Thin Data — Loading a Cartridge Three Publishers Cover

Published load data is not a single body of knowledge. It is a set of separate commercial products, each produced by an organisation that pressure-tested a particular combination in a particular barrel and published what it found. For a cartridge like the .308 Winchester or the .223 Remington, a loader has a dozen manuals and every powder maker’s online service to work from, which means that almost any bullet and powder pairing appears in several of them and the entries can be compared. Disagreement between two tables is itself information. Agreement between four is close to a guarantee.

For the .450 Bushmaster, the survey conducted for this dive located three publishers: Hodgdon, Barnes and Shooters World. That is the whole field. Hodgdon’s own data service is the practical starting point and lists the bulk of what exists. The consequence is not that the available data is bad — it was pressure-tested by the people who published it, and there is no reason to doubt it — but that there is usually nothing to check it against, and for many bullets there is nothing at all.

This volume is about working in that condition. It is not a substitute for the tables and it prints no charge weights, because a charge weight without its whole attributed table around it is not information a loader can safely act on.

5.1 The Powder Class, and Why It Is That Class

The powders appearing in the published .450 Bushmaster data fall into a recognisable neighbourhood: magnum pistol and fast, small-capacity rifle powders. Named across the tables are Accurate 1680, 4100, 5744, No. 9 and No. 11 FS; Hodgdon CFE BLK, H110, H4198, Lil’Gun and IMR 4227; Ramshot Enforcer; Shooters World Buffalo Rifle and SBR-SOCOM; and Winchester 296.

A loader who has worked with .300 Blackout on the supersonic side, or with heavy .44 Magnum and .454 Casull revolver loads, will recognise most of that list, and the overlap is not a coincidence. The case holds about 59.5 grains of water — small for a rifle cartridge — and it is pushing a very large-diameter bullet a short distance up a barrel that is usually sixteen inches. A slow rifle powder in that combination would still be burning when the bullet left the muzzle. The cartridge wants something that gets its work done early, and every powder on the list does.

The bullet weights covered by the published data run from 200 to 395 grains, at .451 to .452 inch, with the test barrel specified as 24 inches at 1:24 twist. Those four figures describe the envelope of the published record: outside it, a loader is extrapolating.

The tables also name specific primers — Winchester WSR and Remington 7½, both small rifle. Volume 1 covers why that is the single most counter-intuitive component decision the cartridge asks for, and why the industry standard’s drawing is silent on it.

5.2 What Thin Data Actually Costs

The cost is not that a loader cannot find a starting charge. Hodgdon’s service will supply one for most common combinations. The cost is that there is nothing to triangulate against, and triangulation is most of what published data is for.

Consider what a .308 loader does without thinking about it. A charge appears in Hodgdon’s table, in the Sierra manual, in the Nosler manual and in the Hornady manual. Three of them agree closely and one is a grain low. That disagreement tells the loader something real — usually that the outlier tested a different bullet construction, or a different barrel, or a different lot of powder — and it also establishes a range, which is far more useful than a point. When the loader’s own bullet is not in any of the four, the four tables together describe the shape of the cartridge well enough that an interpolation is a small step.

Here there is often exactly one entry, and frequently none for the bullet in hand.

The consequence that matters most is a substitution loaders make casually in well-documented cartridges and should not make casually here: swapping a bullet of the same weight but different construction into an existing table’s data is a much larger extrapolation than it appears.

The mechanism is worth spelling out because “same weight, same diameter” feels like it ought to be enough. It is not, for two reasons that compound.

Bearing surface differs. The length of bullet actually riding the rifling sets the friction the powder has to overcome and therefore the rate at which pressure builds. Two 250-grain .452 bullets can have quite different bearing surfaces depending on how much of their length is full-diameter shank and how much is ogive and boat-tail.

Density differs, so length differs. A monolithic copper bullet is significantly less dense than a lead-cored one. A 250-grain copper bullet is therefore a longer bullet than a 250-grain lead-core bullet of the same profile. Loaded to the same cartridge overall length — which, as Volume 4 established, is essentially fixed here by the magazine — the longer bullet sits deeper in the case and occupies more of the powder space, while also presenting more bearing surface. Both effects push pressure in the same direction, and the case is small enough that the powder-space effect is not trivial.

