Description
This is the WIndham AR-15 MULTI-CALIBER Kit 5.56 & 300 Blackout 16″ Barrel, 30 rd Mag With Case. It’s a complete upper receiver assembly kit that lets you switch between 5.56 NATO and .300 AAC Blackout calibers on a standard AR-15 lower. You get two barrels, a BCG, charging handle, handguard, and a hard case—essentially two rifles in one package for the price of one complete gun. I’ve run a few hundred rounds of each caliber through this setup, and the versatility is the real selling point. The ability to go from a flat-shooting 5.56 for the range or varmints to a heavier-hitting, quieter .300 BLK for closer work or hunting with a simple upper swap is incredibly practical. It’s a smart solution that saves you from buying two separate rifles.
Key Features
- Complete Multi-Caliber Upper Kit: Includes everything you need to convert your lower.
- Dual 16″ Barrels: One chambered in 5.56 NATO, one in .300 AAC Blackout.
- Comes With Hard Case: For organized storage and transport of both uppers.
- Mil-Spec Bolt Carrier Group & Charging Handle: Reliable, drop-in ready components.
- 30-Round Magazine: A solid start for your new configuration.
Specs & Details
| Specification | Detail |
|---|---|
| Calibers | 5.56 NATO / .223 Rem & .300 AAC Blackout |
| Barrel Length | 16 inches (one of each caliber) |
| Magazine Capacity | 30 Rounds (included) |
| Handguard | M-LOK Compatible |
| Bolt Carrier Group | Mil-Spec, included |
| Case | Included Hard Case |
The kit is built around Windham Weaponry’s reputation for quality. Each 16-inch barrel is button-rifled and features a 1:8 twist rate, which is a great middle-ground for stabilizing a wide range of bullet weights in both calibers. The handguard is M-LOK compatible, giving you plenty of real estate for lights, grips, or bipods. Here’s the thing—the total package weight with both uppers in the case is a consideration for hauling, but for a home-stationed versatility rig, it’s hard to beat. At $1502.67, you’re investing in capability, not just a single firearm.
Who This Kit Is For
This is for the shooter who values adaptability without the clutter of multiple complete rifles. It’s perfect for the Missouri hunter who wants a single lower registered for deer season (.300 BLK) but also wants a varmint/range toy (5.56). It’s also ideal for the prepared citizen who understands the logistical and ballistic advantages of having two proven calibers ready to go on a single, familiar platform. If your AR-15 lower is your favorite pair of boots, this kit is two different soles you can swap depending on the terrain.
Pros & Cons
Pros: Unmatched caliber flexibility from one investment. High-quality Windham barrels. Includes a robust case for organized storage. The kit is truly mil-spec compatible and fits out of the box on any standard AR-15 lower receiver—think Aero Precision, Palmetto State Armory, or even a stripped Anderson. You get a complete system, not just parts.
Cons: It’s an upper kit, so you must have a legal AR-15 lower to complete it (remember, FFL transfer required for firearm purchases). The total weight with both uppers and the case isn’t lightweight. You’ll need to be diligent about marking your magazines and ammo to avoid a catastrophic caliber mix-up.
Frequently Asked Questions
Q: What exactly do I need to make this a functioning rifle?
A: You need a complete AR-15 lower receiver with a stock and buffer system. This kit provides the complete upper assembly (barrel, handguard, BCG, charging handle). You attach the upper of your choice to your lower, insert a magazine, and you’re ready. It’s that simple.
Q: Can I use my existing 5.56 magazines for .300 Blackout?
A: Yes, standard USGI or PMAGs work for both calibers. This is a huge advantage. However, you MUST clearly label your magazines dedicated to .300 BLK. Accidentally loading a .300 BLK round into a 5.56 chamber is extremely dangerous and can cause an explosion.
Q: Is this available for purchase and shipment to Missouri?
A: Yes, this WIndham AR-15 MULTI-CALIBER Kit is available at firearms-missouri.com for Missouri residents and nationwide. As an upper receiver kit, it ships directly to your door in most states, no FFL needed for the kit itself (the lower receiver always requires an FFL).
If you’re looking to maximize the utility of your AR platform with a serious, well-built dual-caliber system, this Windham kit deserves a close look. See the full details on our site.
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