Case capacity gained with improved cartridges

[Len Backus]

After doing some more careful measuring, it looks as if my 7 Lapua Improved project will net me somewhere around a 6 grain increase in nominal powder capacity over that of a 7 RUM. That's good enough for me, especially with the advantage gained in brass quality.

The increase gained with the various 30/338 Lapuas of our members over the parent case seems to be somewhere around 7 grains. I would be most interested to learn of the amount of gains achieved with other common "improved' cartridge projects. Who has good data on the 300 Tomahawk project, before and after?

 

[Darryl Cassel]

Len

Tomahawk #1 can give you more info on that.
He and Ray Romain was instrumental in the design of that one.
I simply used the reamer that was ordered.

The neck jucture looks the same length but the shoulder is blown out and the shoulder angle is 35 degrees.

Tom #1 will be around soon I'm sure.

Later
DC

 

[tomahawk #1]

Len

The shoulder on the 300 ultra is .520 and the shoulder on the Tomahawk is .535 which gives you a 5 degree body taper with a 35 degree shoulder. Neck length is .286
Hope this info helps with your reamer development

Tomahawk#1

 

[Len Backus]

Tomahawk

Thanks!

I assume the shoulder/body junction had to stay where it was?

 

[dwm]

Len,

What powder are you planning to burn in this really big 7mm case?

Are you planning on having a long neck on the case to keep the throat from buning out or counting on really slow burning powder to help out with that problem?

I have read some posts on here on LRH.com about rapid throat errosion with the big 7mms.

Just wondering if you had a plan in the design of this case which would help this issue out.

Doug

 

[Len Backus]

dwm

I'll use one of the slowest powders, of course. And I'll just re-barrel every few hundred rounds.

 

[4mesh063]

Len,

if you want to increase the capacity and are looking for a big "improvement", just increase the diameter at the shoulder and you'll gain a ton more than all this 40 degree crap. (do that too) Do away with all but about 3-4 thou of taper per inch and the cases wil eject just the same and the capacity will be LOADS bigger.

 

[Darryl Cassel]

Len

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.

Later
DC

[ 01-09-2004: Message edited by: Darryl Cassel ]

 

[Len Backus]

Darryl

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?

 

[Len Backus]

Next chapter:

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.

Or am I missing something here?

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

 

[Darryl Cassel]

Len

As I mentioned before in another post.
Depending on the lot of brass, I have had an increase of 6 grains of capacity over the standard 300 RUM case when going to the Tomahawk.

The sharper shoulder of the Tomahawk also decreases overall case stretching when fired which is an advantage.

To me, it's worth going to the Tomahawk over the standard case and it doesn't take much to fireform it either.

If the new brass is really brass instead of nickle plated, it will be a bit of an improvement. The new lots of Remington brass seems to be better then the older stuff.

I shoot up to 104 grs, 106 grs, and 108 grs of powder in the 300 Tomahawk depending on the bullets fired.

I may go with a 7MM Tomahawk to see what it will do? I do want a longer barrel if I go that route though.

Later
DC

[ 02-21-2004: Message edited by: Darryl Cassel ]

 

[jhilsen]

Len-

Where did you get a copy of the 300 Tomahawk reamer drawing (if you don't mind my asking)?

Jim

 

[Len Backus]

Jim

From Dave Manson

 

[jhilsen]

Thanks Len.