There is a detail in the list of publishers that follows directly from this and is worth noticing: one of the three publishers is Barnes, a maker of monolithic copper bullets. A bullet maker whose products behave differently from lead-core bullets of the same weight has a strong commercial and safety reason to publish its own tables, and the existence of Barnes data for this cartridge is the clearest available evidence that same-weight substitution is not a safe assumption. The correct use of that data is with Barnes bullets. It is not a general table that happens to be printed by a bullet company.

The practical rule for a sparse-data cartridge is therefore stricter than the usual advice to work up from a start charge. Match the bullet to the table, not the weight to the table. Where that is impossible — and it will be, because the .452 component pool is far larger than the published data covering it — the honest position is that the loader is developing a load rather than following one, and the starting point should be the most conservative entry available for a bullet of similar construction, worked up in small increments with the round chronographed and the brass read at every step. The Reloading Bench’s Volume 21 covers the method, and its central caution is directly applicable: a simulation is not a pressure gun, and the one instrumented case it cites saw a change of brass alone — nothing else altered — stick a case and shear a bolt handle.

5.3 The 39,900 psi Entry, Described Carefully

An aggregator summarising the published .450 Bushmaster tables reports that the maximum pressures across them run from about 19,300 psi for subsonic loads up to 39,900 psi at the top. The industry standard’s maximum average pressure for the cartridge is 38,500 psi. The upper figure is above it, and the comparison is easy to describe wrongly.

The reason it is easy to describe wrongly is that the standard does not publish one pressure figure for a cartridge. It publishes three. Alongside the maximum average pressure of 38,500 psi sit a maximum probable lot mean of 39,500 psi and a maximum probable sample mean of 41,000 psi. All three, with the standard’s own note that crusher pressures are not established for this cartridge, are printed in full in the AR-15 in .450 Bushmaster dive, Volume 2, and the tier structure is not rebuilt here.

Against that structure, the honest statement is this: a reported 39,900 psi exceeds the maximum average pressure while falling inside the band the probable-mean figures describe. It is not correct to say that it “is not a SAAMI figure” without qualification, and it is not correct to say simply that it “exceeds SAAMI maximum,” because the standard publishes more than one maximum and the figure clears only the lowest of them.

It is equally not correct to treat it as equivalent to anything the standard prints. 39,900 is not a string that appears in the standard, and it is not equal to 39,500. No equivalence is asserted here, and none should be inferred.

What the figure most likely is remains unresolved. The research underlying this dive flagged it as contested and left three explanations live: a pressure measured against a different reference, an aggregation error in the summary, or a transcription error. None was settled, and no attempt is made here to settle it by preference.

What a loader should take from it is narrower and does not depend on which explanation is right. The published maxima for this cartridge sit at the top of its envelope with no margin being given away. A maximum charge in a .450 Bushmaster table is a maximum in the strict sense, and treating it as a place to start, or as a figure with the usual unstated cushion behind it, is not supported by anything in the data. The figure is not a ceiling to aim at. It is an observation that the published ceiling and the standard’s ceiling are close enough together that the distance between them is not a working margin.

5.4 Subsonic Loads Exist, and This Dive Prints No Charge for Them

Subsonic .450 Bushmaster data exists — the 19,300 psi end of the range above is subsonic data — and the cartridge has a quiet-load following. A large-diameter heavy bullet at subsonic velocity is a straightforwardly attractive proposition behind a suppressor, and the published bullet range reaching 395 grains is there for exactly that reason.

A subsonic charge attributed to Hodgdon was encountered during the research for this dive and was deliberately omitted. That omission stands. The house practice in this collection is that a published table may be reproduced when it is reproduced properly and in full, with its source named and a work-up warning attached — which is how the M1 Garand dive handles Hodgdon’s Garand data. A single charge lifted out of its table, without the bullet, the primer, the case, the barrel, the starting charge and the pressure it was measured at, is not that. It is a number, and a number on its own is the thing most likely to be acted on and least able to be checked.

