Author Topic: forming brass and capacity (Read 390 times)

clodbuster · « **on:** February 16, 2006, 03:48:34 PM »

Just formed 300 Savage brass from Federal and Winchester 308 brass. When I weighed it the FC averaged 177.3 gr and the Win 167.7 gr. My question is how much adjustment should be made for the 10 gr difference in brass weight. I've heard both 1 gr and 2 gr of powder for each 10 gr of brass difference. Is either right?

JBMauser · « **Reply #1 on:** February 16, 2006, 04:56:38 PM »

Well, first off you don't know what the weight of the shell the load data you are looking to adjust from was or do you?. Second, if you want to check the volume which is what you are concerned with. there is a method of checking volume with water. I would assume that the heavier brass will have slightly less volume and it may add to the load density and improve ignition and performance then again if the powder you have chosen does not fill 80% of the case or so, I don't think it will matter one bit. I would note the weight of each type of brass and I would load them the same and then compare the results and see if the load is better in one than the other. I think adjustments at this point have no basis. Shoot them and develop your own imperical data for that rifle. IMHO JB

Reed1911 · « **Reply #2 on:** February 18, 2006, 12:12:03 PM »

You can compare the volume difference by finding the water weight capacity. Take and empty primed case and weigh it, then with tap water and a drop of soap (gets rid of the miniscus) carefully fill it to the top with an eyedropper. Weigh the filled case and subtract the dry weight. You'll need to do this with 5-10 cases to gain an avarage. Keep the manfacturer seperate and do it to both, there may be a slight difference. I think the difference will be so small that you can begin with regular starting loads, but you'll need to look at the cap. info first.

I show the nominal cap. for the .300 Savage as 51.42g

PaulS · « **Reply #3 on:** February 18, 2006, 07:48:53 PM »

Here is the math and the reasons for reduction of powder charge in heavier cartridges:

Powder reduction in military brass: (It also works for making brass for another case from a parent case)

Use this and all reloading information at your own risk. Always start with a reduced charge and work your way up watching for signs of excessive pressure. NEVER exceed maximum loads as recommended by powder and or component manufacturers. The information provided is for Ball Powders only.

You must have fire-formed brass, trimmed and sized to equal external dimensions:

Weigh a lot of once fired trimmed standard brass and record the average weight.
Weigh a lot of your trimmed and fire-formed military brass and record the average weight.
Subtract the weight of the standard brass from the weight of the military brass.
Divide that difference in weight by 6.
That is how many grains to drop your powder charge in the military brass.

for every increase of 6 grains in cartridge weight the powder charge
will be reduced by 1 grain to keep pressures similar.

WHY?

Well it takes just a bit of math and some known values to find the key!
The Specific Gravity of solids is a measurement of how much something weighs for a given volume as compared to water. If we take a container that holds 1 pound of water and fill it with smokeless double-base powder it will hold 1.4 pounds. Fill the same space with brass and it holds 8.45 pounds. This shows us that the specific gravity of brass is 8.45 and the specific gravity of Ball Powder is 1.4 that leads us to this formula:
For each 8.45 units (grains in this case) of brass the same space is taken up by 1.4 units (grains) of Ball Powder. To simplify this we take 8.45 and divide it by 1.4 and get 6.0357. Round it off to the nearest 1/10 and it is a volume that holds 6 grains of brass will only hold 1 grain of powder.

If the cartridges are fire-formed in the same chamber and trimmed to the same length, any difference in weight is the internal volume. The heavier the case is, the smaller the inside space, and now we know how much smaller.

Specific gravity information supplied by:
St. Marks Powders (manufacturer of Hodgdons line of Ball propellants)
Sweezy physical properties of materials (a handbook for metallurgists)
Formulas and math provided by Rev. Paul R. Stephens

clodbuster · « **Reply #4 on:** February 21, 2006, 12:24:51 PM »

Thanks Paul exactly the info I was looking for with a logical backup explanation.

JBMauser · « **Reply #5 on:** February 21, 2006, 06:28:29 PM »

PaulS, I read your post with interest but I must confess I do not understand how these computations which are held to specific standards and references perform in a rifle when we all know that the volume of the cartridge is a starting point for pressure. We all know that the projectile releases from the cartridge well before peak pressure is achieved (the primer can dislodge it) and now the leade and in most cases up to 2 inches of barrel create the actual system volume at or near peak pressure. In some loads there are actually powder grains which are not consumed in the reaction by the time the containment is the total volume of the chamber and the barrel. This is all known. So, where in this computation does the minute difference in case volume when compared with the total volume of the chamber, the barrel the leade make any difference that will require adjustment. It would seem to me that all of your math assumes the nothing happens past the neck volume. And where does the known fact that increased load density which is proven to improve performance without negitive pressure issues come into play. I will admitt that I am light on a lot of the interior ballistic math and all but all variables must be considered and I understand that in analysis things that are hard to factor in can sometimes be left out. But it does not add up to me to make adjustments based on only part of the containment. JB

JBMauser · « **Reply #6 on:** February 21, 2006, 06:47:32 PM »

Oh, and one more question. where is the connection or relationship of 1.4 gr. of powder and energy as it relates to the available empty space in the case. How does a factor of 6 relate to gases released by 1.4gr of powder? Nd how can one make a statement the 6 gr. in weight which relates to volume not weight as that is the true factor. can be related to 1 grain of powder, like which powder, Unique, bullseye, 3031. each has it's own burn rate and will have it's own pressure curve. the more I think about this the less this specific gravity, volume weight thing makes any sense.
Any clarity would be appreciated. JB

PaulS · « **Reply #7 on:** February 21, 2006, 10:58:56 PM »

JBMauser,

You are confusing yourself. We are talking about one rifle with one cartridge of known value and comparing it to the same gun with the same kind of cartridge made from a parent cartridge that was different on the inside and outside.
The "standard cartridge" is say a 308. The case is a Wichester case and weighs xx grains. It has been fire formed and sized and the standard load has a tested amount of space and operates at a "standard pressure" with a given powder (in this case lets say H380). Now we take a 358 Winchester case and size it down for a 308 (or we take a military 7,62 Nato case). We fireform it with a mild load in the same gun and size it in the same die. The outside of both cases are the same but our newly formed case (or the NATO case) weighs 12 grains more than the Winchester 308 case. That means that the brass of the formed case is thicker in spots than the factory 308 case. The thicker brass has less room on the inside than the factory case. Since the alloy used in cartridges is the same it weighs six times as much per unit of measure than the H380 does. (this formula only works for double base powders because their specific gravity is more consistant than the single base powders) The heavier case needs 2 grains less powder to keep the loading density and the resulting pressures close to the same as the "standard case" with the "standard load".
All the rest of your questions have nothing to do with what we are talking about. We are using the same gun, the same bullet, the same powder and the same primer. Whatever happens when we pull the trigger on the new round has happened when we pulled the trigger on the factory round.
The only change is the internal volume of the case. We know that the internal volume is 12 grains of brass heavier (smaller) because it weighs that much more and the outside dimensions are the same. We know that the space of 12 grains of brass and 2 grains of H380 are the same. So by taking our reduced load in the heavier brass the powder takes up the same space as the full load in the factory brass. The pressure will be the same in both cases - the reduced charge will have a bit less velocity but that is what you get from a smaller case. We are trying to protect the firearm and the shooter not keep the velocity the same - that would require an over-pressure load.