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Author Topic: Loading by Density (Part 3)  (Read 99 times)

Offline Dutch-Hunter

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Loading by Density (Part 3)
« on: August 31, 2019, 10:40:44 AM »
Loading by Density (Part 3)


Most reloaders have known that the wrong amount or type of powder will not produce consistently accurate results no matter how much care has been taken to make each round. Powders certainly are not the same. Some shooters/reloaders leave nothing to chance while searching for the optimum load and will test every powder that can be safely fired in a particular caliber. This can be costly and time consuming. Loading uniformly by density is a method of loading that will help identify powders worthy of testing.

LOAD DENSITY THEORY

Powders with similar burn rates (quickness) as listed in a reloading manual or burn rate chart often will not deliver consistent results. There is a simple reason for this. Burn rate is only one powder characteristic and may not be the primary factor which contributes to consistent accuracy in a particular caliber let alone a specific rifle. Composition and granule shape also affects the powder's performance and can vary between powders of similar burn rates. The burn rate of a powder is controlled by its base composition, granule size, granule shape and often granule coating. These design features also change the density of powder so that for a given weight there is more or less gas energy produced for a given volume that the powder occupies.

Ballisticians have long recognized that the volume of powder in a cartridge affects the accuracy of a load. A powder charge that completely fills the space under the bullet is often more accurate. It is theorized that this helps promote more consistent ignition and rise in pressure behind the bullet, more consistent velocities and therefore greater accuracy. Indeed this can be seen with instrumentation that measurers the pressure curves.

The percent of the available powder space under the bullet filled by the powder is called the "load density". As the load density is reduced in other words the powder doesn't fill case. The point of peak pressure moves toward the muzzle and velocity falls off. Low load density can cause another problem called detonation (discussed in Part 1). With a partially filled case the powder charge can shift to change ignition and the shape of the pressure curve. Think about it, powder is loaded into the case vertically, the cartridge is loaded into the breech horizontally. Increasing the load density moves the point of peak pressure backward toward the chamber and increases velocity. Since the powder cannot shift within the cartridge, ignition and pressure is more uniform, resulting in more consistent velocities and better accuracy. What is being accomplished is having the bullet leave the muzzle at the same position during barrel “whip”, with peak pressure being attained just before the bullet exits.

In addition most “good” loads the powder is burned a few inches beyond the point of peak pressure, but if the firearm has a short barrel and low load density, the peak pressure may move further toward the muzzle resulting in excessive "muzzle flash". Excessive muzzle flash happens when unburned powder ignites at the muzzle, causing turbulence behind the bullet with a detrimental effect on accuracy.

There is often a direct relationship between the burn rate (quickness of the powder) and the space a safe powder charge will fill. Powders can be made to burn quicker by reducing the granule size, which increases its density so a given charge weight takes up less space. Switching to a quicker powder may not resolve a muzzle flash problem if the powder has a high density and doesn't fill the case. Instead, filling the case completely with a lower density, but slower powder may hold the point of peak pressure nearer the chamber for a complete burn, less muzzle flash and better accuracy. This is why some reloaders prefer slight compression loads. As I said before I personally have never had reliable results with compressed loads.

CALCULATING POWDER and LOAD DENSITIES

If you do not own software that will calculate load densities there is a manual method that will work. First it is necessary to determine the powder capacity of the case. The method I use to determine case capacity was discussed in Part 2. Weigh the case empty and full of water. Then the volume of water displaced by the seated bullet is subtracted from the difference of the “dry” case from that of the “wet” case.  This is the net powder capacity of the cartridge in grains of water.

Now you must know the density of the powder(s) you want to try. Some powder companies publish bulk densities and some don’t. So you may be forced to find the bulk density yourself. Here’s how you determine a powder’s density factor. The bulk density of powder is most commonly expressed as grams per cubic centimeter (g/cc). Since the weight of one cc of water is nearly equal to 1 gram (the actual factor is .9971). A bulk density of 0.970 g/cc means the powder is 97% as dense as water. Bulk density is therefore also the powder's weight in any volume divided by the weight of water filling the same volume. The procedure to find the bulk density for a powder first is as follows. First find a container to use as a unit of measure, as large as practical for your reloading scale to accurately weigh. A spent and cleaned 45 caliber pistol case works quite nicely. Weigh the container empty to establish the empty weight or zero the scale and continue. Weigh the container filled level full with powder record and empty. Then fill it with water and weigh it again. Divide the weight of powder by the weight of the water. The result is the bulk density of the powder and should be 1.000 or less.

