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How about with a Ruger 10/22 that has the hammer and bolt modified and a new sear made?
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In a conversion like you described, most in the past have registered the sear, although a better choice would have been to register the trigger frame, which also probably required modifications, because it was the largest and most durable piece of the conversion parts set.
Think of an AR-15 DIAS - the body is the largest and most durable part, even though the sear is also different and a distinct, new part from a GI sear, so absent a BATFE directive, either could have wound up being the marked, registered part
M2 carbine conversion kits could register any one of the seven conversion parts, but most used the disconnector, which became the "machinegun", and the other parts could be replaced. Original M2-marked carbine receivers were considered RRs, and all the parts were just replaceable spare parts for M2 RR owners.