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Building a 3-Gun 930 from a Field Gun


Not every 3-Gun 930 begins life as a JM Pro. Our first Mossberg 930 competition guns were built from the SPX or Turkey models, long before the JM Pro came on the scene. In fact, we still run the SPX for night matches and have a Turkey model pulling duty for the occasional match.

In an ongoing series of work being done to convert a 930 Snow Goose for 3-Gun, we will look at some issues that can crop up with adopting a different model than the JM Pro for competition.

While legal (if not practical) out of the box for Open, the 930 Snow Goose model needs some changes to run in Limited or Tactical Optics.

Before the JM Pro, there weren't any 930 models with longer than an 18" barrel that weren't also ported. This is an instant kick to Open in most 3-Gun rule sets. The common solution was to cut 4"-6" from the end of the barrel to eliminate the ports and re-thread the gun for chokes. This can create a problem on a gas shotgun.

A 28" barrel on a Mossberg 930 has different sized gas ports than a 22" model and these cut barrels would sometimes create cycling issues. The reason is simple, but often overlooked. The dwell time has changed and the amount of gas available for the gun to cycle has been reduced as a result.

It was common thought that the cure was to run hotter loads. Unfortunately, a 'hotter' load is sometimes only a faster load, with the same powder charge and lighter shot weight, so the actual dwell time is reduced even more.

It is important to have properly sized gas ports in relation to the barrel. Keep this in mind if you go the cut and re-thread route on a longer barreled Mossberg 930.


For this installment, we have cut a spare barrel for this gun down to 22". While 24" is more common and possible to do after fully eliminating the ports, this gun will be run in Limited (or Factory) with its 8-round magazine tube restriction. This should end up as a compact, easily maneuverable 930.

Stay tuned!


I'm Your Huckleberry
Staff member
Global Moderator
I have that same reamer/tap set. It really is worth the investment if you foresee yourself threading 3 or more barrels.


Today we look at improving a rough factory chamber with a couple of inexpensive chamber hones and making sure that we don't have any of our gun's finish gumming up the gas works.


Sticking shells on extraction are often caused by thin hulled shells (and their aluminum bases) expanding and grabbing on to a rough chamber on the way out. If you can easily deform a shell's hull by pinching it between your thumb and index finger, the hull and base are likely going to contribute to cycling issues in ANY shotgun, pump OR autoloader. In the first picture below you can even see a factory ding in the shell base that can contribute to problematic chambering in the first place. Inexpensive shells like these Winchester Universals are a great value for extended practice sessions, but it is worth function testing, patterning and stepping up to a higher quality shell for match use.


The most common 'user induced' malfunctions we see with Mossberg 930s are with those that have had custom finishing work done. Many refinishers (and factory applicators) don't properly mask the gas system (piston, cylinder, magazine tube) and the resulting overspray (or full intended coating) can turn a gas gun into a single shot. It is also important to keep these coatings (Duracoat, Cerakote, Krylon) off of the bolt, carrier (slide assembly) and the internal rail cuts in the receiver. These finishes change tolerances and add friction to the gas system, which is often enough to cause cycling issues.


Our Snow Goose model came with a generous coating of camo substrate overspray inside the gas cylinder that needed some work to remove. Rather than risk changing the tolerances of the cylinder, we chose the chemical stripping approach and kept the friction to a minimum.

The full chamber smoothing process itself is beyond the scope of a forum post, but something to consider for any shotgun that is expected to cycle these shells reliably. Beyond following the instructions provided with a set of chamber hones, taking the polishing even finer through use of oiled #000 then #0000 synthetic steel wool (wrapped around a bore brush) will go a long way to keep these inexpensive shells cycling smoothly.

Up next, we'll get into the loading port and forearm modification. While many of you are probably picturing the typical butchering of the port and finish for the sake of quicker loading, we'll actually be doing everything we can to retain the factory look while still making the gun friendly to traditional 'weak hand' and twins loading.


We realized we hadn't posted an 'after' picture of the cylinder after opening the gas ports and removing the overspray…


Our next post for the project will cover the loading port and forearm modifications.


Since the gun will be used primarily for local matches by a traditional weak hand loader (that is working their way into the twins method), we will be making modest modifications to the loading port and forearm, keeping the gun looking as factory as possible.

We began by moving the end of the port forward and profiling it a bit more 'thumb-shaped' to enable easier loading. Being able to keep the thumb straight while clicking a round past the shell stop is as far as we needed to go.


This was done with a CNC 3-axis mill and a toolpath file for the process. Most loading port work tends to focus on external access, while internal tapering between the end of the port and entry to the magazine tube is actually more critical for smooth loading. This was done by hand with a Dremel.


The forearm was also CNC profiled with our Twins Cut. This service (along with our Quad Cut) is currently available on the OR3GUN web site under the Services tab:


Once the CNC process on the port and forearm was finished, these areas were hand sanded and masked for inclusion with a larger Duracoat job.


Once all parts were completed and finished, the gun was assembled with our Cold Competition Combo replacing many of the factory internal parts.


Next, we will be adding a different magazine tube extension (and attempting to match it) for compliance with an 8-round division limit. We will also need to get some sights back on the barrel.