No modern handgun is built to be customized like the Sig Sauer P320, and the reason is a legal one as much as an engineering one. The serialized part is the fire control unit, not the frame and not the slide. That single fact decides what shows up at your door and what has to go through a dealer, and it’s why the P320 aftermarket runs deeper than anything else on the market.
Below are the ten P320 parts we reach for first, ordered the way we would actually spend the money. Before the picks, two things worth knowing: which part is the gun, and why a Holosun doesn’t simply bolt onto an X-series slide. The second one is a correction to the earlier version of this page.
Table of Contents
How we tested: Every pick here is ranked against our testing methodology. Minimum round counts, accuracy and reliability protocols, the failures that disqualify a gun.
Sig P320 Upgrades at a Glance
| Category | Our Pick | Why It Makes the List |
|---|---|---|
| Night sights | Night Fision | Cheapest change to how the gun shoots |
| Flat trigger | SIG or Apex | Consistent finger placement, under $50 |
| Grip module | Wilson Combat WCP320 | Transforms the feel, no FFL needed |
| Weapon light | Streamlight TLR-7 | Slim 500 lumen light for duty holsters |
| Red dot | Holosun 507C X2 | Value benchmark, but check your footprint |
| Magazines | SIG and ETS 21-round | Cheap capacity, translucent practice mags |
| Holster | IWB Kydex, several makers | Buy for the gun you assembled |
| Match barrel | True Precision | Tighter groups, threaded options |
| Slide | Grey Ghost Precision | Optics ready, ships to your door |
| Fire control unit | SIG Custom Works | The part that actually is the gun |
Which Part of a P320 Is Actually the Gun
Every other pistol on the market is serialized at the frame. The P320 is serialized at the fire control unit, the small chassis holding the trigger group. Strip the gun and that chassis lifts out, and it’s the part with the serial number on it. Everything wrapped around it is a component.
You don’t have to take our word for that, because the retailers say it themselves. Run the fire control unit card at the bottom of this page and the rows come back tagged as an FFL item. Run the grip module, the slide or the barrel card and not one row anywhere carries that marking. Same retailers, same page, different legal class of object.
What that means when you’re buying
Grip modules, slides and barrels arrive at your house. No transfer, no fee, no wait. A $305 optics ready slide ships like a pair of boots. That’s why a P320 build costs less in friction than the equivalent Glock build, where a slide is also just a part but the culture around it is different.
One FCU legally becomes several guns. Buy a full size module and a compact slide and you own one pistol that reconfigures in about ninety seconds. Buy a second FCU and you own two, and they can share every other component you already have.
The FCU is the part that needs thought on the used market. It carries the serial, the history, and the question of whether it has been through SIG’s Voluntary Upgrade Program, which revised the trigger, sear and striker and added a mechanical disconnector on earlier pistols. A used slide is a used slide. A used FCU is a used gun.
The optic footprint trap
This is the correction. It’s easy to assume an optics ready P320 takes the same dot as an optics ready anything else, and the X-series doesn’t. SIG’s own P320 X-Carry page describes the “Optic Ready XSeries Slide” as “Compatible with ROMEO1PRO, and Deltapoint Pro”. Those are the two patterns the factory pocket is cut for.
The Holosun 507C, the 407C and the Trijicon RMR are all the RMR footprint, which is a different pattern with different screw spacing and a smaller body. Where a slide carries RMR holes as well, an RMR pattern optic will mount, and a filler plate exists to close the gap the smaller optic leaves at the front of the pocket. Where it doesn’t, you need an adapter plate or a slide cut for what you own.
None of that makes the 507C a bad pick, and it stays on this list. It does mean the honest budget for a dot on an X-series gun is the optic plus a plate, not the optic alone.
1. Night Fision Night Sights
- Fits: Sig P320, all sizes
- Type: Tritium, steel bodies
- Heights: Standard and optic co-witness
- Street price: Around $54 to $107
| Capability gained | 4 / 5 |
| Ease of install | 3 / 5 |
| Value | 4 / 5 |
| Durability | 5 / 5 |
| Availability | 4 / 5 |
Pros
- Tritium glows through a whole night shift
- Optic height options co-witness a dot
- Steel replaces the plastic factory sights
Cons
- Needs a sight pusher
- Front sight needs thread locker
- Costs more than plain steel sights
Sights are the cheapest thing on this page that changes how the gun shoots, and on a P320 they’re also the upgrade most owners need. Night Fision sets are bright, they glow all night, and they come in standard and optic co-witness heights so they still work if you add a dot later.
