Mossberg 51M(a) Sights

Discussion in '.22-Rimfire Forum' started by Rockgineer, Oct 4, 2010.

  1. Rockgineer

    Rockgineer New Member

    Oct 4, 2010
    I picked up a Mossberg 51M(a) this weekend and it is missing the rear peep disk and the windage adjusting screw that holds the disk.

    I have done some research and found that the gun came origianlly with the S100 peep site. My gun appears to have the S104 peep site. I also saw that the 51M had some variations in the sites but I have found no record of what sites were used when. Is it possible that my gun came equiped with the S104 site from the factory or was the S104 installed by some one in the past 70 years?

    Also I also found that the front site should be a No. 1A Ramp, while it is very similar to the ones found at this link it does not match it exactly. The front site on my gun has a much narrower site than the one in the picture.

    I will try to post more pictures later today.


    Thanks for you help,

  2. Jay

    Jay Active Member

    Mar 26, 2003
    Go here and get in touch with Vic Haviln. Vic pretty much wrote the book (More Gun For Your Money) about Mossbergs. He has parts, too.

  3. Rockgineer

    Rockgineer New Member

    Oct 4, 2010
    Thanks, just sent them an email.
  4. FW!

    FW! New Member

    Jun 21, 2009
    Auburn Hills, MI
  5. S-100 was 1st put on the 51M(b) models.

    The 51M(a) came with the S-104 (#4) Like the original 51M...

    Also the front 1A-Ramp only came on the original model 51, the S-101 was on the 51M, 51M(a) and 51M(b)
    Last edited: Oct 4, 2010
  6. Rockgineer

    Rockgineer New Member

    Oct 4, 2010
    Great thanks for the info guys. I have been able to find an aperture but the windage adjustment screw is harder to find.

    Did this gun come with the adjustable aperture?

    It looks like I might be able to get a complete assembly from ebay but i'd prefer to find just the parts if i can save some money.
    Last edited: Oct 4, 2010
  7. Did not come with the adjustable 4A aperture...

    Sight adjustment screws are pretty hard to find... seems like everyone has what you have... with the same missing parts...:(
  8. Rockgineer

    Rockgineer New Member

    Oct 4, 2010
    Thanks to everyones help I actually think I managed to find everything I need!

    I found the aperture and adjustment screws for the #4 peep site at

    I found the stock extension screw at

    and I found the Cocking Lever and Cocking Lever Sleeve at Jack First Distributors (605-343-9544)

    I am in the process of finding the Mainspring Cap Assembly and think I found a source.
  9. Rockgineer

    Rockgineer New Member

    Oct 4, 2010
    So even though I need a new main spring assembly because the dentent broke off (see picture below to see problem) I have put a few rounds through the gun. It does fine single shot however there are a few problems when trying to fire with more than one round in the mag. I am new to troubleshooting gun problems and would like to know if these problems are or may be a result of the bad main spring assembly or should i repair or order more parts while I wait for my new main spring to arrive.

    When loading the first round from the mag the round enters the chamber perfectly every time. The second round however often gets smashed or part of the bullet sliced off by the edge of the chamber (see picture below)

    The rounds I am currently using are hollow point so the tip is very blut. That is a suspect in the cause of the problem and I will try some other ammo in the next few days.

    Is there anything else I should check?
  10. gunplumber

    gunplumber New Member

    Apr 14, 2010
    From the picture I would say that the carrier is worn at the "stop" and lifting the round a bit too high. Marlin 60s have a bad habit of this also, after much use. If the situation occurs with several types of ammo, I would look there first.
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