Re: Question for you gunsmiths , new bolt or modify old one
Little bit of a tangent to the original topic, but...
If the outside diameter of the locking lugs remained the same while you increased the body diameter on the bolt to .800 for example, yes you have essentially made one of the dimensions (lug depth) smaller that determines its shear strength. Increasing the lug diameter another .100 would compensate for the change in bolt diameter though.
An .800 diameter bolt could also have a wider (viewed with lugs vertical in lockup) lug though, because of its OD increase, and thus has increased strength over a .700 OD bolt.
A bolt's lug width is limited to 50% (180 degrees) of the bolt's circumferance due to the fact that it needs the exact same width cut through the lug abutment area, 2, 3, 9 lug design, makes no difference, neither has any advantage over the other in this area when this alone is viewed. However, this 50% lug contact area is further reduced by the closing cam ramps cut into the lug abutments or lugs themselves. This is where the two lug design is superior to all others with identical bolt diameter and extraction cam.
The 30 degree angle or whatever it happens to be on the extraction cam, and also the amount of rearward travel upon extraction dictates what the closing cam needs to be in order to follow it in time and decides how much less than the 50% is left available for lug contact surfaces. The number of closing cam ramps affects the amount of contact area left also.
The degree of rotation that the lugs ride the closing cam, and the number of lugs and cams cut dictate the remaining rotation left for lug contact then this determines lug and raceway width.
A two lug design may use up ~50 degrees (25x2) of the 360* surface as the bolt rides down the closing cam ramps.
A three lug design may use up ~75* on the other hand because three cuts are done for ramps not two. The available degrees left over for lug abutments and raceways cutouts would be the remainder of the 50* or 75*, or 310 and 285 degrees respectively. The 310* would be divided 4 ways for a two lug and give 77.5 degrees of lug width at the root, and the three lug is divided 6 ways and that 285* gives 47.5* per lug.
So in the end, the three lug design gives up nearly 10% in lug contact area to the two lug design. In other words, it's not as strong.