One buying note that cost this page an error until today. Searches for P320 sights pull in P365 sets constantly, because the two model names sit one character apart in a product title. They aren’t interchangeable. Read the full listing name before you click buy, not the heading above it.
Best For: Any P320, and the first hundred dollars worth spending.
2. Flat Trigger, SIG or Apex
- Fits: Sig P320 fire control unit
- Face: Flat, 90 degree break
- Makers: SIG factory and Apex Tactical
- Street price: Around $26 to $44
| Capability gained | 4 / 5 |
| Ease of install | 4 / 5 |
| Value | 5 / 5 |
| Durability | 5 / 5 |
| Availability | 5 / 5 |
Pros
- Flat face makes finger placement repeatable
- Under fifty dollars at most retailers
- Drops in without touching the sear
Cons
- Doesn’t lighten the pull much
- Feels different from the factory curve
- Apex and SIG versions differ slightly
The curved factory trigger is the part most P320 owners replace first after sights, and a flat shoe is the cheapest way to make the break feel consistent. SIG sell their own 90 degree flat trigger and Apex sell the Flat Advanced, and both land under fifty dollars.
Be clear about what this does and doesn’t do. A flat shoe changes where your finger sits and how square the press feels, which is worth real money. It doesn’t meaningfully lighten the pull. If weight is what you’re after, that’s a sear and spring job, and on this platform that means Grayguns territory at three to four times the price.
Best For: Almost everyone. Highest satisfaction per dollar on the whole list.
3. Wilson Combat WCP320 Grip Module
- Fits: Sized per model, carry and compact common
- Ships: To your door, no FFL
- Feature: Higher grip, 1911 style angle
- Street price: Around $50 to $67
| Capability gained | 5 / 5 |
| Ease of install | 5 / 5 |
| Value | 5 / 5 |
| Durability | 5 / 5 |
| Availability | 4 / 5 |
Pros
- Higher grip lowers felt muzzle rise
- Better texture than the factory module
- Ships to your door, no FFL
Cons
- Sized per model, check yours
- Costs more than a SIG module
- Grip angle takes adjustment
This is the upgrade the P320 exists for. Because the frame is a grip module wrapped around the fire control unit rather than the gun itself, swapping it changes how the pistol feels without changing what you legally own. The WCP320 gives you a higher grip relative to the bore, better texture, and a 1911 style angle that settles the muzzle down.
Drop your FCU in and you have effectively a new pistol for the price of a part, which isn’t a sentence you can write about any other mainstream handgun. Check the size before ordering though, these are cut per model and a carry module isn’t a compact one.
One gotcha worth knowing before you spend: Primary Arms note in their P320 size guide that X-Series grip modules are “not compatible with any fire control units that have a manual safety”. If your FCU has a thumb safety, that rules out a whole shelf of modules.
Best For: Anyone whose factory module doesn’t fill their hand. Nothing changes the gun more per dollar.
4. Streamlight TLR-7 Weapon Light
- Fits: Any rail-equipped P320
- Output: 500 lumens on the TLR-7 X
- Battery: CR123A
- Street price: Around $127 to $133
| Capability gained | 5 / 5 |
| Ease of install | 5 / 5 |
| Value | 4 / 5 |
| Durability | 5 / 5 |
| Availability | 5 / 5 |
Pros
- Slim head clears most duty holsters
- Ambidextrous switching for either hand
- Undercuts SureFire on price
Cons
- Needs a light bearing holster
- CR123A batteries cost more than AA
- Less throw than the TLR-1
A duty or home defense P320 needs a light, because you can’t shoot what you can’t identify and a weapon light keeps both hands on the gun while you find out. The TLR-7 is the slim body standard: a compact head that clears most holsters, ambidextrous switching, and a price well under the SureFire equivalent.
