What This Dive Is For — and What Three Other Dives Already Answer

Handloading the .450 Bushmaster is not difficult, but it is consistently misjudged, and it is misjudged in a specific direction. The case is short, fat and straight-sided. It takes a bullet of the same diameter as the .45 ACP. It sits in a magazine designed for an intermediate rifle cartridge and it runs at a pressure closer to a magnum revolver’s than to a .308’s. Everything about its appearance says pistol cartridge, scaled up, and a loader who owns a progressive press and a set of carbide pistol dies will reasonably expect to load it the way pistol ammunition is loaded: no lubricant, high throughput, minimal fuss.

Almost none of that transfers. The governing fact is one dimension, and it is small: the case carries roughly twenty thousandths of an inch of taper from head to mouth. A carbide sizing ring is a straight cylinder and cannot impart taper, so no carbide sizer for this cartridge exists, so every case must be lubricated, so lubrication and lube removal become process steps, so sizing effort rises, so stuck cases become a live risk, and so the press that makes pistol loading fast stops being the right machine. The .450 Bushmaster is a rifle-loading process wearing a straight-wall case’s shape. That sentence is the thesis of this dive and Volume 2 is the argument for it.

This volume is deliberately mostly a map. Three other dives in this collection already answer the questions a reader is most likely to arrive with — what the cartridge is, why it headspaces the way it does, and how the general bench process works — and they answer them from primary sources, at length. Reproducing that material here would duplicate it, and duplicated technical writing drifts apart until a reader cannot tell which copy is current. So the pointers below are the service, not an apology for one.

1.1 The Small Rifle Primer, Because It Is Ordered First

One point of fact belongs at the very front, because it is a purchasing decision made before a single case is touched and because getting it wrong means a box of primers that cannot be used.

The .450 Bushmaster takes a small rifle primer. The published load tables specify it by name — Winchester WSR and Remington 7½ appear across the data — and the point is corroborated between sources rather than resting on one.

This is genuinely counter-intuitive, and the reason it is worth stating flatly is that the reflex runs the other way. Nearly every .45-calibre cartridge a handloader is likely to have met takes a pistol primer: .45 ACP, .45 Colt, .454 Casull. A case this large on a small primer, and a rifle primer at that, is the sort of thing a loader assumes they have misread. Ordering large anything is the predictable error.

The AR-15 in .450 Bushmaster dive, Volume 6, states that the industry standard’s drawing does not specify a primer size and asserts none. That is correct about the drawing and it should stay correct — the standard specifies the cartridge and the chamber, not the components loaded into the case. The primer size comes from the organisations that pressure-tested the loads and published the tables, and that is the authority under which it is stated here. The two statements complete each other rather than conflicting: the drawing is silent, the load data is not.

1.2 What the Rifle and Cartridge Dive Already Answers

The AR-15 in .450 Bushmaster dive, at /actual-firearms/rifles/ar-15-450-bushmaster/, is six volumes and owns the cartridge as an object and the rifle as a machine. It is built on a reading of the industry standard’s page images rather than on secondary summaries, and it is the reference for anything dimensional.

Volume 1 covers the origin: Jeff Cooper’s Thumper concept, Tim LeGendre’s .45 Professional, Bushmaster’s acquisition of the design in 2007 and Hornady’s shortening of the case around a 250-grain bullet, plus who currently makes rifles and factory ammunition.

Volume 2 is the specification volume and carries most of what a handloader will want to look up. Its dimension table is read off the drawing, including the bullet, head, rim and case-length figures and the two overall-length limits. Its treatment of the rebated rim explains why the cartridge exists at all — a rim turned down to the .308 and .30-06 head diameter on a case body far larger than either. Its section on headspacing on the case mouth identifies the headspace dimension on the drawing, gives the chamber’s minimum and maximum, places the cartridge in the same gauge block as the .30 Carbine, and sets out the three consequences that follow for a loader: case length is the headspace dimension, over-trimmed brass headspaces long, and anything that deforms the mouth is deforming the datum surface. Its pressure section prints all three of the figures the standard publishes — the maximum average pressure and the two probable-mean figures — together with the standard’s explicit statement that crusher pressures are not established for this cartridge. Its section on the 24-inch test barrel is the one every reader of published velocity figures needs, and Volume 5 of this dive returns to its handloading consequence. Its twist section explains why 1:24 is correct rather than lazy.

