Mossberg says it has sold more than 12 million 500-series shotguns, and the receiver has kept the same basic shape the whole time, so the aftermarket for it is enormous. That is also why so many parts get bought for the wrong gun. "Mossberg 500" is really three closely related shotguns, and the difference that matters for upgrades is not the finish or the barrel length. It is the magazine tube.

This guide sorts the three first, then walks the upgrade categories using only parts we can name and price from the maker or a major dealer as of this writing. Prices move; treat them as a starting point.

500 VS 590 VS 590A1

THE MAGAZINE TUBE DECIDES EVERYTHING

On a 500 the magazine tube is closed at the front. The barrel is held on by a takedown screw that threads into the end of the tube, and Mossberg's owner's manual describes removing the barrel on 500, 505, 510 and 535 models by backing that screw out until it clears the tube. On a 590 the tube is open at the muzzle end. The barrel's takedown ring slides over the tube and a screw-off magazine cap holds it in place. Mossberg calls this the clean-out magazine tube, and every 590 has one.

That one design choice drives the compatibility rules for the whole family. A 590 barrel needs a 590-length open tube to seat on, so it does not go on a 500 receiver with a 500 tube, and Mossberg's manual is blunt about it: accessory barrels are interchangeable only within a specific model, gauge and magazine capacity. The 590A1's 9-shot magazine tube (part 11241P, $55 direct from Mossberg) carries the same kind of note, compatible only with 590A1 20-inch barrels. Match barrel, tube and capacity as a set, or nothing seats.

The open tube is also why the 590 is easier to live with: unscrew the cap and the spring and follower come out without pulling the barrel.

WHAT THE 590A1 ADDS

The 590A1 takes the 590 and hardens it. Mossberg lists a heavy-walled barrel, a metal trigger guard and metal safety button, a Parkerized finish and a MIL-SPEC 3443G rating, none of which it lists on the standard 590, which ships in matte blue. Both share the open tube, the top-mounted ambidextrous safety and the twin-action-bar receiver.

The price gap is real. Mossberg lists the 590 9-Shot (#50693, 20-inch barrel, 8+1, ghost ring sights, heat shield) at $647 and the 590A1 9-Shot (#51663, 20-inch barrel, 8+1, ghost ring sights) at $910. If the gun lives in a safe and goes to the range, the 590 is the same shotgun where it counts. If it rides in a truck, the A1's metal controls and thicker barrel are what you are paying for.

CAPACITY AND THE OTHER VARIANTS

A standard 500 holds 5+1 with 2-3/4- or 3-inch shells per Mossberg's manual. The 20-inch 590 and 590A1 hold 8+1. The 590S Optic-Ready (#51604, $742) cycles 1.75-, 2.75- and 3-inch shells interchangeably without an adaptor, which Mossberg rates at 7+1, 8+1 or 13+1 depending on shell length, and its receiver is cut for a Shield RMSc-footprint optic. The 590 Shockwave (#50659, $592) pairs a 14.375-inch heavy-walled barrel with a bird's-head grip at an overall length of 26.37 inches and 5+1 capacity, and gets its own legal section below.

STOCKS AND FORENDS

The 500-versus-590 question bites hardest here, on the forend.

SIGHTS AND OPTIC MOUNTS

A factory bead works for most defensive distances. Ghost rings change how the gun shoots, because they keep the target and the front sight in view together and they can be adjusted for slugs. Mossberg sells its own kit, the Ghost Ring Sight Kit for the 500/590, 12 Gauge (#95300, $119 direct), with the rear sight, front sight and base for non-ribbed barrels. Mossberg notes that gunsmithing is required for the front sight, so plan on a shop visit or buy a model that ships with them.

For a red dot, the cleanest path is a receiver cut for one, and the 590S Optic-Ready ships with a Shield RMSc footprint. Other 500-series receivers are drilled and tapped, which takes a rail or a plate. Gear Guy's Mossberg catalog does not yet carry a named rail or plate we have verified, so there is no recommendation until it does.