Buy the holster and the light together. A bare gun holster won’t close around a light, and that’s the single most common way people end up with a two hundred dollar setup they can’t carry.
Best For: Any P320 with a defensive job. Not optional for those roles.
5. Holosun 507C X2 Red Dot
- Footprint: RMR, not the factory ROMEO1PRO
- Reticle: 2 MOA dot, 32 MOA circle, or both
- Power: Solar backup plus shake awake
- Street price: Around $237 to $310
| Capability gained | 5 / 5 |
| Ease of install | 2 / 5 |
| Value | 4 / 5 |
| Durability | 5 / 5 |
| Availability | 4 / 5 |
Pros
- Shake awake saves the battery
- Multi reticle suits dot or circle
- Solar backup if the battery dies
Cons
- Needs a plate on X-series slides
- Not the factory ROMEO1PRO footprint
- Open emitter can collect rain
The 507C X2 is the value benchmark in pistol dots: a rugged open reflex with shake awake, a multi-reticle system, a solar backup and battery life measured in years. On hit probability at speed it is the biggest upgrade on this page, once you have put in the reps to find the dot on the draw.
Now the correction, because the earlier version of this page got it wrong and so does most of the internet. The X-series factory cut isn’t an RMR cut. SIG’s own P320 X-Carry page says the “Optic Ready XSeries Slide” is “Compatible with ROMEO1PRO, and Deltapoint Pro”. The 507C is the RMR footprint, which is a different pattern.
In practice that means one of three things. If your slide carries RMR screw holes alongside the ROMEO1PRO pocket, the optic will bolt down, and a filler plate closes the gap the smaller optic leaves at the front. If it doesn’t, you’re buying an adapter plate or a slide cut for what you own. Either way, budget for it rather than discovering it when the box arrives.
Best For: Optics-cut slides, once you have confirmed which pattern yours is.
6. Sig P320 21-Round Magazines
- Fits: Full size and carry grip modules
- Capacity: 21 rounds of 9mm
- Makers: SIG factory and ETS translucent
- Street price: Around $11 to $48
| Capability gained | 3 / 5 |
| Ease of install | 5 / 5 |
| Value | 5 / 5 |
| Durability | 4 / 5 |
| Availability | 5 / 5 |
Pros
- Twenty-one rounds for range and defense
- Translucent ETS bodies show the count
- Cheaper than factory for practice
Cons
- Extends below a carry grip
- Polymer feed lips need watching
- Factory mags still preferred for duty
Full size P320 magazines hold 17, and the 21 rounders are cheap insurance for the range and the nightstand. Factory SIG magazines are excellent. The translucent ETS bodies cost a fraction of that and let you read the round count without stripping one out of the gun.
The price spread on this card is wide and worth understanding rather than ignoring. The headline slot only ever draws from a handful of trusted retailers, so a factory SIG magazine can sit at the top while an ETS at a third of the price sits below it. Both are correct products. Read down the list before you decide.
Best For: Buy several. Magazines are the cheapest reliability insurance you can own.
7. IWB Kydex Holster
- Fits: Match to your exact P320 configuration
- Material: Molded Kydex
- Options: Bare gun, light bearing, optic cut
- Street price: Around $32 to $62
| Capability gained | 4 / 5 |
| Ease of install | 5 / 5 |
| Value | 5 / 5 |
| Durability | 4 / 5 |
| Availability | 5 / 5 |
Pros
- Molded Kydex with adjustable cant
- Options for light and optic cuts
- Several makers fit the same gun
Cons
- Must match your exact configuration
- Kydex wears a gun’s finish
- Compact and full size differ
A gun you can’t carry comfortably is a gun you leave at home. The P320 is well served here, with C&G, DeSantis, Rounded Gear, Alien Gear and 1791 all cutting shells for it, and the honest advice is to buy for your configuration rather than for a brand.
That matters more on this platform than most, because the P320 is four or five different guns depending on which module and slide you assembled. A compact shell won’t take a full size slide. Work out what you actually built, then buy the holster for it.
Best For: Daily carry. Match the shell to the gun you assembled, not the gun on the box.
