Gen 5, 2.0, Mk IV: Why Guns Get Generations and Which Matter

Why guns get generations

A popular pistol is hard to rename. Police departments have it on contract, holster makers have molds for it, and buyers ask for it by name. So when a maker improves the gun, it keeps the model and adds a label: Gen 5, M2.0, Mark IV.

That's mostly good news. The gun gets better without losing its name. But it also means two guns with the same model number can be quite different under the hood, and the used market is full of every generation at once. Knowing which one you are holding is the difference between a smart buy and a box of magazines that don't fit.

This guide is an editorial reference built from published histories and maker information. We cover four common families, then the questions to ask about any used gun with a generation on the slide.

Glock: Gen 1 through Gen 6

Glock is the textbook case. Wikipedia's history of the company lays out the generations:

What matters to a used buyer? Holsters mostly carry over within a model size, but rails and grip shapes differ, so check the holster maker's fit list. Magazines are broadly shared within a model and caliber, but compatibility across generations isn't universal, so check the maker's notes before you stock up on spares for a different generation. Optics are where generation matters most: a MOS slide takes plates for a red dot, a plain slide needs milling, and the new Gen 6 optic system is different again. Our red dot footprints guide explains why that matters.

The Glock 19 Gen 5 at Ammunition Depot is listed with a polymer frame, black nDLC slide and interchangeable backstraps. Matching Glock 19 Gen 5 factory magazines are sold separately.

How to tell which Glock you have

Gen 3 and later are generally easy: look for the finger grooves and rail, or for "Gen4" or "Gen5" marked on the slide. Gen 2 guns weren't marked, so you identify them by the checkered grip and the lack of a rail. If you're unsure, the serial number and a call to the maker settle it.

Smith & Wesson: M&P to M2.0

The M&P arrived in 2005 and gave Smith & Wesson a polymer service pistol with interchangeable grip inserts. In 2017 the company launched the M2.0, which Wikipedia describes as adding a full-length steel chassis, a more aggressive grip texture and the improved trigger from the Pro series.

The compact Shield line has its own history: the original Shield in 2012, a .45 ACP version in 2016, the double-stack Shield Plus in 2021 (after which the original Shield 1.0 was discontinued), and the Shield X in 2025.

For a used buyer the practical questions are holster fit, which differs between the 1.0 and M2.0 frames in many holsters, and whether the slide is cut for an optic. The M&P Shield M2.0 9mm listed at Ammunition Depot is the single-stack M2.0 Shield with no manual safety, a good reference point if you're comparing against an older Shield on a used rack.

Ruger Mark: Standard to Mark IV

Ruger's .22 pistol shows how long a generation line can run. Wikipedia's account goes like this: the Standard in 1949 as Ruger's first product; the Mark II in 1982, adding a last-round bolt hold-open and stainless options; the Mark III around 2004, adding a loaded chamber indicator, a magazine disconnect and a relocated magazine release; and the Mark IV in 2016.

The Mark IV's headline change is one-button takedown: a hinged upper receiver that tips up for cleaning. Anyone who has wrestled an older Mark pistol back together will tell you that alone is worth the generation bump.

The Ruger Mark IV Hunter pairs laminate wood with stainless steel. For a budget alternative, see our GSG Firefly vs Mark IV piece.

SIG P320 and the modular twist

The P320 doesn't use generation numbers, but it has evolved just as much. SIG launched it in January 2014 as its first striker-fired pistol for the commercial market. The design is modular: the removable fire control unit can move between grip modules and calibers. A customized P320 won the U.S. Army's Modular Handgun System competition in 2017 as the M17 and M18. In 2021 SIG introduced the AXG, the first metal grip module.

The generational detail that matters most here is the 2017 voluntary upgrade program, which SIG announced after drop-safety concerns. It changed the trigger, sear and striker and added a mechanical disconnector. On a used P320, ask whether the upgrade was done.

For balance: reports of P320 discharges without a trigger pull continued after the upgrade, several agencies stopped using the pistol, and litigation has produced mixed outcomes. SIG maintains the pistol cannot fire without a trigger pull. Read the current reporting and decide for yourself.

Generation, variant or series?

Gun counters throw around three words that sound alike and mean different things. Getting them straight saves a lot of confusion on used listings.

• Generation is a redesign of the same model over time. A Gen 3 and a Gen 5 Glock 19 are the same size and role, built to different designs. Think model year.
• Variant is a sibling sold at the same time: compact versus full-size, a different caliber, a model with or without a manual safety, a factory optic cut. A Gen 5 Glock 19 and a Gen 5 Glock 17 are variants of the same generation.
• Series or line is marketing grouping: "Pro Series," "Performance Center," "Hunter," "Target." It tells you about features and finish more than design history.

A used listing that says "M&P9 Pro Series M2.0 Compact" is telling you all three at once: the line, the generation and the variant. Break it apart before you compare prices, because a listing that looks like a bargain may simply be an older generation or a less-equipped variant.

Why makers don't just rename the gun

From the outside, a new name for each redesign might seem cleaner. From the maker's side, a familiar name is worth a lot. Agencies that standardize on one pistol can move to the newer generation without re-doing their whole approval process from scratch. Holster and accessory makers already know the outline. Shooters who liked the last one walk in asking for the same name.

The cost of that approach lands on used buyers, who have to learn which details changed. That is why this guide exists, and why a few minutes of reading before you buy beats a box of parts that don't fit.

What to check on any used generation

1. Read the markings. Find the model and generation on the slide or frame, then the serial number. Don't trust a listing title alone.
2. Look up what changed. A quick search of "[model] [generation] changes" tells you what that version does and doesn't have.
3. Check recalls and upgrades. Makers post recall pages with serial ranges. Confirm any work was done before you buy.
4. Confirm parts fit. Magazines, holsters, sights and optic plates can be generation-specific. Price them in before you agree on the gun.
5. Inspect the gun itself. Generation tells you the design; condition tells you the gun. Use our used gun inspection checklist, and as always, verify it's unloaded before you touch anything else.

Does the newest generation always win?

No. A newer generation usually has better ergonomics or easier optics mounting, but older guns can be excellent buys. Plenty of shooters prefer finger grooves, or simply want a cheaper pistol that runs. The point is to know what you're getting.

A few rules of thumb:

• If you want a red dot, newer generations with factory optic cuts usually save money compared with milling an older slide.
• If you already own magazines and holsters for one generation, staying in it can be the smart choice.
• If a generation had a recall, a fixed gun is fine. An unfixed one is a phone call away from fixed, but don't skip the call.

Whatever you buy, budget for range time with practice ammo, such as 9mm range ammo or .22 LR for a Mark pistol, and confirm the gun runs before you trust it.

The takeaway

Editorial note: This is an independent editorial overview built from manufacturer specifications, retailer listings and published sources. We have not performed hands-on testing of the products discussed, and nothing here is a performance guarantee. Verify specifications, compatibility and the law where you live.

Affiliate disclosure: Some links on this page are affiliate links to Ammunition Depot. We may earn a commission if you buy through them, at no extra cost to you. Our editorial choices are not influenced by commissions. Check current price, stock and specifications directly with the retailer.