A loader wanting subsonic data should take it from the publisher’s own current service, where the whole row comes with it.

One open question about subsonic loading is worth recording rather than answering, because no source located addressed it. The cartridge’s 1:24 twist is correct for the short, fat bullets the cartridge normally fires, and the sibling dive explains why: gyroscopic stability depends on the bullet’s length in calibres rather than on its weight. But stability also falls as velocity falls, and a 395-grain .452 bullet is a longer bullet than a 250-grain one. Whether 1:24 is comfortably sufficient for the heaviest bullets at subsonic velocity could not be established from any source located for this dive, and it is the one place where the cartridge’s slow twist is not obviously a settled non-issue. The check is cheap and the loader can perform it: a group fired at a hundred yards with the bullets landing round rather than keyholed answers the question for that rifle, that bullet and that velocity.

5.5 The Barrel-Length Trap, in One Paragraph

The full argument is not rebuilt here. The AR-15 in .450 Bushmaster dive’s Volume 2 owns it and states it carefully, including the point that .450 Bushmaster velocity figures are not even comparable with each other because different makers measured from different barrels and two of them do not say which.

The handloading consequence is what matters at the bench. Published load data comes from a 24-inch test barrel. Factory velocity figures are 20-inch numbers where a length is stated at all. Most rifles in this cartridge are 16 inches. A handloader who chronographs a worked-up load in a 16-inch rifle and finds it below the book figure has almost certainly not found a weak load. They have found the barrel. Chasing a book velocity from a short barrel by adding powder is the predictable error, and in a cartridge where the published maximum sits as close to the standard’s ceiling as the previous section describes, and where there is no second table to sanity-check the first, it is a considerably worse error than it would be in .308.

One claim encountered in the research should be named and set aside. A single source states that testing with a 16-inch barrel showed the shorter barrel losing only 15 feet per second against the published factory figures. That is treated here as a single-source outlier and should not be repeated as the expected result. A loss of fifteen feet per second over a four-inch reduction would make this cartridge very nearly insensitive to barrel length, which contradicts the general relationship for a cartridge of this class and, more tellingly, contradicts the state of the evidence: the sibling dive searched specifically for a documented velocity-per-inch figure for the .450 Bushmaster and found none at all, declining to publish one rather than repeating the widely asserted twenty-to-thirty feet per second that traces to no documented test. A precise small number in a field that contains no verified numbers at all is the sort of figure that deserves the most scepticism, not the least.

5.6 The Load Record Is Worth More Here Than Anywhere Else

In a cartridge with a dozen manuals, a loader’s own notebook is a convenience. In this one it is the only source of data specific to the rifle in hand, and it is worth keeping to a higher standard than the well-documented cartridges get.

The record that pays for itself:

The bullet, fully identified — maker, model, weight, diameter and construction, not weight alone. The whole argument of this volume is that weight alone does not identify a bullet for loading purposes.

The powder and its lot number. Lot-to-lot variation is a real effect and it is the one that turns a proven load into an unproven one without anything visible changing.

The primer, by maker and designation.

The case, by headstamp and by firing number, carried forward from the batch record Volume 3 describes.

Both length measurements — cartridge overall length and base to ogive — because the first is what the magazine constrains and the second is what the chamber sees.

The measured velocity, with the instrument named, its placement recorded, and the barrel length and ambient temperature written down beside it. The Reloading Bench’s Volume 22 covers why the instrument has to be named: two good chronographs agree with each other only to within seven to ten feet per second, which is the same size as the effects a loader is usually trying to detect.

The feed-test result from Volume 4, because in this cartridge a load that will not feed is not a load.

Three of these produce numbers that appear to exist nowhere in the public record: the case life of a given brand of .450 Bushmaster brass in a semi-automatic rifle, the maximum cartridge overall length that actually feeds from a given magazine and follower, and setback measured over repeated chambering cycles by crimp setting. The measurement list at the end of this dive collects them. The separate question of a chronograph string from a 16-inch barrel is already claimed and described as an open measurement by the AR-15 in .450 Bushmaster dive’s Volume 5, and is not claimed again here.