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

Figure 1 — .450 Bushmaster on the left, .223 Remington on the right. The whole design problem is in one frame: a case half an inch across the body has to work through a receiver built around the slim cartridg… — Figure 1 — .450 Bushmaster on the left, .223 Remington on the right. The whole design problem is in one frame: a case half an inch across the body has to work through a receiver built around the slim cartridge beside it. Not an original photograph. Source: commons.wikimedia.org, File:450 BM + 223.jpg, Hellbus, CC BY-SA 4.0.

This dive is about one rifle type: an AR-15 chambered in .450 Bushmaster, and in particular mine, which I built on a Black Rain Ordnance lower. When it was first published on 17 September 2026 its first three volumes also carried the cartridge’s history, its SAAMI specification, its factory loads and ballistics, and the Michigan law that made it popular. Those subjects now have their own dive, .450 Bushmaster, which re-read every source in October 2026. Volumes 1 to 3 here are now short summaries that link to it, and keep only what is about the rifle.

1.1 What these volumes argue

Four claims run through the eight volumes.

A .450 AR-15 is a 5.56 AR-15 with four different parts: the barrel, the bolt with its extractor and ejector, the magazine follower, and usually an ejection port opened out to clear the case. Volume 4 is the build, Volume 7 the mechanism and how to tell the parts apart, Volume 8 the maintenance and repair.

It is a 200-yard rifle, and the reasons are arithmetic, not opinion: an 8-inch point-blank range of about 190 to 210 yards, a 10 mph wind moving the bullet about ten inches at 200, and, for the one supersonic bullet whose maker publishes it, an expansion floor reached at about 200 yards. Volume 5.

The Michigan rule that sold most of these rifles has been repealed. Volume 3.

What the repeal does not do is make the rifle pointless. It recoils about a third less than the slug gun it replaced, does the job inside 200 yards, and is already built. Volumes 5 and 6.

1.2 The cartridge in one paragraph

The .450 Bushmaster is a straight-walled rifle cartridge firing a .452-inch bullet from a 1.700-inch case, with a .473-inch rebated rim under a .500-inch case head, a maximum overall length of 2.260 inches, and a SAAMI maximum average pressure of 38,500 psi. It headspaces on its case mouth. SAAMI’s current standard (Z299.4-2025) lists reference loads from 250 grains at 2,250 fps to a 395-grain subsonic at 1,050, all from a 24-inch test barrel. Everything about it, read off the current drawings and every maker’s own page, is in the cartridge dive’s Volumes 2 to 6.

1.3 Where it came from, in brief

The lineage is usually traced to Jeff Cooper’s “Thumper” concept: a large, heavy bullet from a self-loading rifle. Tim LeGendre of LeMag Firearms designed the cartridge as the .45 Professional and licensed it to Bushmaster Firearms International, and it was standardised by SAAMI as the 450 Bushmaster; SAAMI’s test-barrel drawing is dated 9 September 2007 and its cartridge drawing 1 February 2008.¹² The case head is the .284 Winchester’s, line for line on the SAAMI drawings, which is where the .473 rim comes from.²

The company it is named after now lists no .450 rifle, and no trademark registration for “450 BUSHMASTER” was found; Hornady, Federal, Remington, Winchester and smaller loaders make the ammunition. The full account, with what can and cannot be pinned to a dated document, is the cartridge dive’s Volume 1.

1.4 Correction: what the first edition said that its source does not

⚠ Correction (19 October 2026). The first edition of this volume (17 September 2026) told the cartridge’s history from the Wikipedia article on the .450 Bushmaster, and attributed it to “Philip Massaro, ‘Behind the Bullet: .450 Bushmaster,’ American Hunter, 16 November 2022,” which that article cites. Massaro’s article, read on its own page in October 2026, does not contain most of what was attributed to it.¹ It supports: the Thumper lineage, LeGendre and LeMag, the .45 Professional name, the licence to Bushmaster, a 2007 date, the .284 Winchester parent case and the small rifle primer. It does not contain:

Table 1 — ⚠ Correction (19 October 2026). The first edition of this volume (17 September 2026) told the cartridge's history from the Wikipedia article on the .450 Bushmaster, and attributed it to "Philip Massaro, 'Behind the Bullet: .450 Bushmaster,' American Hunter, 16 November 2022," which that article cites. Massaro's article, read on its own page in October 2026, does not contain most of what was attributed to it.[^1] It supports: the Thumper lineage, LeGendre and LeMag, the .45 Professional name, the licence to Bushmaster, a 2007 date, the .284 Winchester parent case and the small rifle primer. It does not contain

