While the name "Mossberg" typically conjures up images of a twelve-gauge shotgun -- at least if you ask a shooting enthusiast -- Mossberg & Sons actually manufactures a wide variety of long guns. The Mossberg model 151m, for instance, is an older-style auto-loading rifle chambered for the .22LR round. As with any firearm, it is important that you know how to disassemble the 151m for maintenance and repair if needed. Because it is an older-style rifle, the takedown process is slightly more involved than it would be with more modern firearms.
Remove the magazine from the rifle and pull the bolt back to eject any live round that may be in the firing chamber. This should always be your first step when you are working on a firearm.
Loosen and remove all the screws on the underside of the rifle with flat and Phillips screwdrivers, starting with the screw nearest to the muzzle of the weapon and ending with the large screw just forward of the trigger guard.
Lift the barrel and receiver assembly off the stock and set the stock aside.
Ensure that the striker is in the "Fired" position, then unscrew the endcap at the rear of the barrel assembly and set it aside.
Pull the bolt backward in the assembly until the bolt handle lines up with the larger opening at the back of the receiver, then pull the bolt handle out and away from the receiver.
Pull the bolt assembly out the back end of the receiver.
Drift the cross-pin in the bolt -- located immediately behind the bolt handle hole -- using a small punch, until it releases the pressure on both the striker spring and the bolt spring. Be careful here, as both springs are under pressure and may come out with force if you are not prepared.
Drift out the vertical pin on the right side of the bolt, near the back, again using a small punch. This will allow you to lift the extractor and coil spring off of the bolt assembly.
Tap the firing pin out the front of the bolt assembly using a small punch and -- if necessary -- a small hammer.