Sanding or grinding a radius onto the bottom rear of the bolt is a popular performance enhancement for the 10/22. Properly done, the radius creates a ramp effect which reduces the force required for the bolt to cock the hammer. (Remember our simple machines from elementary school science?) This can help the bolt cycle fully and prevent stovepipes from short-cycling especially with standard velocity or low-velocity “quiet” ammunition.
Here is a comparison of an OEM bolt and one that has been radiused:
The video below shows what happens:
A question came up recently as to whether this modification could cause the bolt to open prematurely. The simple answer is “no.” As the video shows, once the hammer hits the firing pin, it rests against both the upper part of the rear face of the bolt and the lower edge of that face, below the cutout for the bolt stop pin. It is important that both the recoil spring and the hammer spring work together to resist the bolt’s opening immediately after the shot. The radius does raise the bolt’s point of contact with the hammer away from the hammer’s axis of rotation – which increases the leverage the bolt has on the hammer. The initial resistance to blowback gas pressure is therefore slightly reduced from a that of an unmodified bolt. However, the difference is small enough that the bolt does not open prematurely. Once the bolt is pushed back a little, the rounded section glides easily over the hammer.
It is critical to leave that small edge on the lower part of the bolt’s rear face. It is needed to prevent the hammer from hitting the firing pin if the bolt is not fully in battery. The photos below from The 10/22 Companion show how this works:
If the bolt is back even a few hundredths, the lower edge catches the hammer. If the radius job is done wrongly and this edge is removed, the hammer could hit the firing pin while the bolt is slightly open.
That lower rear face is also important for keeping the correct resistance to gas pressure to maintain the correct timing of the action. You do not want the bolt to open too far while pressure remains high in the barrel, or the case could rupture. If the only point of contact were above the stop pin cutout, the leverage on the hammer would probably be too great, making the bolt cycle too fast. But I’m not going to sacrifice a good rifle to test that hypothesis.
So just as with trigger work, a bolt upgrade done right improves the rifle. One done wrong means buying new parts and starting over.