The Cartridge, as It Bears on the Rifle — A Summary of the Cartridge Dive, With Corrections

2.1 Where the cartridge material went

The first edition of this volume (17 September 2026) read the SAAMI drawing, surveyed the factory loads, fitted a ballistic model and discussed terminal behaviour. All of that is now done better, against the current standard and every maker’s own page, in the .450 Bushmaster cartridge dive:

Table 1 — The first edition of this volume (17 September 2026) read the SAAMI drawing, surveyed the factory loads, fitted a ballistic model and discussed terminal behaviour. All of that is now done better, against the current standard and every maker's own page, in the [.450 Bushmaster cartridge dive](/ammo-and-ballistics/450-bushmaster/)

Subject	Where it lives now
The SAAMI 2025 drawing, the .284 parent, pressure, the neighbours	Volume 2
Every factory load, on its maker’s page, with prices	Volume 3
Test barrels, barrel length, twist, subsonic loads	Volume 4
Trajectory, wind, range	Volume 5
Terminal performance on deer, hogs and bears	Volume 6

What stays here is what the cartridge does to the rifle, and the corrections this volume owes (§2.7).

2.2 The numbers a builder needs

From ANSI/SAAMI Z299.4-2025 (approved 10 February 2025), cartridge and chamber drawing for 450 Bushmaster, as read in the cartridge dive:¹

Table 2 — 2.1 The numbers a builder needs

Dimension	Value (in)	Why the rifle cares
Bullet diameter	.4525 −.0030	.452 handgun-bullet family, not .458; the muzzle device must clear it
Groove / bore	.450 / .442	barrel
Case head	.500	why the magazine single-stacks (Volume 4 §4.6)
Rim	.473 −.010	why the rifle needs a .450/.458 bolt (§2.2)
Extractor groove	.409 −.010	the extractor
Rim bevel	35° +20° (2015 edition: 45° ±10′)	none in practice
Case length	1.700 −.010	is the headspace dimension (§2.2)
Chamber headspace	1.700 min / 1.710 max	the datum a headspace gauge checks
Overall length	2.000 min / 2.260 max	the AR-15 magazine well
Twist	24 in, R.H., marked OPTIONAL	§2.4
Maximum average pressure	38,500 psi (transducer only)	a modest load on bolt and barrel

SAAMI’s current reference loads are 250 gr at 2,250 fps, 260 gr at 2,150, 300 gr at 1,900 and 395 gr at 1,050; the last two are new in the 2025 edition and the 395 is subsonic.¹

2.3 The rebated rim, the bolt, and the mouth

The case head is .500 and the rim .473. A rim smaller than the head is a rebated rim, and it is the decision that makes the cartridge work in an AR-15. The .473 dimension is the .284 Winchester’s rim, which is also the size of the .308 family’s bolt face, so a .450 bolt is a face size the industry already tools for. It is not a 5.56 bolt face, which is .378. The rifle needs its own bolt, with its own extractor and ejector (Volume 4 §4.2, Volume 7 §7.2). The .458 SOCOM shares the .473 rim, so one bolt serves both.

It headspaces on the case mouth. The ⊗ headspace symbol on the drawing sits on the chamber’s mouth step, 1.700 to 1.710, and SAAMI’s headspace gauge table lists the .450 Bushmaster beside the .30 Carbine, the other mouth-headspacing straight wall in the rifle standard.¹ Three consequences for the rifle:

• Case length is headspace. An over-trimmed case sits deeper and headspaces long.
• A heavy roll crimp deforms the datum surface.
• Carbon on the chamber step sits on the datum (Volume 8 §8.3).

What a headspace number means, and how to measure one, is in the Headspace dive; the cartridge dive’s Volume 2 reached the same reading of the drawing.

2.4 The test barrel is 24 inches, and almost nobody’s box is

This was the first edition’s central correction and it still stands. SAAMI’s velocity and pressure test barrel for the .450 Bushmaster is 24 inches, 1:24 twist.¹ Hornady publishes from a 20-inch barrel and says so; Federal and Remington state no barrel length at all.² So published .450 velocities are not on one basis, unlike the .45-70’s, which all trace to SAAMI’s 24 inches.

The cartridge dive’s Volume 4 sharpens it: inside SAAMI itself, the .350 and .400 Legend are standardised on 16-inch test barrels and the .360 Buckhammer on 20, so a box of Legend ammunition describes a carbine and a box of .450 describes a barrel no AR owner carries.

For my rifle, with its 16-inch barrel, every published .450 velocity is an upper bound (Volume 5 §5.5). No maker publishes a velocity loss per inch for supersonic .450, and none is given here.

2.5 Twist: the makers have not converged

⚠ Correction (19 October 2026). The first edition said “in practice the industry has converged on 1:24.” It has not. On the makers’ own spec sheets in October 2026, Ruger’s whole current .450 line is 1:16; Savage’s is 1:24, as are SAAMI’s reference barrel, Bear Creek’s .450 barrels (including mine) and Tromix’s 16 and 18-inch .450 barrels. Tromix’s 12.5-inch barrel is 1:20.³⁴⁵

For supersonic factory loads it makes no difference. The cartridge dive computes Miller stability factors of about 4.0 at 1:24 for Federal’s 260 gr Fusion and 300 gr Power-Shok, against a working minimum of about 1.4.

For the heavy subsonic loads it may be the whole question. Steinel states that its 425-grain copper subsonic “will not stabilize with 1:24”. Hornady gives no twist for its 395-grain Sub-X; the arithmetic says a 395-grain bullet at 1,050 fps stays above 1.4 at 1:24 only if it is shorter than about 1.24 inches, and at 1:16 up to about 1.66.³ So my 1:24 barrel is right for the 250-grain FTX it is set up for, and whether it will stabilise the Sub-X is a question only a target at 100 yards can answer (owner question 4).