Claim in the first edition	Status now
Original .45 Professional case 1.772 in, overall 2.362 in	Not in Massaro. Unverified; not repeated.
Hornady asked for the case to be shortened to suit its existing 250 gr bullet	Not in Massaro. Plausible, unverified.
LeGendre built an AR-10 in .45 Professional and delivered it to Cooper	Not in Massaro. Unverified; not repeated.
Rights sold to Cerberus Capital Management in 2006	Not in Massaro. Unverified; withdrawn.
Franklin Armory bought the cartridge and company in 2021	Not in Massaro. Unverified; withdrawn.
SAAMI cartridge drawing “issued 02/01/2008”, cited to the 2015 standard	The date is right but is printed on the 2025 drawing; the 2015 drawing carries no visible issue date.
Rifles chambered by Mossberg, Remington bolt guns, Ruger No. 1, Ruger AR-556 MPR, Franklin XO-26, Bushmaster	Not current on the makers’ pages in October 2026. Ruger (bolt guns, 1:16) and Savage (bolt guns, 1:24) are; see §1.5.
The parent case is “usually given” as the .284 Winchester, unconfirmed	Confirmed by SAAMI’s drawings: the case head is the .284’s.

The two sections that told the ownership story (§1.4 and §1.5 of the first edition) and the list of makers (§1.6b) are therefore removed rather than corrected. The cartridge dive’s Volume 1 sets out what is documented and what is only plausible.

1.5 Who makes rifles and ammunition for it now

Rifles, on the makers’ own pages in October 2026: Ruger’s American Gen II, American Ranch Gen II and Hawkeye Hunter (1:16 twist, 11/16-24 thread) and Savage’s 110 Trail Hunter Lite V2, Ridge Hunter and Scout V2 (1:24, 11/16x24). AR-15s are mostly parts-built: an upper with a .450 barrel and bolt on any lower. Bear Creek Arsenal and Tromix sell .450 barrels, bolts and complete uppers; Ruger made an AR-556 MPR in .450 from 2018 but lists none now.³⁴⁵

Ammunition: Hornady (four loads, 245 to 395 grains), Federal (three), Remington (three), Winchester (two) and Steinel, with Starline making brass.³ Volume 6 has the prices.

1.6 The problem it had to solve in an AR-15

An AR-15 is built around the 5.56×45mm: a .378-inch case head that stacks in two staggered columns. A case big enough for a 250-grain .452 bullet at deer velocities is about .500 inch across. Two things give:

The rim is rebated to .473, the .284 Winchester’s rim and the .308 family’s bolt-face size. The bolt is not an AR-15 bolt, but its face is a size the industry already tools for, and the .458 SOCOM uses the same one, which is why bolts are sold as “.450/.458”. Volume 7 §7.2 draws the faces to scale.

The magazine goes to a single column. A .500-inch body will not stagger in an AR-15 magazine, so the rounds stack in one centred column: four in a ten-round body. It needs its own follower. My rifle runs Lancer four-round magazines with Tromix followers, and Volume 4 §4.6 explains why the follower is not optional.

Figure 2 — The two magazines seen from above: staggered .223 on one side, a single column of .450 Bushmaster on the other. Not an original photograph. Source: commons.wikimedia.org, File:223+450 magazine top.… — Figure 2 — The two magazines seen from above: staggered .223 on one side, a single column of .450 Bushmaster on the other. Not an original photograph. Source: commons.wikimedia.org, File:223+450 magazine top.jpg, Hellbus, CC BY-SA 4.0.

1.7 Michigan, in one paragraph

For twelve years Michigan’s “limited firearms deer zone” allowed only straight-walled rifle cartridges of .35 calibre or larger with a case between 1.16 and 1.80 inches, and the .450’s 1.700-inch case fitted that rule almost exactly while the .45-70’s 2.105 did not. The zone was repealed effective 12 September 2026, and the order now says so. The six-shell limit on semi-automatics is unchanged. Volume 3 has the rule as it was, the repeal, and what still binds this rifle; the cartridge dive’s Volume 7 covers Michigan and the four other straight-wall states.

