.450 Bushmaster AR-15

Building, shooting and keeping a home-built AR-15 in .450 Bushmaster: the four parts that differ from a 5.56, what the barrel and bolt makers publish about gas, buffer, ports and threads, a corrected zero, and the Michigan rules that still bind a semi-automatic deer rifle after the straight-wall zone's repeal.

Firearms

.450 Bushmaster AR-15 · Deep Dive

.450 Bushmaster AR-15

Three makers publish three different muzzle threads for .450 Bushmaster barrels, and Bear Creek lists its .450 barrel with a 5/8 x 32 thread that many .30-calibre muzzle devices will screw onto.

Building, shooting and keeping a home-built AR-15 in .450 Bushmaster: the four parts that differ from a 5.56, what the barrel and bolt makers publish about gas, buffer, ports and threads, a corrected zero, and the Michigan rules that still bind a semi-automatic deer rifle after the straight-wall zone's repeal.

Photo: 223+450 magazine top by Hellbus · CC BY-SA 4.0

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    Volumes

    1. Vol 1 Why the .450 Bushmaster Exists — and the Rifle These Volumes Are Written Around The cartridge's history now lives in its own dive; this volume keeps the short version, corrects what the first edition attributed to a source that does not say it, and sets out my rifle part by part
    2. Vol 2 The Cartridge, as It Bears on the Rifle — A Summary of the Cartridge Dive, With Corrections The SAAMI numbers a builder needs, the rebated rim that dictates the bolt, the mouth that sets headspace, why box velocities are not comparable, what twist the makers really use, and the expansion floors the first edition said nobody publishes
    3. Vol 3 The Michigan Question — The Rule This Cartridge Was Bought For No Longer Exists The limited firearms deer zone's rule as it was posted, its repeal as the order now prints it, and the rules that still bind a .450 AR-15 in Michigan: six shells, the quiet period, transport and legal hours
    4. Vol 4 Building and Feeding One — The Bolt, the Barrel, and the Magazine Problem A .450 AR-15 is a calibre conversion: what carries over from a 5.56, the four parts that do not, what the barrel and bolt makers now publish about gas, buffer, ports and muzzle threads, and why the follower is not optional
    5. Vol 5 Shooting It — Recoil, Optics, Zero, and What the Accuracy Claims Are Worth The zero that suits a 200-yard cartridge, recomputed with the sight height actually applied; recoil against the slug gun it replaced; wind; and the measurements my rifle still owes
    6. Vol 6 Owning One — Brass, Cost, and the Alternatives Now That the Rule Is Gone What a .450 AR-15 costs to feed on the makers' own October 2026 prices, the brass that is now confirmed on its maker's page, and how it compares with the .350 and .400 Legend, the slug gun, the .45-70 and the other big-bore AR cartridges
    7. Vol 7 How the .450 AR Works — and Telling Its Parts From 5.56 The AR-15's cycle with the four places a .450 changes it: the bolt face, the barrel, the ejection port and the magazine; and how to tell a .450 upper, bolt or magazine from the 5.56, .350 Legend and .458 SOCOM parts it is mixed up with
    8. Vol 8 Keeping It Working — Field Strip, Detail Strip, Maintenance and Repair Only what differs from a 5.56 AR-15: the bolt that is headspaced to its barrel, the big-bore extractor that wants no extra tension, the chamber step that is the headspace datum, and a malfunction table that reads ejection differently