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  1. #1
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    Default Cock-on-opening problem with Mossberg 185KB 20-gauge bolt-action shotgun

    Good afternoon, all.

    First of all, looking at the back of the receiver and bolt, as though looking through the barrel, a peg protrudes from the striker southwest, and runs back and forth in a long groove inside the receiver. At the back of the sleeve-like rear half of the bolt in which the striker slides, there is a right-triangle-shaped cut in said "sleeve" facing west when the bolt is closed. Upon firing, the sear lets go of the peg, which slides down its groove in the receiver above the "base" of the right triangle cut. As the bolt handle is lifted to open the action, rotating the back half "sleeve" part of the bolt, the right triangle cut, being part of the "sleeve", cams the peg (and therefore the striker) along its "hypotenuse" side back to catch in slight notch where the "hypotenuse" side meets the back edge of the "sleeve". (This last side of the triangle is open). The triangle cut is now facing roughly south; the bolt handle is north. As the bolt is pulled back, the peg (and hence the cocked striker) is held in that notch. As the bolt goes home, the peg enters the back of its groove, and is held back at the groove's end by the sear when the bolt is fully home. The bolt "sleeve" part is now just a hair further forward, leaving the notch no longer engaging the peg. The bolt handle is turned closed, going north to east, moving the right triangle likewise from south to west to leave the peg resting above the back end of the right triangle's "base" side. That is how the action functions.

    Moving on, the gun now mis-cycles in these two ways: Whilst shooting some trap (not my hobby; was patronizing a fire company's fundraiser), after perhaps thirty or so shells, the striker did not cock as I cycled the bolt, leaving the firing pin protruding from the face of the bolt; this made it impossible to fully close the bolt and turn the handle down (the resistance felt mushy, no doubt from the striker spring). I fixed that after a look at the bolt (the peg, the right triangle cut and its notch, are all open and visible with the bolt either in or out of the gun), turning the peg back into the notch by hand (the striker does not cock right if the bolt is put back in the gun uncocked). Several shells later, the problem happened again, and I choose not to finish the last several of my fifty shells for the shoot. Some days later at home, I completely disassembled the bolt for the first time since getting the gun, and cleaned and oiled it as best as I could. I did oil both outside of the bolt and its inside, where the striker slides (No oil on the bolt face, of course). The firing pin spring rests its back against the solid front of the back half of the bolt; its front is held in by a nut screwed onto some threads further forward on the firing pin. The back of the nut has four protruding rectangular lugs, like castle crenellations. I did "torsion" the spring with these (a matter of fact, not something I was doing for any reason other than it seemed how it might have to be) when screwing the nut back on against the spring's resistance. If it matters any, the trap shoot shells were seven-eighths ounce trap loads, I believe the type with that cheap plated steel or aluminum low brass; the gun is a push feed, and I was dropping a shell into the barrel and closing the bolt on it.

    The second sort of mis-cycling came up later at home; whilst doing some rapid dry cycling and dry-firing (both with and without dummy rounds/snap caps [A-Zoom aluminum ones]), the peg, which is supposed to stay in the same line (pointing southwest) all through the action cycle, would somehow wind up, from time to time, [I]behind[I] the back of the bolt and receiver, anywhere from south to east, at the end of the cycle. When I pulled the bolt back, the peg would still be behind the back of the bolt (the "sleeve") and would stay there, until I manually corrected it again.

    Now, what is going on, and how can it be fixed and prevented? Was the initial failure-to-cock event at the shoot simply due to old gummified oil or fouling inside the bolt (that was what an older man there hypothesized)? Were those low-power (and possibly cheapo-type) trap shells sending fouling back past the shell into the insides of the bolt or action? Does the second problem come from having put oil inside the bolt (which I later read is a no-no, because it gums up eventually or as the gun heats with firing, and attracts dirt, dust, and fouling; I tried to clean it off later)? Did the apparent torquing of the firing pin spring on reassembly make the striker rotate on cycling, permitting the peg to travel to other compass points than the southwest? Did I screw the nut too far, making the striker able to go back farther than necessary, and allowing the peg to wind up behind the receiver and bolt? (The second problem only happens when cranking the bolt rapidly, as in actual use; not when cranking deliberately).

    On another thought, I rather doubt that these specific malfunctions would happen were the action cock-on-closing.

  2. #2
    Join Date
    Jan 2019
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    (Chester County)
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    Default Re: Cock-on-opening problem with Mossberg 185KB 20-gauge bolt-action shotgun

    I think that I have solved the problem. I screwed the tighter tighter (and therefore further) down the firing pin. The peg no longer jumps behind the receiver whilst cycling, and seems to be staying deeper in its groove at the beginning and end of the cycles.

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