To finish the calculation use these formulae: Cartridge Capacity in gr. of H20 * Powder's Bulk Density = Cartridge's Powder Capacity Powder Charge in gr. / Cartridge's Powder Capacity = Load Density

The Sierra load manual is ideally suited for load density analysis because powder charges are arranged under an estimated velocity column. Select a velocity column that is close to what you hope to achieve and calculate the load densities using the indicated powder charges.

INTERPRETING DATA

Most powder companies will admit that the bulk density of production lots may differ by as much as ±10%. You should consider this limitation when comparing load densities. Powders that produce load densities within 10% of each other may provide similar results and should not be eliminated from consideration. However, all rules are made to be broken. The theory of load density related to accuracy is not entirely foolproof, particularly when applied to certain cartridges and/or free bored chambers. Any safe charge producing a load density of 80% or more may be worth testing if powders with higher densities do not produce the desired result.

 FINALLY DONE! ##$%#1118 @--0--0123  @--0--0118  @--0--0123  ##$%#117

Now that wasn’t too bad was it? I’m still working out a few bugs in the Excel spreadsheet to assist in computing this information. I have passed it off to Bo for more comprehensive and thorough evaluation. ‘Cause we all know…  “Bo Knows”. Just as soon as we are satisfied with it one of us will post it. I wanted to make sure that any recommended loads that are computed in the spreadsheet won’t be dangerous to use. I am confident that all suggested starting loads are a very good place to start your loads. Then using incremental load or ladder loading to test charge weights would be an excellent way to come up with “THE” load for your gun. After establishing a charge weight range to help zero in on “THE” load for your gun then you can play with jump space to fine tune it.

Dutch out!
« Last Edit: August 31, 2019, 10:42:52 AM by Dutch-Hunter »
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Loading by Density (Part 3)
« on: August 31, 2019, 10:40:44 AM »
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Offline BoBallistic

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Re: Loading by Density (Part 3)
« Reply #1 on: September 04, 2019, 02:37:21 PM »
Dutch as always - you are spot on when it comes to ballistic and loads and shooting at long distances...man are you good......will take a look at "Load Book" and give make modifications if any on your spreadsheet and we can go from there....I have been shooting a long time (since my teens) and not as long as you have. I have slowly over the years built my own hand written library on the loads I prefer and the loads your rifle likes....

A couple of things that I have noticed is that once your rifle shoots say about 200 rounds down the barrel, you will have to tweak the loads again maybe add or subtract a grain or two, just depends on the rifle....your lans certainly changes and you shoot. The more you shoot the more they it changes and you certainly shoot them out bit by bit...little by little.....but then after you shoot your rifle 500 times, you rifle certainly changes again and again you will have to change your load density and factory...you may even have to change bullets said go heavier (and longer) bullets, say in my case of a Winchester Model 70A in 270 Win went from 130 gn and after 453 rounds gone to a 140 gn bullet. I have notice that its groups had slowly been getting larger and larger (i would have taken any deer with the larger round at 200 yard we are talking about from 1/2 MOA to 1 MOA over the years using the same load. So when I went to the 140 gn on the 454 round it went back to 1/2 MOA....

I have notice this on several rifles and different calibers, like the 308....I have gone from a 165 to a 180 gn on some of them. But could this be me or the rifle or could it be the velocity and the twist rate combo...I know from recent shootings of my Cooper M2012 in 308 that it can shoot the 155 gn just as good as the 165 gn bullet....but what give here? since I have plenty of vintage bullets for both the 150 and 155 gn to the 178 and 180 gn...I think it all comes down to the rifle and what it likes....that is why I am in favor of a "Variable Pack" of ammo to help the average hunter/shooter decide on what his gun likes...say for example, a 30-06 with 6 rounds each in 150 gn and 165 gn and 180 gn bullets that way you buy the one that is right for you rifle....

Got to run now, but this is food for thought.......Bo 
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