Case capacity gained with improved cartridges

[Len Backus]

Darryl

Respectfully.

It's not that hard to calculate a rough approximation of percentage increase in case capacity based on the differences in the reamers. The shoulder diameter is increased by .009 and the length from base to bottom of shoulder is increased by .020. Ignore the neck and shoulder. Multiply the new, longer base to shoulder dimension by the new diameter at the shoulder. Divide this by the same of the original, smaller reamer. That calculation shows 4.5% more capacity but then divide that in half because the increase in body diameter is only at one end of the case. The base end stays unchanged. So, about 2.25 % increase times 110 or so grains equals just under 3 grains increase.

Compare these numbers with with those of two 30-338 Lapua variants whose cases I have. Here the shoulder is blown out nearly 3 times as much, the base to shoulder dimension is more, also. And the result is still only in the range of 8 grains increase when actually weighed.

In addition, the one fired, unsized 300 Tomahawk case and the one new, unfired 300 RUM case actually showed only 4 grains of water difference. If I take one of my new 7 RUM cases and measure this same case both before and after firing it I get about 3 grains more water capacity with the now fired, unsized case.

Obviously the two .308 caliber cases, when new, started out with different case capacities before the Tomahawk one was fireformed. Even then, the capacity gain was so small as to not even exceed, or exceed by much, the normal variation in poor brass volume consistency of the 300 RUM case.

You are right, I think the sharper shoulder is a good thing.

[ 02-21-2004: Message edited by: Len Backus ]

 

[Jay Kyle]

Not sure if this helps or if you already know about this, but Neconos.com just released a new program called QuickDesign that ties into the Quickload and QuickTarget programs. QuickDesign is a modelling program for creating wildcat cartridges, which you can push through to Quickload for internal ballistics analysis, then over to QuickTarget for external ballistics.

AB

 

[subsonic]

Hi Len,
I wanted to build a little baby cartridge that I could carry lots of ammo when hunting Wallabies on steep hills and a light rifle to carry as well. I started with a std hornet and new the K was an option the case taper went from 20 thou to 15 thou and the shoulder angle was significantly steepened this increased case capacity from 14gn's water to 15.2gn's (+8.6%).
I then reduced case taper to 5 thou and shifted the shoulder forward slightly and my case capacity went up further to 16.5gn's (8.6% over K or even better 17.9% over the hornet). I ran this through quickload and it gave me 3300fps with the 40gn V-max versus a factory load 45gn hornet at 2600fps.
When I chronographed the Ruger 77/22hornet that had been specifically throated to suit the V-max (I had to lengthen the mag slightly internally to allow the V-maxs to touch the lands) the velocity was within 3fps of what quickload had predicted.
I now have a little rifle that behaves like a 223 (if I ignore bullet weight) with a 50gn bullet as far as trajectory goes but I can carry a lot more ammo easily.

 

[Len Backus]

AlbertaBound and subsonic

It seems I should look into Quickload. Thanks.

 

[Darryl Cassel]

Hello Len

You mentioned;
"It's not that hard to calculate a rough approximation of percentage increase in case capacity based on the differences in the reamers. The shoulder diameter is increased by .009 and the length from base to bottom of shoulder is increased by .020. Ignore the neck and shoulder. Multiply the new, longer base to shoulder dimension by the new diameter at the shoulder. Divide this by the same of the original, smaller reamer. That calculation shows 4.5% more capacity but then divide that in half because the increase in body diameter is only at one end of the case. The base end stays unchanged. So, about 2.25 % increase times 110 or so grains equals just under 3 grains increase"

__________________________________________

Something to consider here Len

A rough approximation is "not" what I did when weighing the two cases with full water content. It was done actual with the used primer put in the cases upside down, weighed, then filled with water to the top and weighed again.

I have found that the brass makes a differance as per thickness from lot to lot.
Did I have thinner brass then the one or two sent you, I don't know?

Also with the shoulder angle change to 35 degrees it does change shoulder "edge to base" overall diminsions in my case's. It all depends on how the reamer was made to and what specs are used.

There are also different versions of the Tomahawk out there already just like the variations of the Gibbs cartridges. Some Gibbs rifles were chambered for the 06 brass size and others used the reamers made for 270 brass which does make a differance since the 270 brass is about .050" longer then 30/06 is.

Anyway, good luck with your project and hope it works out for you.

Later
DC

 

[Len Backus]

Darryl, thanks.

Now, since I have a long, long way to go to ever know half of what you do about wildcatting,(sounds wicked

) please describe in detail how you would go about actually measuring the difference in case capacities with something like the Tomahawk case compared to its parent case.

Before I order my 7-338 Lapua reamer I will then go back and measure my 338 L and 30-338 L cases again.

[ 02-23-2004: Message edited by: Len Backus ]

 

[Darryl Cassel]

Len

If your measuring water weight for capacity here is the way I have done it for many years.

Take two fired primers and put them in upside down in the Wildcat case and one in the parent case.
Weigh EACH case seperatly and write that number down.
Now fill BOTH cases up to the top of the neck with water and weigh each one seperatly and write that number down.

Subtract the empty case weight from each one that you filled up with water. Empty and filled parent case and empty and filled wildcat case.

Now subtract the lower number weight from the highest and you will have the case capacty difference of the wildcat against the parent.

Many times two "fired" cases can be used for this measurment as this is the chamber diminsions and many shooters will only neck size their cases when loading them. This way it gives the "full" capacity of both case's.
The grain "difference" between the two will still be the same, however.

Later
DC