Quoted:
Will a 5.56mm extractor grab a 7.62 rim enough for reliable extraction? Any idea what metallurgical difference there would be in the 7.62 bolts that lets their lugs break like that?
The case is larger in 7.62 and so the extractor has to be cut deeper. The radius' are different. However, like AR15performance is doing, it can be made to work. I haven't seen one of their bolts. But tinkering with a 5.56 extractor in a 7.62 bolt I had problems with the extractors being too tight with any spring I tried. I'm thinking if you milled a shallow hole in the bolt to recess the spring a little it would work. Again, just guesses.
Regular 5.56 bolt break under heavy use. With the larger case of the 7.62x39, the sides of the bolt are much thinner surrounding the head of the case so the situation is much worse. Making engineering changes will help such as doing stress relief cuts on the lugs like LMT has done. Changes to stronger material also helps. LMT has done BOTH. I have no personal experience with any other 7.62 bolt that has both advantages which is why I stick to using those. I run mine very hard. All full auto from a 7.5" barrels using 75 round drums.
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