1. The locking lug helix (surface contour) of the Chinese bolt does not agree/match the locking lug helix of the receiver. Thus, the bolt locking lug surfaces only partially contact the receiver locking lug surfaces. As you fire the rifle, the bolt will "collapse" back onto the locking lugs of the receiver until "enough" surface contact is engaged to stop the rearward force caused by the 50,000 PSI or so that is produced on each firing. As the bolt moves rearward, headspace is lost. Often/mostly/always massive headspace loss occurs. I have seen Chinese bolts that close & move back & forth with a field gauge. I have witnessed headspace readings estimated (no gauge long enough to be certain) at greater than 20 thous. beyond Go. G.I. bolt's will also "collapse" when receiver locking lugs are not correct (commercial receivers sometimes have poor locking lug surfaces which is why we at Fulton Armory always hand lap every bolt to every commercial receiver to ensure excellent engagement before we install/set headspace), though they move rearward more slowly due to better heat treat & steel. The fact that the Chinese bolts are very soft, amplifies the problem. BTW, the locking lug helix of the Chinese receiver does agree/match the helix of the G.I. M14 bolts quite nicely. I have never had to hand lap a G.I. bolt for more than say, 15-20 minutes to get a beautiful engagement. It's quite odd. It's as though someone ground on the Chinese bolt lugs with a handtool.
2. The Chinese bolt is "too long" as it relates to the firing pin bridge of the receiver, which retracts the firing pin on loading. As the Chinese bolt moves reward, the firing pin tail also moves reward. So much so that the firing pin bridge does not effectively/at all retract the firing pin! I have seen Chinese rifles where the firing pin did not retract at all. This is quite dangerous. Add to this bolt "geometry" problem the fact that the trigger & sears, & hammers are also way too soft (hammer fall will occur) one can see the train wreck ahead.
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