Crunch time: powder speed vs bullet weight

[spitballer]

Okay guys I've got a question that has been vexing me for some time, and I wonder if anyone can explain it satisfactorily (explaining anything satisfactorily on this website is a tall order and that's EXACTLY why I'm throwing this question out there). It has to do with powder speed and bullet weight and load data for the .223 Remington (and just to be clear I have NO beef with data publishers, in fact I'm grateful for access to load data for safety reasons, obviously). Okay the question goes like this:

Let's take the example of H4895 - there's a vary narrow window of 25 to 26 grains for a 55 grain bullet which, according to my reloading manual, indicates that the propellant is approaching borderline with regard to pressure. Yet FASTER propellants like BL(C)-2 don't share the same narrow window of usefulness - in fact BL(C)-2 can be packed with 27.5 grains with that same 55 grain pill!

What am I missing here? Does the BL(C)-2 simply handle pressure better than H4895? Or does H4895 produce more pressure because it's speed of expansion more closely matches the speed of the projectile, producing sustained pressure or something? I've always wondered about this, and why certain propellants are more forgiving than others when it comes to pressure (and anecdotal evidence indeed supports the load data, they're not just making this stuff up). Thanks in advance to anyone who can shed some light on this phenomenon. -TH

 

[ATLDave]

I don't know if this is the explanation for your specific example, but sometimes case fill is a limiting factor. Some powders in some cartridges will literally run out of room before they reach max pressure for the cartridge... others may become highly compressed and get a sudden spike or non-linearity in pressure curves that renders them unsafe.

 

[hdwhit]

spitballer wrote:
What am I missing here?

I don't think you're really "missing" anything. It is just that the reality of what is happening is more complex than things like "burn rate charts" make it appear.

When a smokeless propellant is ignited, it undergoes a combustion and thereby transforms from a solid state with a particular volume to a gaseous state displacing a much greater volume. This occurs very rapidly which accounts for the pressure and the projectile being propelled down the barrel.

The combustion process does not proceed at a uniform, linear pace, rather it accelerates. And increases in both temperature and, separately, pressure, can influence the rate of that acceleration. Thus (speaking generally) the more confined a propellant is, the faster it will combust.

Burn rate charts are generally compiled by combusting a fixed amount of powder in a pressure vessel of a certain size and measuring the rate of pressure rise. And if your cartridge case had the same volume as the pressure vessel used in the test, then the rates should correspond closely to what is reflected in the load data. But, since powders run the gamut from loading 25 ACP cartridges to 50 BMG cartridges, the burn rates will cease to closely correspond the farther the cartridge's volume is from that of the pressure vessel.

So, the bottom line answer is that in 223, there is no a close correlation between the burn rate charts and the cartridge. In a different cartridge you would expect different results.

 

[Load Master]

Great questions and one that I have wondered about as well. The IMR website shows the "Burn Rate Chart", with no explanation of how the data was acquired. Was the burn rate measured in a common volume, open air, etc... They provide data with no explanation of how to best use it.

 

[taliv]

if you're specifically looking at AR15 load data, keep in mind that faster powders can result in unreliable cycling because they may effectively reduce the "dwell time". there may be reasons unrelated to pressure that cause a powder to be unsuitable

 

[VoodooMountain]

Burn charts aren't set in stone

Different companies use different components and equipment to measure pressure

Data is continually allowing for a larger safety margin

Different powders handle certain situations differently, ;
ball vs extruded or single vs double base

Different lots of powder will have variations

 

[hdwhit]

spitballer wrote:
Yet FASTER propellants like BL(C)-2...

Just following up on my prior post, please note that where a powder ends up on the burn rate chart is, in part, a function of how it is tested. On the Hodgdon burn rate chart (https://www.hodgdon.com//wp-content/uploads/2017/09/burnratecolor.pdf) H4895 comes in at 88 while BL-C(2) comes in at 104 meaning that when Hodgdon tested them using its testing protocol, BL-C(2) was SLOWER than H4895.

In contrast, the burn rate chart in the Vihta Vuori Reloading Guide #12 places BL-C(2) as a FASTER powder than H4895.

Different testing protocols produced a different ordering of powders by burn rate. This is why every burn rate chart says they are for reference only and should not be used to develop loads.

 

[Laphroaig]

Different testing protocols produced a different ordering of powders by burn rate

I have always thought that burn rate charts are a qualitative ordering of powders based on general experience and personal bias, not instrumented laboratory testing. Please correct me if there is evidence to the contrary such as a description of the test apparatus.

As for the OP's inquiry, loading data is based on instrumented testing. Different powders have burn characteristics that generate pressures differently depending on the cartridge. Even that is not an exact science, as evidenced by variations in min and max loads between manuals.

A good example are the reduced loads that Hodgdon recommends with H-4895. I asked a technical rep why a person couldn't use IMR-4895 for similar reduced loads, and his response was that different powder chemistry results in a different pressure curve between those 2 powders.

 

[Rule3]

As to burn rate charts, remember that the "burn rate " is all relative to ALL the 150 or so powders on the chart. It is not divided up between pistol and rifle powders. So it is relative from the fastest to the slowest of all powders.

Kinda like the top speed of a Ferrari and a Yugo or something.

 

[243winxb]

Powder type different - flattened ball vs cylinder(tubular)

Ball can and does produce pressure spikes when getting close to maximum. IMO. So start lower on powder with less velocity and work up.

The 223 has a SAAMI maximum average pressure rating of 52,000 CUP.

http://www.ilrc.ucf.edu/powders/sample_detail.php?powder_id=73

edit add. Ball powder needs a magnum primer, even more so using starting loads. IMO.