1.8 The rifle documented here

I built this rifle at home, on a Black Rain Ordnance lower. Its configuration as recorded in my inventory in September 2026:⁶

Table 2 — 1.8 The rifle documented here


Lower	Black Rain Ordnance
Upper	Aero
Barrel	Bear Creek Arsenal .450 Bushmaster, 16 inch, 4150 chrome-moly vanadium, parkerized, heavy profile, 1:24 twist
Bolt carrier group	Black nitride .450/.458 BCG, MPI-marked bolt
Handguard	15-inch M-LOK free-float rail
Stock	Luth-AR MBA-3 with cheek riser
Recoil pad	Luth-AR 3-axis, soft 35 durometer
Trigger	RISE Armament RAVE 140 drop-in
Bipod	Harris, M-LOK (shared between rifles)
Optic	Vector Optics Paragon 3-15×50, 30 mm tube (optics move between rifles)
Optic mount	Primary Arms offset mount
Magazines	Lancer four-round
Followers	Tromix, marked “must use”
Ammunition	Hornady .450 Bushmaster 250 gr FTX

Not recorded: the gas block, gas tube length, buffer and spring, the muzzle thread, the upper’s ejection-port type, the BCG’s maker and the Lancer magazines’ date code. Volume 7 §7.6 lists what each would tell.

⭐ The barrel is 16 inches. An earlier record listed the rifle as an 18-inch build while its barrel line named a 16-inch Bear Creek barrel; the 16 inches is the confirmed figure. That is four inches shorter than the 20-inch barrel Hornady publishes its velocities from and eight shorter than SAAMI’s 24-inch test barrel, so every published velocity overstates what this rifle does, by an amount nobody has measured (Volume 5 §5.5).

Bear Creek Arsenal’s own listing for its current barrel of this exact description (16-inch, parkerized, heavy, 1:24) gives a carbine-length gas system, a 0.875-inch gas block journal and a 5/8 x 32 muzzle thread, and recommends Hornady BLACK 250-grain ammunition.⁷ Whether my barrel, bought earlier, matches that listing on every point has not been checked (Volume 4 §4.3).

1.9 Reading order

Table 3 — 1.9 Reading order

Vol	Subject
1	Why the cartridge exists, and the rifle (this volume)
2	The cartridge as it bears on the rifle: a summary of the cartridge dive, with the corrections
3	Michigan: the rule, its repeal, and what still binds the rifle
4	Building and feeding one: bolt, barrel, gas, buffer, magazine, threads
5	Shooting it: zero, recoil, optics, wind, and the measurements still owed
6	Owning one: brass, cost, and the alternatives now the rule is gone
7	How it works, step by step, and telling .450 parts from 5.56, .350 Legend and .458 SOCOM parts
8	Field strip, detail strip, maintenance and repair: what differs from a 5.56

Handloading is a separate dive, Loading the .450 Bushmaster.

1.10 References (Vol 1)

Philip Massaro, “Behind the Bullet: .450 Bushmaster,” American Hunter (National Rifle Association), posted 16 November 2022, read on its own page by the cartridge dive, 19 October 2026. URL: https://www.americanhunter.org/content/behind-the-bullet-450-bushmaster. ↩ ↩²
ANSI/SAAMI Z299.4-2025, 450 Bushmaster cartridge and chamber drawing (printed p.156; issued 1 February 2008, revised 23 August 2022) and velocity and pressure test barrel (p.369; issued 9 September 2007, revised 2 August 2023), as read in the cartridge dive, Volumes 1 and 2. URL: https://saami.org/wp-content/uploads/2026/04/SAAMI-Z299.4-CFR-2025-Centerfire-Rifle-Approved-2-10-2025-2026-04-27.pdf. ↩ ↩²
Makers’ pages for rifles and ammunition, read 19 October 2026, as listed in the cartridge dive’s Volumes 3 and 8. ↩ ↩²
Tromix Corporation, “.450 Bushmaster Barrels, Uppers & Parts” listings, read 19 October 2026. URL: https://tromix.com/product-category/450-bushmaster/. ↩
Sturm, Ruger & Co., news release “AR-556 MPR Now Chambered in .450 Bushmaster”, 26 September 2018, read 19 October 2026. URL: https://ruger.com/news/2018-09-26a.html. ↩
My inventory, mirrored without serial numbers in the hub’s inventory file, September 2026. ↩
Bear Creek Arsenal, “BC-15 .450 Bushmaster, Parkerized 16” Heavy Barrel, 1:24 Twist, Carbine Length Gas System”, SKU 1817-B450CHB16124(P).875, read 19 October 2026. URL: https://www.bearcreekarsenal.com/450-bushmaster-16-parkerized-hb-1-24-twist-carbine.html. ↩