Will a .450 Bushmaster Upper Fit a .223 Lower?

When it comes to customizing and building your AR-15 rifle, you may find yourself wondering about the compatibility of different caliber uppers and lowers. One common question that arises is “ Will a .450 Bushmaster upper will fit a .223 lower? “ In this article, we’ll dive into this topic to provide you with a clear understanding of caliber compatibility.

Understanding the AR-15 Platform

The AR-15 is a versatile firearm platform known for its modularity. This modularity allows gun lovers to swap out various components, including upper receivers, barrels, and magazines, to create rifles tailored to their specific needs and choices.

Caliber Compatibility

The compatibility of an upper and lower receiver largely depends on the caliber of the upper and the specifications of the lower. Let’s break down the factors involved in determining Will a .450 Bushmaster Upper Fit a .223 Lower.

Magazine Well Compatibility

The magazine well on an AR-15 lower receiver is designed for a specific caliber. A .223/5.56 lower typically has a smaller magazine well to accommodate .223 Remington or 5.56x45mm NATO cartridges. In contrast, a .450 Bushmaster cartridge is significantly larger in diameter.

Buffer and Spring

The .450 Bushmaster cartridge generates more recoil and requires a different buffer and spring setup than the .223/5.56. Therefore, the buffer and spring in a .223 lower may not be suitable for the increased recoil of the .450 Bushmaster.

Bolt and Bolt Carrier Group

The bolt and bolt carrier group in a .450 Bushmaster upper are specifically designed to handle the larger cartridge. Using a .223 bolt carrier group with a .450 Bushmaster upper is not recommended, as it may result in malfunctions or damage.

So, Will They Fit?

In most cases, a .450 Bushmaster upper will not fit directly onto a .223 lower without modification. The differences in magazine well size, buffer and spring requirements, and bolt carrier group make these two calibers incompatible without adjustments.

Considerations for Compatibility

If you are interested in building a rifle that can switch between .450 Bushmaster and .223/5.56, you have a few options:

Multi-Caliber Lower: Some manufacturers offer multi-caliber lower receivers that are compatible with both .450 Bushmaster and .223/5.56 uppers. These lowers have a larger magazine well to accommodate the .450 Bushmaster, but they can still accept .223/5.56 uppers.

Dedicated Uppers: Alternatively, you can have separate uppers—one for .450 Bushmaster and another for .223/5.56—and switch them out as needed. Just ensure you have the appropriate lower and components for each caliber.


In summary, while the AR-15 platform is highly customizable, caliber compatibility is a critical consideration. A .450 Bushmaster upper typically won’t fit directly onto a .223 lower due to differences in magazine well size, buffer and spring requirements, and bolt carrier group specifications. To build a rifle that can handle both calibers, consider multi-caliber lower receivers or dedicated uppers to ensure safe and reliable operation.

Before making any modifications or purchasing components, it’s essential to consult with firearm experts or manufacturers to ensure compatibility and safety.

Please note that firearm regulations can vary by location, so always ensure you are in compliance with local laws and regulations when modifying or building firearms.