enough to remove that sharp edge. Don't worry about removing too much as long as that beveled surface is no wider than 3/32in. After that take the fine file and round the new edges you just created. And then run the file along the bottom of the bolt to be sure there is no burrs left over. You now have completed the mod. Just oil everything (don't forget the firing pins home) the way you like it and reinstall everything. You can polish the bolt up if you want but it's not necessary. A unpolished surface holds oil better than a polished one. Enjoy removing your mags without the hassle.