I have gained up to 6 grs of powder over the standard 300 RUM with the Tomahawk chambering.
Many of us just neck size the case so it's easy to see the gain on both cases AFTER they have been fired.
Simply fire the standard and IMP case, resize the neck, put a fired primer in upside down and fill with water. Weigh and deduct the difference between the two.
If you full length size "BOTH" standard and imp cases the "difference" will still be close to the above procedure as per the difference.
You can get the "maximum" case capacity by neck sizing only.
Most athorities on this subject feel that a 6% gain is worth going to the Imp. case.
I agree with that after shooting many Imp cartridges over the years.
Thanks. I am in the final stages of designing my reamer for the project. With your Tomahawk project, how much did you change shoulder location, neck length, body diameter at shoulder, etc? Would you design the reamer any differently today?
As you know, I am going to chamber a barrel for my Nesika Bay/HS Precision platform which is now in the 7 Dakota cartridge at 3,150 fps with the Cauterucio 176 grain bullet. I have tweaked my 7-338 Lapua reamer design some more. Now it looks like I will gain 7.5 grains of capacity over that of the 7 RUM. I am guessing at a max of 10 fps of velocity gain per 1 grain of case capacity for 75 fps. To reach this "max 10 to 1" conclusion I studied two things: the difference between my own 7 Dakota and my 7 RUM: and Hogdon's online loading data, with comparable pressures, for the 300 RUM and the 300 Short Action RUM.
Compared to my existing 7 RUM with its 30 inch barrel I should obtain about 3,400 to 3,450 fps with the Cauterucio 176 grain (.750 BC) vld bullet. That's plenty of gain (total of 120 fps) over my existing 7 RUM and 250 to 300 over my 7 Dakota, for me with my project. I will also benefit by going to the superior Lapua brass. Also, this harder brass may allow slightly (note: slightly) higher operating pressures and velocity since the harder brass probably reaches the point of tight extraction and, more certainly, loose primer pockets a little later.
Now here's a curious issue.
I have never worked on developing a wildcat cartridge of my own before so I have been going slowly and delberately in my analysis. I own a 7 RUM rifle which is based on the same case as the 300 RUM. So...I wondered how much capacity gain is achieved with the 300 Tomahawk project and even considered, for about an hour, a 7mm Tomahawk since it was mentioned on another post that Federal is now offering premium 300 RUM brass.
I obtained one unfired 300 RUM case and one fired 300 Tomahawk case. I also got a copy of the drawing for the 300 RUM reamer and one for the 300 Tomahawk reamer.
My conclusion is that I do not think the 300 Tomahawk improves case capacity by more than 3 grains of water. Resultant velocity gains would therefor be in the neighborhood of 25 to 30 fps. This hardly seems enough to justify all the work and expense, especially since you still are using the Remington brass. I never saw posted an opinion of how much velocity is gained and I guess I just wrongly assumed it was much more. I realize we all do these projects more for the fun of creation than for the actual measureable gains. I wonder, though, if some Tomahawk shooters would be disappointed to learn how little velocity is probably actually gained.