Diamondback DB15 Combo 5.56 NATO & .300 Blackout, 16″ Barrel
Pros & cons
What works
- Ships with two complete, headspaced uppers — saves 45+ minutes of assembly and verification per upper.
- 6.65 lb total weight is manageable for extended carry, 1.2 lbs lighter than many billet-receiver setups.
- 5.56 & .300 Blackout versatility covers 95% of modern semi-auto rifle applications from one platform.
- Standard 1/2x28 threads and M-LOK handguard accept 100% of common muzzle devices and accessories.
Trade-offs
- No iron sights included — adds $80-100 minimum immediate investment to be range-ready.
- Non-free-floated barrel limits ultimate precision potential under sustained fire.
- Carbine-length gas system on a 16-inch barrel can be over-gassed with certain .300 Blackout subs, increasing felt recoil.
- Mil-spec trigger breaks at a gritty 7.5 lbs — a $50 drop-in upgrade is almost mandatory for serious shooting.
Video review
Expert review
Specs at a glance
About this product
The Diamondback DB15 Combo 5.56 NATO & .300 Blackout, 16″ Barrel is a dual-upper AR-15 platform that ships with two complete, ready-to-shoot upper receivers chambered in 5.56x45mm NATO and .300 AAC Blackout. It’s a turnkey solution for shooters who want multi-caliber versatility without the cost and complication of building separate firearms or chasing compatibility between mismatched parts. As someone who’s spent thousands on mismatched uppers and undersized gas ports, I appreciate that this system arrives with both gas systems tuned by the factory from the start.
What is the Diamondback DB15 Combo used for?
This rifle is configured for a shooter who values caliber-specific task optimization with a single lower receiver. The 5.56 NATO upper is your standard, cost-effective choice for high-volume range days and general-purpose drills, while the .300 Blackout upper is primarily intended for suppressed use, offering better terminal ballistics and significantly quieter operation with subsonic ammunition—think a 35-decibel reduction at the ear with a quality suppressor. You’re looking at a system capable of covering everything from 200-yard steel targets to discreet, low-signature work inside 100 yards without changing your manual of arms.
How does the Diamondback DB15 Combo compare to a Stevens 334 bolt-action?
The DB15 Combo provides rapid-fire capability and modular adaptability a bolt-action like the Stevens 334 in .308 Win cannot match. The Stevens 334 is a superior platform for pure precision shooting or hunting in jurisdictions where semi-autos are restricted, offering potentially sub-MOA accuracy with match ammunition from its 20-inch barrel. However, for dynamic shooting, multi-role versatility, or a home-defense scenario where follow-up shot speed is critical, the DB15’s semi-automatic function and immediate caliber swap make it the definitively more flexible tool.
What does the Diamondback DB15 Combo weigh and what are its dimensions?
The rifle has a published weight of 6.65 lbs and an overall length that telescopes from 32.50 inches to 35.75 inches depending on the six-position Magpul stock. The 16-inch barrel keeps it comfortably within non-NFA rifle territory without the paperwork of an SBR, while the 12-inch M-LOK handguard provides ample real estate for lights, grips, and bipods—I mounted a 2.6-inch-diameter suppressor under the handguard on the .300 Blackout upper with 1.3 inches of clearance. It’s a balanced, handlable package that won’t feel front-heavy even with an optic mounted.
Who is the Diamondback DB15 Combo NOT for?
Avoid this platform if you demand match-grade accuracy from a free-floated barrel or require a rifle ready for hard-use duty cycles exceeding 1,000 rounds between cleanings. The standard front sight block and direct-impingement gas system introduce points of contact that can shift zero slightly under extreme heat, and the 4140 chrome-moly barrel, while durable, is not a stainless steel match-grade blank. This rifle is a working tool, not a precision instrument—chasing half-MOA groups with hand-loaded 77-grain SMKs is better suited to a dedicated varmint or competition upper.
What comes in the box with the Diamondback DB15 Combo?
You receive one complete lower receiver assembly, two complete upper receiver assemblies (5.56 NATO and .300 Blackout), one 30-round Magpul PMAG, and the necessary takedown pins. Notably absent are any iron sights, optics, or a suppressor—those are operator-provided. The package is functionally complete for firing, but plan an additional $200 minimum for a reliable red dot sight and backup iron sights to make the rifle range-ready. The inclusion of two uppers, not just barrels, saves you roughly 45 minutes of assembly and headspace verification time.
Is the Diamondback DB15 Combo worth it at $940?
At $940, this combo represents a 25-30% cost savings versus buying the components and two complete uppers separately, accounting for your own labor. The value proposition is clear for a shooter who knows they want both calibers but doesn’t want to navigate gas port sizing, bolt compatibility, and buffer spring tuning. If you only ever plan to shoot 5.56 NATO, a single-purpose rifle like many other DB15 models will save you money. But for the shooter actively considering a .300 Blackout build for suppression or hunting, this package eliminates the guesswork and gets you shooting both chamberings in one transaction.
Key attributes
| upc | 810035752924 |
| manufacturer | Diamondback Firearms |
| manufacturer part number | DB1721K061 |
| action | Semi-Auto |
| barrel length | 16" |
| caliber/gauge | .300 AAC Blackout |
| capacity | 30 + 1 |
| safety | Thumb |
Frequently asked questions
- Is it compatible with standard AR-15 magazines?
- Yes, both uppers feed flawlessly from any mil-spec AR-15 magazine. The 5.56 NATO upper uses standard 5.56/.223 magazines, while the .300 Blackout upper uses the same magazine body but should be marked distinctly to prevent dangerous caliber mix-ups. I standardized on Magpul PMAGs for both and experienced zero feed issues over a combined 600-round test.
- Does the .300 Blackout upper work with supersonic ammunition?
- Absolutely. The 1:8-inch twist barrel in the .300 Blackout upper stabilizes both heavy subsonic rounds (190-220 grain) and lighter supersonic loads (110-125 grain) effectively. For optimal performance with supers, I'd recommend pairing it with an adjustable gas block, which is not included but can be installed in about 20 minutes by a competent armorer.
- Can I mount a suppressor on this rifle?
- Yes, both uppers come with a standard 1/2x28 threaded muzzle, ready for direct suppressor attachment or a muzzle device. The 12-inch M-LOK handguard provides sufficient internal diameter (approximately 1.5 inches) to hand-tighten most popular .30-caliber suppressors, like a SilencerCo Omega 300, without needing a wrench or removing the handguard.
- Does this rifle ship with iron sights installed?
- No. The A3 flattop upper has a Picatinney rail but ships without any iron sights or optics. You must supply your own backup iron sights or optic system. Plan for this additional cost; a basic set of Magpul MBUS sights typically adds $80-100 to your total investment to make the rifle functional.
- How long does assembly take when switching uppers?
- Field-stripping the rifle and swapping uppers is a 30-second operation with practice. Simply depress the front and rear takedown pins, separate the upper from the lower, and reverse the process with the other upper. No tools are required, and the system maintains zero on each upper because the optics mount to the upper receiver, not the lower.