SIDE-SADDLES

On-gun shell storage is the cheapest capacity upgrade there is, and it does not touch the tube.

The carrier is the permanent answer; the card costs almost nothing and tells you whether you actually want shells on the receiver.

LIGHTS

Without a light you cannot identify a target at night, and a bare 500 or 590 forend has nowhere to put one. The two clean paths are an M-LOK forend, meaning the MOE above on a 590 or 590A1 or the 590S's factory M-LOK forend, and a clamp on the barrel or tube.

Check what the light has to share space with. Mossberg's 500/590 Barrel Heat Shield (#95066, $59 direct) is listed as not compatible with the 590A1, the 590M, the Shockwave, breacher barrels, ghost ring sight barrels, or 590 7-Shot models with the +1 extension, and the Magpul forend does not clear it either. Pick the forend and the heat shield together or you will buy one twice. The catalog does not carry a named light we have verified for this family, so we are not naming one.

MAGAZINE TUBES

The catalog carries two factory tubes, the 500's 5+1 and the 590's 8+1, and no aftermarket extension, for the reason above. On the 500 the barrel is retained by the takedown screw in the end of the closed tube, so capacity is tied to the barrel-and-tube pairing Mossberg ships. On the 590 the open tube runs out under the barrel with the barrel ring riding over it, which is why the 20-inch 590 comes as an 8+1 from the factory. If you want more shells in the gun, start with a 590. Capacity limits vary by state; verify your state law.

FOLLOWERS AND SAFETIES

A high-visibility follower and an oversized or metal safety button are common upgrades and worth doing. They are also internal or near-invisible small parts, and Gear Guy models what you can see at bench zoom, so they are not bench slots and this article does not rank them. Keep the receipts and note them on the gun's record.

THE SHOCKWAVE AND STOCKS

Mossberg's 590 Shockwave is sold as a "firearm," not a shotgun. Mossberg states that BATFE designated it a non-NFA firearm with no tax stamp required for transfers, because it never had a shoulder stock and its overall length is 26.37 inches. Federal law defines a shotgun as a weapon designed to be fired from the shoulder, and treats a shotgun with a barrel under 18 inches, or a weapon made from a shotgun with an overall length under 26 inches, as a short-barreled shotgun under 26 U.S.C. § 5845.

Put a shoulder stock on a 14.375-inch Shockwave and you have made a short-barreled shotgun. As of January 1, 2026 the federal tax is $0, but registration is still required. Going the other way, a pistol grip on an 18.5-inch gun that started life with a stock is a federal question about that 26-inch overall length and a state question. These are federal rules only; verify your state law before changing either end of the gun.

ON THE BENCH IN GEAR GUY

Gear Guy renders the 500 and 590 in 3D from real named parts, so the stocks, forends, sights, side-saddles and magazine tubes above can be configured on the bench before anything is ordered. The compatibility engine carries the family's rules: a 590 barrel on a 500 receiver draws a warning that it will not seat without the matching magazine tube, and a shoulder stock on a Shockwave flags the short-barreled shotgun problem instead of letting the picture look fine. A finished configuration shares as a link at gearguyapp.com/b/.

The gun itself lives in the Armory, an AES-256-GCM encrypted vault on the device with no account and serials masked by default. The range log tracks round counts and deducts ammunition as you shoot, the Capability sheet shows whether a configuration carries a white light, Identify reads a photo of a gun and stands it up on the bench, and Premium users can export a PDF insurance and estate report of everything in the vault.

FINAL THOUGHTS

Know which Mossberg you own before you buy anything for it. The 500 is a fine shotgun with a closed tube, and its upgrades are stocks, sights and shell cards. The 590 is the one to buy for the open tube, the capacity and the forend options, and the 590A1 is the 590 for people who are hard on equipment. Spend on a light and a way to mount it, then sights, then shells on the gun. Leave the magazine tube alone unless you bought the receiver designed to have it changed.