Mossberg 940 JM Pro: Common Problems & Solutions

by Jhon Lennon 49 views

Hey guys! So, you're looking at the Mossberg 940 JM Pro, huh? It's a slick shotgun, no doubt about it. Designed with input from Jerry Miculek himself, this thing is built for speed and performance in competitive shooting. But like any piece of gear, especially one pushed to its limits, it can sometimes throw a curveball. Today, we're diving deep into the Mossberg 940 JM Pro problems that shooters sometimes encounter, and more importantly, how to fix them. We want you to get the most out of your scattergun, so let's get this sorted.

Understanding the Mossberg 940 JM Pro

Before we jump into the nitty-gritty of issues, let's take a sec to appreciate what the Mossberg 940 JM Pro is all about. This gas-operated semi-automatic shotgun is a step up from previous Mossberg models, featuring a self-regulating gas system that can handle a wide range of loads, from light target loads to heavier defensive rounds. The "JM Pro" designation means it's kitted out with features specifically for competitive shooters, like an extended magazine tube for increased capacity, an oversized bolt release and safety, and a beveled loading port for faster reloads. It's designed for action, and when it works perfectly, it's a joy to shoot. However, pushing the envelope in speed and reliability means sometimes you might run into hiccups. The good news is that most of these Mossberg 940 JM Pro problems are relatively minor and often stem from a few common areas: break-in period, ammunition choice, and maintenance. Understanding these nuances will help you keep your 940 JM Pro running smoother than a greased weasel.

Break-In Period and Initial Issues

Alright, first things first, let's talk about the break-in period. Most firearms, especially semi-automatics, benefit from a proper break-in. This means cycling a good number of rounds through the gun to allow the moving parts to wear in and mate up perfectly. For the Mossberg 940 JM Pro, this is particularly true. Some shooters report initial issues like failures to feed, eject, or short-stroking right out of the box. This is often not a sign of a major defect but rather the gun needing to be "broken in." Think of it like breaking in a new pair of boots – they might be stiff at first, but with some use, they become comfortable and perform perfectly. To properly break in your 940 JM Pro, we recommend firing at least 100-200 rounds of quality ammunition. Don't skimp here. Use a reputable brand and a moderate load. During this phase, pay close attention to how the gun functions. If you experience a failure, try to note whether it was a failure to feed (round not getting into the chamber), a failure to eject (spent casing not clearing the receiver), or a short-stroke (bolt not cycling fully to the rear). This information is crucial.

Why is the break-in important? It allows the gas system components, bolt carrier, and receiver rails to polish themselves. Minor manufacturing imperfections that might cause friction or resistance are smoothed out, leading to a more reliable cycle. It's also a good time to give the gun a light clean and lubrication after the break-in period. Over-lubrication or under-lubrication can also be a culprit, so finding that sweet spot is key. Some guys prefer specific lubricants, so experimenting with a good quality CLP (Cleaner, Lubricant, Protectant) or a dedicated gun oil can also make a difference. Don't be discouraged if you have a few issues early on; it's often just the gun telling you it's ready for some action and a bit of attention. Proper break-in is the foundation for a reliable Mossberg 940 JM Pro, so don't skip this crucial step.

Common Mossberg 940 JM Pro Problems and Fixes

Let's get down to the brass tacks, guys. We've heard about a few common Mossberg 940 JM Pro problems, and luckily, most have straightforward solutions.

1. Failure to Feed/Chambering Issues

This is probably the most frequently reported issue. You pull the trigger, hear a click, but there's no round chambered, or the round is jammed halfway.

  • Cause: Often related to the break-in period, as discussed. Can also be caused by weak ammunition, a dirty gas system, or improper magazine feeding. Sometimes, the follower in the magazine tube can hang up.
  • Fix:
    • Complete Break-In: Ensure you've run at least 100-200 rounds through the gun. This is your first and best solution.
    • Ammunition: Try different brands and types of ammunition. The 940 JM Pro is designed to be versatile, but some very light target loads might not have enough power to cycle the action fully, especially during the break-in. Stick to reliable 2 ¾" shells from reputable manufacturers, typically in the 1150-1300 fps range.
    • Clean Gas System: The gas system is the heart of this shotgun. If it's dirty, it won't cycle properly. Disassemble the barrel and forend, remove the gas pistons and rings, and clean them thoroughly with a good solvent. Ensure the gas ports in the barrel are clear. A pipe cleaner or a small brush works wonders here. Reassemble and lightly lubricate.
    • Magazine Follower: Check the magazine follower for any burrs or rough spots. It should slide smoothly. Some aftermarket followers are available if yours seems problematic.
    • Proper Loading Technique: Ensure you are loading rounds firmly into the magazine tube and that they seat properly against the follower. Sometimes, a poorly seated round can cause feeding issues.

2. Short Stroking (Failure to Cycle Fully)

This is when the bolt doesn't travel all the way to the rear, preventing the spent casing from ejecting and the bolt from locking open or picking up a new round. It's like the gun just can't