The Sig Sauer M400 SDI Sentry 5.56 NATO 16in ROMEO5 Combo is a factory-ready AR-platform rifle built for ranchers and rural workers who need a reliable tool that hits the mark without a bunch of fuss. This combo pairs the M400-SDI X chassis with a ROMEO5 Gen II red dot and a Foxtrot-MSR weapon light, all mounted on a free-floating M-LOK handguard with a vertical foregrip. You get ambidextrous controls, a 6-position Magpul MOE-SL stock, and a 16-inch chrome moly barrel with a 1:7 twist that handles standard 5.56 NATO and .223 Remington rounds. One 30-round polymer magazine and a SIG flash hider-compensator round out the package. At $1,749.99, this rig is ready to work straight out of the box.
| Manufacturer | Sig Sauer |
|---|---|
| Model | M400 SDI Sentry |
| Material | Chrome moly barrel, anodized aluminum upper/lower receiver (per manufacturer specs) |
| Compatibility | 5.56 NATO / .223 Remington |
| Finish | Hardcoat anodized black |
| Weight | 6.8 lbs (unloaded, per manufacturer specs) |
| Condition | New |
Key Features
- Factory-Equipped ROMEO5 Gen II Red Dot — Why it matters: You don’t waste time mounting and zeroing a separate optic; this 2-MOA dot is ready for varmints, coyotes, or steel targets at dawn or dusk.
- Foxtrot-MSR Full-Size Weapon Light — Why it matters: When you’re checking a dark barn or tracking a wounded animal after sunset, that 500-lumen beam cuts through shadows so you can see what you’re shooting at.
- Free-Floating M-LOK Handguard with Vertical Foregrip — Why it matters: Keeps your support hand locked in for steady shots off-hand or over a fence post, and the free-float design improves accuracy by not torquing the barrel.
- Ambidextrous Controls — Why it matters: Left-handed or right-handed, you can drop the bolt or flip the safety without shifting your grip—handy when you’re carrying a fence stretcher in one hand.
- Magpul MOE-SL 6-Position Stock — Why it matters: Adjusts to fit heavy winter coats or light summer shirts, and the slim profile doesn’t snag on brush or truck seats.
Who It’s For
This combo is for the rancher who needs a carbine that pulls double duty: predator control and property defense. It’s also for the rural shooter who wants a complete setup—optic and light—without piecing together parts. If you work livestock, patrol fence lines, or chase coyotes, the M400 SDI Sentry is your truck gun.
Pros & Cons
- Pros: Ready to shoot out of the box; ambidextrous controls for lefties; free-float handguard improves accuracy; durable chrome moly barrel; includes quality optic and light.
- Cons: The included vertical foregrip is polymer and can feel a bit flexy under hard use—consider swapping for an aluminum grip if you’re rough on gear.
Comparison
| Feature | Sig Sauer M400 SDI Sentry | Geissele Super Duty 16″ | Zev Technologies OZ9C |
|---|---|---|---|
| Caliber | 5.56 NATO | 5.56 NATO | 9mm (pistol) |
| Barrel Twist | 1:7 | 1:7 | N/A |
| Optic Included | Yes (ROMEO5 Gen II) | No | No |
| Light Included | Yes (Foxtrot-MSR) | No | No |
| Weight | 6.8 lbs | 7.0 lbs | 2.1 lbs |
| Price | $1,749.99 | $1,999.00 | $1,699.00 |
FAQ
What ammunition does the Sig Sauer M400 SDI Sentry 5.56 NATO 16in ROMEO5 combo use?
It chambers 5.56 NATO and .223 Remington. Stick with 55-77 grain projectiles; the 1:7 twist stabilizes heavier bullets for better terminal performance on predators.
Is the ROMEO5 sight included in the combo?
Yes, the combo ships with a ROMEO5 Gen II red dot sight mounted and co-witnessed with the factory iron sights. It runs on one CR2032 battery and has a 50,000-hour battery life.
Can I swap the handguard for a different M-LOK rail?
Yes, the free-float handguard uses standard M-LOK slots, so you can add rails, bipods, or replace the whole handguard if you want a different length or profile.
Does this rifle come with a case or extra magazines?
It includes one 30-round polymer magazine and no case. You’ll want to grab a soft case or a hard case for transport.
Trust Block
- ✅ Nationwide delivery (all 50 states)
- ✅ Ships 1-2 business days
- ✅ 30-day returns
- ✅ Secure checkout
Browse more AR rifles like this in our AR Rifles category, or check out Optics and Tactical Lights for upgrades.
Last updated: April 2026
FFL Transfer & Firearms Laws for Sig Sauer M400 SDI Sentry 5.56 NATO 16in ROMEO5 Combo
All firearms, including the Sig Sauer M400 SDI Sentry 5.56 NATO 16in ROMEO5 Combo, must be shipped to a licensed Federal Firearms License (FFL) dealer. You will be required to complete a background check and any necessary paperwork before taking possession.
How It Works
- Place your order online.
- Provide your preferred FFL dealer information.
- We ship the firearm directly to your FFL.
- You complete the background check and pickup at the FFL.
Legal Compliance
We comply with all federal and state firearm laws. It is your responsibility to ensure the Sig Sauer M400 SDI Sentry 5.56 NATO 16in ROMEO5 Combo is legal in your state. Some states may have restrictions on certain firearm models or features.
Helpful Resources
Need assistance? Contact our Miami store for help with FFL transfers and firearm compliance.
Frequently Asked Questions About This Product
Why buy Sig Sauer M400 SDI Sentry 5.56 NATO 16in ROMEO5 Combo from Miami Shooters Supply?
We’re a trusted firearms dealer offering fast shipping, competitive pricing, and 100% compliance with U.S. federal firearm laws. All orders ship from our licensed facility with full tracking and expert customer service.
Where can I buy Sig Sauer M400 SDI Sentry 5.56 NATO 16in ROMEO5 Combo?
You can purchase this item directly on our website and have it shipped to your local FFL dealer. You can also visit us in-store in Miami, Florida.
Is Sig Sauer M400 SDI Sentry 5.56 NATO 16in ROMEO5 Combo available now?
Availability is shown live on this page. If the “Add to Cart” button is visible, this item is currently in stock and ready to ship.
What category does this product belong to?
This product is listed under: AR Rifles.
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Is this product legal in my state?
It is your responsibility to ensure this item is legal in your state or locality. We ship only to valid FFL dealers in compliance with U.S. laws. Always check your local regulations.
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FFL Transfer & Firearms Laws
All firearms, including the Sig Sauer M400 SDI Sentry 5.56 NATO 16in ROMEO5 Combo, must be shipped to a licensed Federal Firearms License (FFL) dealer. A background check and any required local paperwork must be completed before you can take possession.
How It Works
- Place your order online.
- Send us your preferred local FFL dealer info.
- We ship the firearm to your FFL after verification.
- Complete background check and pick up from your dealer.
Legal Compliance
We comply with all U.S. firearm laws. It is your responsibility to ensure the Sig Sauer M400 SDI Sentry 5.56 NATO 16in ROMEO5 Combo is legal in your state. Some states (e.g. CA, NY, NJ) have restrictions on magazine size or certain features.
Helpful Links
Need help? Contact our team — we’ll guide you through selecting a compliant firearm and the FFL transfer process.
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