Build Sheet
SIG P320 Build Sheet
The P320 is built around a stainless chassis the size of a matchbook. That chassis is the gun. Everything else (grip, slide, barrel) ships to your door and swaps in minutes.
9mm · Modular · Beginner · ~1 hr · Tools: Punch set
What this build is
The P320 is the most genuinely modular handgun on this list, and it is modular in a way that matters legally. The serialized firearm is not the frame you hold. It is the Fire Control Unit, a small stainless chassis that drops out and moves between grip modules. One FCU can be a full-size duty gun today and a compact carry gun next month.
That makes it the cheapest platform to change your mind on. A grip module costs about the same as a decent holster. If you buy one FCU and later decide you want a shorter grip, a different color, or a manual safety, you are buying a part, not another gun.
The assembly itself is quick, around an hour, and requires a punch set and nothing else. There is no fitting, no headspacing, and no press work.
One thing we are not going to skip, because this site would be worthless if we did: the P320 has been the subject of years of reports, lawsuits and agency suspensions over uncommanded discharges, and the litigation is still live. SIG has consistently denied a defect and points to its 2017 voluntary upgrade program, which addressed drop-related discharges on early guns. Reasonable, informed people have landed on both sides of this. If you are buying a used or older FCU, find out whether it predates that upgrade program, and read the current reporting yourself before you commit. This is the part you are meant to keep through every future configuration, which makes it the one worth an hour of research.
The parts list and what it costs
Every part of the firearm itself, with a ballpark price for each. Optics, mounts and extra magazines go on top of this. The estimated total for this build is $535–$725, the sum of the parts below with a ±15% band around it, because prices move.
| Part | Ballpark | Notes |
|---|---|---|
| Fire Control Unit (FCU) · FFL | $180 | The regulated part - the actual firearm |
| Grip module | $45 | Any size/color, no FFL |
| Stripped slide | $150 | Optic-cut options common |
| Slide completion kit | $70 | Striker, extractor, plates |
| Barrel | $80 | Standard or threaded |
| Recoil spring assembly | $22 | |
| Sights or optic plate | $40 | Skip if slide came cut + capped |
| Magazine - 17rd | $45 | Check your state limits |
These are ballparks, not quotes. Prices move weekly and we do not invent them. The deal board links straight to the retailer's own product page so you see the live price before you buy.
The part that needs an FFL
The Fire Control Unit is the firearm. It carries the serial number, it goes on the paperwork, and it ships to a licensed dealer for a background check. The grip module, the part that actually looks like a pistol frame, is legally a plastic accessory and ships to your house, as do the slide, barrel, recoil assembly, sights, and magazines. This is the single most misunderstood thing about the platform, and it is worth saying plainly: you can own a drawer full of grip modules and slides with no paperwork on any of them.
Where to save
These are the parts where the budget version and the premium version are the same material with a different roll mark:
- Grip module. These are inexpensive and there is no functional penalty to a cheaper one. Buy the size that fits your hand, not the expensive one.
- Sights. Skip if your slide is optic-cut and capped.
- Stripped slide. Blems and closeouts show up regularly and the savings are real.
Where not to save
And these are the ones where cheap costs you twice:
- Fire Control Unit. This is the trigger, the serialized part, and the piece you will keep through every future configuration. Buy it once, buy it right.
- Barrel. Especially if you are going threaded, since thread concentricity is not something to gamble on.
- Slide completion kit. Striker and extractor parts determine whether the gun goes bang every time.
Common mistakes
- Ordering a grip module in the wrong size class. Full-size, carry, and compact modules are not interchangeable with every slide and barrel length, so match the module, slide, and barrel to one size class.
- Assuming any P320 slide fits any FCU. The FCU is universal; the slide and barrel still have to match each other and the intended grip size.
- Buying a slide cut for the wrong optic footprint.
- Forgetting that magazines are also size-specific, and a full-size magazine will stick out the bottom of a compact grip module.
- Treating the grip module as the serialized part when selling or transferring. It is not, and getting this backwards is a legal problem, not a technical one.
Before you load it
Do all of this with the firearm confirmed unloaded and pointed somewhere safe, before it ever sees live ammunition. It takes a couple of minutes and catches almost every assembly error:
- Function check. The hammer or striker holds when cocked, the trigger releases it, and the trigger resets.
- Safety check. With the safety engaged, the trigger does not release the hammer or striker. Check every safety the design has.
- Hand-cycle dummy rounds or snap caps. Watch them feed, chamber, extract and eject. Do it slowly and watch, not fast.
- Slide and barrel fit. With the slide on the grip module, the takedown lever should rotate fully and the slide should run its whole stroke without binding. If it catches, stop and find out why.
- First live rounds one at a time. Load a single round, fire, and inspect the case, then do it again, then load a full magazine.
If anything on this list does not behave the way it should, stop and take it to a gunsmith. That is a cheap afternoon compared to the alternative.
FAQ
What part of a SIG P320 is the actual firearm?
The Fire Control Unit, the small stainless chassis that holds the trigger group. It carries the serial number and is the only part that requires a background check.
Can I buy a P320 grip module without a background check?
Yes, the grip module is legally an accessory, and so are the slide, barrel, and magazines.
Can one FCU be used in multiple guns?
It can be moved between grip modules and slide assemblies, so one serialized FCU can be configured as a full-size, carry, or compact pistol. It can only be in one configuration at a time.
How long does a P320 build take?
About an hour with a punch set. There is no fitting or headspacing involved.
Is the SIG P320 safe?
That is genuinely contested, with years of uncommanded-discharge reports, active litigation, and some agencies suspending the pistol; SIG denies a defect and points to its 2017 voluntary upgrade program for early guns. We are not going to resolve it for you. Read the current reporting, and if you are buying used, check whether the FCU predates the upgrade program.
Price this build out
The deal board tracks parts for this build across the retailers builders actually use. Every listing links straight to the shop's own page for the live price.
Browse the gear →Affiliate disclosure: Cold Bore Index earns a commission when you buy through links on this site, at no extra cost to you. We link to real products at retailers you already use; prices and availability are shown live on their pages. We're not a dealer and hold no inventory. Nothing here is legal advice. Firearm laws vary by state and by city, and checking yours is on you.