2.6 Factory loads, and the floors makers do publish

The full table, with every maker’s page, is the cartridge dive’s Volume 3. The points that change what the first edition said:

• Hornady publishes a G1 BC of .210 for the 250 gr FTX (items 82246 and 82244), and lists four .450 loads, not one: the two FTX loads, a 245 gr InterLock and the 395 gr Sub-X subsonic, all from a 20-inch barrel. The first edition fitted a BC of .2112 to Hornady’s velocity table because the BC was not read; Volume 5 now uses Hornady’s .210.²
• 🔴 Makers DO publish expansion floors. The first edition said Hornady publishes none for the FTX and none was located for any other .450 bullet. True of the FTX; not true of the field. Federal’s pages give a “Terminal Performance Minimum Velocity” of 1,400 fps for the 260 gr Fusion and 1,000 fps for the 300 gr Power-Shok; Hornady says its Sub-X bullet expands at velocities “as low as 900 fps”.² The Fusion reaches its floor at about 206 yards from Federal’s own muzzle velocity, and about 183 if it starts 100 fps slower; the Power-Shok at about 414 yards; the Sub-X at about 243 from 20 inches and under 100 from a barrel 100 fps slower.
• Federal’s prices fell between the two readings: Fusion $76.99, Power-Shok $68.99, American Eagle $39.99 per 20 on 19 October 2026 (first edition: $79.99, $71.99, $41.99).²
• Winchester’s pages now show no ballistics, so the Winchester row of the first edition’s table (2,200 fps, BC .210) cannot be traced to a current maker’s page and is dropped.
• Remington’s AccuTip BC of .144, flagged in the first edition as a possible catalogue error, is consistent with Remington’s own published downrange table: the cartridge dive’s solver reproduces that table with .144. It is the maker’s figure and it is internally consistent.

The first edition’s “factory field” and “energy” figures are withdrawn; the cartridge dive’s velocity figure, with the published floors drawn across it, replaces both.

2.7 Terminal performance

No independent terminal testing (gelatin, recovered weights, expansion diameters, by someone without a product to sell) was found for any .450 Bushmaster factory load, in either dive’s search. What the makers publish, and what the published record says about deer, hogs and bears, is the cartridge dive’s Volume 6. The mechanism argument stands: a .452 bullet starts wider than most rifle bullets end up, and on deer-sized game it tends to exit.

2.8 Corrections to the first edition, in one table

Table 3 — 2.7 Corrections to the first edition, in one table

First edition (17 Sep 2026)	Now (19 Oct 2026)
SAAMI drawing read from the 2015 edition: rim bevel 45° ±10′; two reference loads (250, 260 gr)	2025 edition: rim bevel 35° +20°; four reference loads, adding 300 gr and a 395 gr subsonic
”In practice the industry has converged on 1:24”	Ruger 1:16, Savage 1:24 (§2.4)
G1 BC .2112 fitted for the FTX	Hornady publishes .210; Volume 5 uses it
No maker publishes an expansion floor for any .450 bullet	Federal: Fusion 1,400 fps, Power-Shok 1,000; Hornady Sub-X 900
Only one Hornady load (82246) could be confirmed	Four Hornady loads, including the 395 gr Sub-X
Federal prices $79.99 / $71.99 / $41.99	$76.99 / $68.99 / $39.99 (19 Oct 2026)
Winchester Deer Season XP: 2,200 fps, BC .210	Winchester’s page shows no ballistics; row dropped
AccuTip BC .144 “may be a catalogue error”	Consistent with Remington’s own table
Two circulating dimension drawings (1.703 case, 2.230 OAL) reproduced with warnings	Withdrawn; the SAAMI figures in §2.1 govern

2.9 References (Vol 2)

Footnotes

1. ANSI/SAAMI Z299.4-2025, Voluntary Industry Performance Standards for Pressure and Velocity of Centerfire Rifle Sporting Ammunition, approved 10 February 2025 (file revised 27 April 2026): 450 Bushmaster cartridge and chamber drawing p.156, test barrel p.369, transducer pressure table and headspace gauge table, as read in the cartridge dive’s Volume 2, 19 October 2026. Also ANSI/SAAMI Z299.4-2015, printed pp.147 and 343, read 17 September 2026 for the first edition. 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 ↩³ ↩⁴

2. Hornady, Federal, Remington and Winchester product pages, read 19 October 2026, as cited in the cartridge dive’s Volume 3. Key URLs: https://www.hornady.com/ammunition/rifle/450-bushmaster-250-gr-ftx-black; https://www.federalpremium.com/rifle/fusion/11-F450BMFS1.html; https://www.federalpremium.com/rifle/power-shok/11-450BMB.html; https://www.remington.com/rifle/premier-accutip/29-27943.html. ↩ ↩² ↩³ ↩⁴

3. Ruger and Savage spec sheets, and Steinel’s 425 gr subsonic page, read 19 October 2026, with the Miller stability calculation, in the cartridge dive’s Volume 4. ↩ ↩²

4. Bear Creek Arsenal, “.450 Bushmaster Barrels” category and the 16-inch parkerized heavy barrel listing, read 19 October 2026. URL: https://www.bearcreekarsenal.com/calibers/450-bushmaster/450-bushmaster-barrels.html. ↩

5. Tromix Corporation, “.450 Bushmaster Barrel Only – Lightweight”, read 19 October 2026. URL: https://tromix.com/product/450-bushmaster-barrel-only/. ↩