8. True Precision Match Barrel
- Fits: Sized per slide, compact and full differ
- Threads: 1/2×28 on threaded models
- Ships: To your door, no FFL
- Street price: Around $147 to $190
| Capability gained | 4 / 5 |
| Ease of install | 4 / 5 |
| Value | 4 / 5 |
| Durability | 5 / 5 |
| Availability | 4 / 5 |
Pros
- Match grade accuracy for the money
- Threaded options for a can
- Supported chamber suits reloaders
Cons
- Barrel length must match your slide
- Thread protector often sold separately
- Fitting varies between slide brands
A match grade drop-in tightens groups, gives reloaders a supported chamber, and in threaded form lets you hang a compensator or a can off the front. True Precision are the value pick here, with button rifling and finishes from black nitride through gold TiN if you’re matching a custom slide.
Watch the length. The card below mixes 3.9 inch compact barrels with 4.7 inch full size ones, because retailers list them under the same model name. A barrel has to match the slide it lives in, so read the listing rather than the card heading. Going threaded? Match the pitch to your can, 1/2×28 is standard for 9mm.
Best For: Accuracy work and suppressor hosts, once you know your slide length.
9. Grey Ghost Precision Slide
- Fits: Sized per model, compact and full
- Optic cut: Machined for a dot
- Ships: Stripped, to your door, no FFL
- Street price: Around $305 to $313
| Capability gained | 5 / 5 |
| Ease of install | 2 / 5 |
| Value | 3 / 5 |
| Durability | 5 / 5 |
| Availability | 3 / 5 |
Pros
- Optics ready without milling your slide
- Lightening cuts reduce reciprocating mass
- Ships to your door, no FFL
Cons
- Ships stripped, budget for internals
- Sized per model, compact or full
- No trusted retailer stocks it
An optics ready slide is how most P320 builds go modern, and it’s the clearest demonstration of what this platform actually is. A complete slide assembly isn’t a firearm, so it arrives at your house like any other package. Pair it with your FCU, a barrel and a completion kit and you have an optics ready gun for a fraction of a full custom.
Two practical notes. It ships stripped, so budget for internals on top of the sticker. And decide your optic pattern before you order, because this is the moment you get to pick a cut instead of inheriting one.
Best For: Builds that want an optic cut, or a second configuration off one FCU.
10. Sig Custom Works Fire Control Unit
- What it is: The serialized firearm
- Ships: To an FFL, like any pistol
- Fits: Every P320 grip module size
- Street price: Around $350
| Capability gained | 5 / 5 |
| Ease of install | 4 / 5 |
| Value | 3 / 5 |
| Durability | 5 / 5 |
| Availability | 4 / 5 |
Pros
- The part that is the gun
- One unit builds many configurations
- Skeletonized Custom Works version drops straight in
Cons
- Ships to an FFL, not you
- Costs more than most complete parts
- Manual safety blocks X-series modules
Last on the list because it’s the biggest decision, not the biggest upgrade. The fire control unit is the P320. Everything else on this page is a part; this is the gun, and the listings say so. Look at the card below and you’ll see retailers tagging these rows as an FFL item, a marking that doesn’t appear on a single grip module, slide or barrel row anywhere else on this page.
That’s the whole platform in one line. A second FCU is a second pistol, legally and practically, and it means a household can run one set of slides and modules across two serialized guns. The Custom Works unit is the skeletonized version with a flat trigger already fitted, which is why it sits around $350 rather than the price of a bare FCU.
If you’re buying a used FCU rather than a new one, ask whether it has been through SIG’s Voluntary Upgrade Program. That program changed the trigger, sear and striker and added a mechanical disconnector on earlier guns, and it’s the single thing worth establishing before money changes hands. Our Sig P320 problems piece covers that history.
Best For: Anyone building a second configuration, or replacing an early unit.
