The M1917 Enfield bolt locking lugs had a 4 degree helical angle with matching angles on the receiver lug seats (the technical term is interrupted threading). This means that final head space is not achieved until the bolt handle is turned down all the way. The design probably used helical locking lugs to allow for chambering imperfect or dirty ammunition and that the closing cam action is distributed over the entire mating faces of both bolt and receiver lugs. This is one reason the bolt closure feels smooth. The angled lugs had no tendency to unwind with chamber pressure since the "angle of repose" of smooth, lubricated steel surfaces is approximately 8 degrees. One advantage was that when the bolt handle was turned up the lugs cleared each other immediately so full effort was applied to the extraction cam. The trigger had a mechanical interlock to prevent firing unless the bolt was fully locked. The location of the safety on the right rear of the receiver has also been copied by most sporting bolt-action rifles since, as it falls easily under the firer's thumb. The trigger pull is ≥ . One notable design flaw was the leaf spring that powered the ejector, which could break off and render the ejector inoperable. A combat-expedient repair method was to slip a bit of rubber under the bolt stop spring. A redesigned ejector, incorporating a small coil spring in place of the fragile leaf spring, was developed and can be fitted to the M1917 to remedy this issue.