into its blocking position and hold it there when selector 26 is in positions number one and two as described above with reference to the alternative embodiment shown in fig9 18 and in particular with reference first to fig1 and 18 it is seen that selector unit 26a like selector 26 is rotatably mounted to be turned to and held in three discrete positions determined by detent 70 mounted in a detent hole 71 in receiver 12 detent spring 72 urges detent 70 into recesses 73 74 and 75 in selector unit 26a selector unit 26a includes thumb piece 77 cylindrical body portion 78 including configured foot portion 79 foot portion 79 carries notch 81 providing a configuration for engagement and disengagement with the trigger which will be explained trigger 82 pivotable about pivot axis 83 includes finger piece 84 sear engaging upper trigger arm 86 and trigger extension 87 including extension block 88 also shown in fig1 and 18 are trigger side pieces 89 91 sear 92 sear spring 93 sear pin 94 selector cover plate 95 and rifle sight 96 turning to fig1 13 and 14 it is seen that when selector unit 26a is in position a fig1 and 14 configured foot portion 79 of selector 26a is positioned to accommodate extension block 88 thus permitting.