What a Built P320 Really Costs
The P320 rewards buying in stages, because each stage works on its own. These are the three stopping points most builds actually have, priced off the live retailer figures on the cards above.
| Build level | What is in it | Cost | What you get |
|---|---|---|---|
| Feel and sights | Night sights, flat trigger, grip module | $130 to $215 | Changes how the gun shoots and fits. Nothing here needs a dealer. |
| Defensive setup | Add a TLR-7, a dot and a holster | $525 to $620 | The duty or nightstand gun. Add a plate if your cut is ROMEO1PRO. |
| Second configuration | Add a slide, a barrel and a second FCU | $1,330 to $1,470 | Two serialized guns sharing one parts bin. Only the FCU goes through an FFL. |
The first row is the one we would argue hardest for. Around two hundred dollars of sights, trigger and grip module changes more about shooting a P320 than the optic that costs more than all three together.
How to Prioritize Your P320 Build
The modularity means you can build in almost any direction, which is exactly why people overspend. If we were starting from a base gun, this is the order we would go in.
- Sights first. The cheapest upgrade that pays off every time the light gets low, and it fits every P320.
- A flat trigger. Under fifty dollars, and it changes the press more than the price suggests.
- A grip module if the factory frame doesn’t fill your hand. No transfer, no fee, and it arrives at your door.
- A light if the gun has a defensive role, bought together with the holster that fits it.
- A red dot once you have established which footprint your slide is cut for, and budgeted for a plate if it’s ROMEO1PRO.
- A slide, a barrel, or a second FCU last, when you want a second configuration rather than a better version of the one you have.
Want the background on the platform first? Read our Sig Sauer P320 review, the Glock 19 vs Sig P320 comparison, and our reporting on the Sig P320 problems discussion. For the wider field, see our best full-size 9mm pistols roundup, and don’t forget quality 9mm ammo to feed it.
Frequently Asked Questions
Which part of a Sig P320 is the serialized firearm?
The fire control unit. It is the small chassis holding the trigger group, it carries the serial number, and it is the part the law treats as the pistol. The grip module, slide, barrel and everything else are components. You can see the split in retailer listings: fire control units are tagged as an FFL item, while grip modules, slides and barrels are not.
What red dot fits the Sig P320 X-series?
Fewer than most guides suggest. SIG describe the Optic Ready XSeries Slide as compatible with the ROMEO1PRO and the Deltapoint Pro, and those are the patterns the factory pocket is cut for. The Holosun 507C, the 407C and the Trijicon RMR are all the RMR footprint, which is a different pattern. Where a slide carries RMR screw holes as well, an RMR optic mounts and a filler plate closes the gap the smaller body leaves at the front. Otherwise you need an adapter plate. Confirm your slide cut before you buy the optic.
Do Sig P320 grip modules and slides ship to your door?
Yes. Because the fire control unit is the serialized part, grip modules, slides, barrels, sights, magazines and triggers are ordinary components. They ship like any other package, with no transfer and no dealer fee. Only the fire control unit itself goes through an FFL.
What is the best upgrade for a Sig P320?
Night sights and a flat trigger, which together run about a hundred dollars and change how the gun shoots on the first magazine. After that, a grip module if the factory frame does not fill your hand, since swapping it changes how the whole pistol feels without changing what you legally own.
Can you customize a Sig P320 yourself?
More than almost any other pistol. Grip modules, slides, barrels, sights and triggers are all non-serialized parts you can buy and fit at home, and moving the fire control unit between modules takes a couple of minutes. One catch worth knowing: Primary Arms note that X-Series grip modules are not compatible with fire control units that have a manual safety.
Do I need a gunsmith to install P320 parts?
Not for most of this list. Grip modules, slides, barrels, magazines, lights, holsters and flat triggers all install with basic tools. Sights are easier with a sight pusher. Machining a slide for an optic, or fitting a match barrel to tight tolerances, is where a gunsmith earns the fee.
How many rounds does a P320 magazine hold?
Standard full-size P320 magazines hold 17 rounds of 9mm, and 21-round extended magazines are widely available from SIG and from aftermarket makers such as ETS and Magpul. Capacity varies by caliber and by module size, so check that the magazine you are buying is listed for the configuration you built rather than for the P320 in general.
Should I worry about buying a used P320 fire control unit?
It is the one part where history matters, because the fire control unit is the gun. Ask whether it has been through SIG's Voluntary Upgrade Program, which revised the trigger, sear and striker and added a mechanical disconnector on earlier pistols. A used slide or grip module carries none of that baggage; a used fire control unit does.
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