Volume 3 is the Michigan question in full, including the elimination of the Limited Firearms Deer Zone effective 12 September 2026, what that change does and does not alter, and why most of the popular literature about straight-wall cartridges is now stale on its central premise. Nothing about that belongs in a loading dive, and nothing about it is repeated here.

Volume 4 owns the magazine problem from the rifle’s side — why a follower built to feed two staggered columns mishandles one fat column, what the capacities actually are, and the feeding and setback behaviour that follows. Volume 4 of this dive owns the other half: what that constraint does to load development.

Volume 5 covers trajectory and zero, and already records the missing 16-inch chronograph string as a photograph and measurement debt.

Volume 6 covers brass and reloading from the rifle owner’s side: the .452 component pool and its availability advantage, the existence of Starline brass, the argument that case life should be good at this working pressure, and short statements of the crimp and setback cautions.

1.3 What the Headspace Dive Already Answers

The Headspace dive at /gunsmithing/headspace/ carries mouth-breeching three times over, and it carries it better than a per-cartridge dive could.

Its Volume 2 sets out the four ways a cartridge can be breeched and why the classification matters. Its Volume 11, “Mouth-Breeching Cases — Where Trimming Is Headspacing”, is the canonical treatment: it quotes Forster’s statement that for such cases their headspace is the overall length of the case, and it establishes the working rules — never trim past minimum, do not roll crimp into the datum, sort straight-wall brass by length, and recognise inconsistent ignition as the symptom. Its Volume 14 carries the summary table.

A volume in this dive called “Case Length Is Headspace” would be that volume rewritten, and it is deliberately not written. Where this dive needs the principle — and it needs it in Volume 3, for case gauging, and in Volume 4, for crimp — it states the consequence and points there for the derivation.

1.4 What The Reloading Bench Already Answers

The Reloading Bench, at /reloading/the-reloading-bench/, is twenty-four volumes on the bench process in general, and it is the parent of this dive rather than a sibling. Its Volume 9 covers die families and selection, including carbide and its straight-wall exception. Its Volume 12 covers case preparation, lubricant types and delivery, lube removal, and the stuck case, as general technique. Its Volume 13 covers trimming machines and what each one indexes from. Its Volume 15 covers case life, batch discipline and retirement criteria at scale. Its Volume 20 covers seating, ogive, base-to-ogive measurement and crimp in general. Its Volume 21 covers load-development method and the statistical criticism of node-hunting, and its Volume 22 covers chronographs and the statistics of group size.

Two of those need a boundary drawn precisely, because this dive lives right on it.

Volume 9 and Volume 12 both state that no carbide sizing die exists for this cartridge, and both give the taper as the reason. So the fact is not news and this dive does not present it as news. What those volumes do not do — because they are writing about dies and case preparation across eight chamberings — is work the mechanism through to the end and follow every consequence of it. That is the whole of Volume 2 here.

Volume 12 owns lubricant as technique: which delivery systems exist, what each trades away, the lanolin mixture, and how to remove lube afterwards. This dive does not re-teach any of it. What this dive owns is why this particular chambering punishes an under-lubricated case harder than anything else on a mixed bench, which is a question about surface area and die geometry rather than about lubricants.

1.5 What Is Left

Four volumes, and four is the honest size.

Volume 2 — A Straight-Wall Shape and a Rifle Process. The taper, the reason a carbide ring cannot deal with it, how much case surface is actually being drawn through the die, and the workflow that follows. The centrepiece.