 

[spitballer]

hdwhit and taliv both make good points. The expanding plasma from a fully filled case will obviously propel the bullet more efficiently (and safely) than an expanding gas by itself. And a powder which burns too quickly won't allow the machinery of an autoloader to keep pace.

But aren't we basically talking about elasticity? Aren't some propellants simply able to handle more crunch than others? As an example I find Extreme Extruded powders seem to be less forgiving than others, grain for grain: is this related to formulation, or density or something?

 

[243winxb]

Powder bomb pressure vessel are be used for burn rates.

" Harwood’s unique pressure vessel design of the Powder Bomb Vessel was originally fabricated for use as a closed bomb for ordnance work related to ballistics research. Used as a tool to test the burn rate of explosives and propellants as a function of pressure and initial temperature, it allows for ignition and combustion studies in a controlled laboratory setting – such as necessary for developing formulae for solid rocket propellants. "
https://www.harwoodeng.com/products/special-purpose-vessels/powder-bomb-vessel/

Here is a pdf Australia government test. http://www.dtic.mil/dtic/tr/fulltext/u2/a128560.pdf

 

[243winxb]

Let's take the example of H4895 - there's a vary narrow window of 25 to 26 grains for a 55 grain bullet which, according to my reloading manual, indicates that the propellant is approaching borderline with regard to pressure. Yet FASTER propellants like BL(C)-2 don't share the same narrow window of usefulness - in fact BL(C)-2 can be packed with 27.5 grains with that same 55 grain pill!

BL(C)-2 is slower on my burn rate chart then H4895. Burn rate charts are interesting, but load data is much better.

 

[Varminterror]

Or does H4895 produce more pressure because it's speed of expansion more closely matches the speed of the projectile, producing sustained pressure or something?

This is absolutely NOT the case. It's more complex than you're envisioning, but you can be certain, it has absolutely nothing to do with the flame front velocity matching the projectile velocity - the propellant gases expand considerably more quickly than the projectile. SAAMI cites the propellant gases are traveling 75% faster at the muzzle than the bullet.

Powders are gasified within the first couple inches of travel, and then the gaseous product is burned through the bore - it's not a fuse of powder burning from one end to the next, but rather a mass of gaseous mixture conflagrating throughout the bore.

 

[Archie]

Okay guys I've got a question that has been vexing me for some time, and I wonder if anyone can explain it satisfactorily (explaining anything satisfactorily on this website is a tall order and that's EXACTLY why I'm throwing this question out there). It has to do with powder speed and bullet weight and load data for the .223 Remington (and just to be clear I have NO beef with data publishers, in fact I'm grateful for access to load data for safety reasons, obviously). Okay the question goes like this:

Let's take the example of H4895 - there's a vary narrow window of 25 to 26 grains for a 55 grain bullet which, according to my reloading manual, indicates that the propellant is approaching borderline with regard to pressure. Yet FASTER propellants like BL(C)-2 don't share the same narrow window of usefulness - in fact BL(C)-2 can be packed with 27.5 grains with that same 55 grain pill!

What am I missing here? Does the BL(C)-2 simply handle pressure better than H4895? Or does H4895 produce more pressure because it's speed of expansion more closely matches the speed of the projectile, producing sustained pressure or something? I've always wondered about this, and why certain propellants are more forgiving than others when it comes to pressure (and anecdotal evidence indeed supports the load data, they're not just making this stuff up). Thanks in advance to anyone who can shed some light on this phenomenon. -TH

1. What manual are you using? I just checked two edition of Lyman's manual (45 and 50) and Hodgdon's (10); no listing for H4895 given for 55 bullet in .223 Remington.

2. BL-C(2) is, in most references I check AND in my own experience listed as 'slower' then H4895. (One reference shows it is slightly faster.) Please note, these rates are based on "closed bomb" tests. Even if carved in stone - they're NOT - there is no standard of determining the 'difference' in powder burn rates. There is no system of predicting powder A is 5% faster than powder B and so forth. Just because a powder is seven places down the list from another powder does not mean is is seven 'clicks' slower.

3. As already mentioned, some powders are bulky enough the maximum shown is NOT the maximum pressure safe, but the most one can cram in the case without using a four foot cheater bar on one's press. Usually this situation is indicated by the memorandum "compressed load". In my experience, use of a 'faster' powder will generate higher velocities, still without exceeding safe maximum pressure for the arm. I'm not sure of the volume different between IMR 4895 and H 4895, but 36 grains of IMR 4895 is a compressed load in whatever cases were being used in at least one load book.

4. Guns - like the one used in the data collection for the manual you have - don't run the same as the gun you have. For instance; I have two rifles in 8mm Lebel, one carbine (22" more or less barrel) and a rifle (31" barrel). So far, the short carbine runs notably faster than the rifle. (In this case, probably the bore of the rifle is harder worn and one would not expect that condition in a commercial 'test rifle'.) All rifles respond a bit differently. Not to mention the brass, lot of powder, lot of bullets, air temperature and phase of the Moon have probably altered.

5. Essentially, a certain burning range - not usually ONE powder - is indicated on the basis of the sectional density of the bullet and the expansion ratio of the arm itself. More sectional density (greater weight in a given caliber) requires a slower burning powder, and a greater expansion ratio requires a faster burning powder; so it all gets goofy attempting to "predict" without testing.

Among other things, see if you can find a copy of "Firearms Pressure Factors" by Lloyd Brownell, PhD. Published at one time by Wolfe Press. It is rather in depth and does not require being a genius to understand.