Not all Model 12's were created equal, here is a picture I posted in another forum showing early Model 12 bolts compared to one form a 121. Notice that beside the larger locking lug the 121 bolt has a round firing pin leaving the left side of the bolt solid, the early 12 bolts are milled away there for the flat firing pin undercutting the locking lug which makes them much weaker - around serial number 650,000 the Model 12 bolt was changed to a round firing pin which made them much stronger - those MAY handle HI-Speed fine, or at least better than the earlier ones. I have seen several bolts with the locking lug gone, but never one of the later ones with a round firing pin.
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In the background is an old Numrich catalog, they used to sell a kit with a modified Model 121 bolt for early the Model 12- these had the round firing pin but had the locking lug milled down to Model 12 size and the front of the bolt tapered.