Volume 3 — Sizing, Lube, and the Stuck Case. The consequences at the bench: lubricating a large-diameter tapered case, removing that lubricant from a case whose mouth is a headspace datum, recognising and recovering a stuck case, mandatory case gauging, and case life — where no published figure exists and the volume says so.

Volume 4 — Seating and Crimp Against a Single-Stack Magazine. The binding constraint is the magazine, not the chamber. Overall length limits, the vanishing headroom for seating a long bullet out, why every load must be feed-tested from the magazine rather than chambered by hand, why a heavy crimp is required, why the usual reason given for the type of crimp is wrong even though the conclusion is right, and why the shared .452 bullet diameter is a supply advantage rather than a terminal-performance one.

Volume 5 — Thin Data. Three publishers cover this cartridge. What that scarcity does to the ordinary practice of cross-checking a load between manuals, the powder class the data occupies, one published pressure figure that needs careful handling, and the barrel-length trap that makes a chronograph reading look like a weak load.

1.6 The Component Checklist

What a loader needs in hand before the first case is sized, with the volume that explains each.

Small rifle primers, per the discussion above.

A conventional steel die set. Not carbide — none is made. Volume 2.

Case lubricant, and a means of removing it. Both are process steps here, not refinements. Volume 3, and The Reloading Bench Volume 12 for the products themselves.

A stuck-case removal tool. Cheap, needed rarely, and needed urgently. This is the chambering most likely to call for it. Volume 3.

A case gauge. Not optional on a cartridge that stops on its case mouth and feeds from a magazine into a semi-automatic action. Volume 3.

Bullets of .451 to .452 inch, preferably cannelured. The cannelure is close to a requirement rather than a preference, because this cartridge needs a heavy crimp and an uncannelured bullet crimped hard bulges the case below the mouth. Volume 4.

Brass. Hornady and Starline are the makers identified in the research for this dive; no price is printed for either, because the Starline product page could not be read when it was attempted and an unconfirmed price is worse than none.

A magazine with a single-stack follower, because feed testing is part of load development for this cartridge rather than a range-day formality. Volume 4.

A chronograph, because every published velocity figure for this cartridge comes from a barrel longer than the one most rifles carry, and the gap is large enough to be mistaken for a problem with the load. Volume 5.

1.7 Reading Order

Table 1 — Reading Order

If the question is	Go to
Why does this cartridge need lube when .45 ACP does not	Volume 2
Which die set, and can it run on a progressive	Volume 2
How should the case be lubricated, and how much	Volume 3
A case is stuck in the die	Volume 3
How many firings should this brass give	Volume 3
Why will this load not feed	Volume 4
Roll crimp or taper crimp, and why	Volume 4
The bullet is not in any published table	Volume 5
The chronograph reads well below the book	Volume 5
What the cartridge actually is, dimensionally	The AR-15 in .450 Bushmaster dive, Volume 2
What headspacing on the mouth means	The Headspace dive, Volume 11
Which press, which trimmer, which scale	The Reloading Bench, Volumes 5 to 9, 13 and 19

A reader new to the cartridge should read Volume 2 of the rifle dive first and then come back here. A reader who already loads for it should start at Volume 2 below and can treat Volume 1 as an index.

1.8 What This Dive Does Not Print

Three refusals, stated at the front so the document’s character is clear.

No charge weight appears in this dive’s own voice. Naming a powder that appears in a published table is fine and is done throughout Volume 5. Producing a number without a named published source, or phrasing any number as a recommendation made here, would be a safety defect rather than a helpful addition. Where a published table is reproduced anywhere in this collection it carries its publisher’s name and an instruction to work up from the starting load in the current edition.

No pressure figure is converted between measurement systems. For this cartridge the question does not even arise — the standard prints, in place of a crusher figure, a statement that crusher pressures are not established.

Nothing is stated more confidently than its source supports. Where a figure could not be confirmed, the prose says so. Where two sources disagree, both appear and the disagreement is named rather than averaged into a middle number. There is more of that here than in most writing about this cartridge, because the published record on it is genuinely thin, and the thinness is itself one of this dive’s findings.