Big Seven Case Capacities

[Len Backus]

Later next year I'm thinking of doing a wildcat 7mm cartridge based on the 338 Lapua case. Does anyone know the water capacity of the various existing big sevens. Such as the 7 Rem Ultra Mag, Lazzeroni's Firebird and any others in that category.

 

[Dave King]

Len

You thinking of shortening it to make a Super "Short Magnum", or leave it full length?

Sounds like an interesting project, maybe I'll tag along and get one of these barrel burners... "Cut two (2) chambers please".

 

[Len Backus]

Dave

Full length so I can use a bushel basket full of gun powder.

You're in. I know you need a new gun project about any time your check book balance gets too high.

 

[dbhostler]

Len,

My 7 Ultra holds about 113gr water.

db

 

[Dave King]

Checkbook balance, any amount in the black is too much... I like 0.00 or a red balance... don't want to leave too much if I die.

What action you thinking about? Maybe we can get twins... I really want to try some of those high BC 7mm bullets. (If my friends could hear me now... "Dave with a 7mm... must have bumped his head pretty hard recently!"

 

[Len Backus]

dbhostler

What velocities are you getting with the heavier bullets? Barrel length, etc? How do the published load/velocities compare to what you are achieving after adjusting for barrel length?

Dave

My Nesika Bay action with its HS Precision Pro Series tactical stock is already set up as a switch barrel gun so I will be using that. The BC of the Cauterucio 176 grain really is the high .750 claimed by Bob.

 

[dbhostler]

Len,

Haven't worked with the heavy 7s too much. I picked up the Ultra later this year and began with a 140gr NBTs. Got those to shoot and then sent the rifle to Chris and had an OPS INC brake installed. The only thing I have at this time is 162gr A-Max with 96.0gr Retumbo lit by RP 91/2Ms for 3280fps. This is from a 26" factory bbl. As far as published data, most Ultra data is on the 30 and 338. Very little on the 7. If you can get "Speedbump" to chime in, he has shot the 7 Ultra in a match rifle for some time and has tried about every custom bullet and powder combo you can imagine. He is the one "in the know" on this subject. As a matter of fact, he necked down a 300 Ultra case and made a 7 Ultra before there was a 7 Ultra.

db

 

[speedbump]

Len,

Chiming in as you requested. I've tried 10+ powders in the 7mmRUM + 5 bullets, the most recent being some of 'Shaky's' Groove Bullets. The barrel was starting to slip a little at @850 rounds, but a great many of those 850 were near-max loads doing initial testing. It's been re-set once & I have a Pac-nor 3 groove tube enroute for Spring.

Here are some links previously posted. I'll look for more, but this'll get you started:
www.reloadbench.com/ubb/Forum5/HTML/000103.html
www.reloadbench.com/ubb/Forum29/HTML/000265.html
www.longrangehunting.com/cgi-bin/ultimatebb.cgi?ubb=get_topic&f=4&t=000227&p=2

 

[Len Backus]

Speedbump

You made some very helpful comments on "Big Sevens" over on the LRH Rifle Series thread. Here are my thoughts. I have next to zero experience turning necks and forming cases for wildcats. I have been approaching my goal of a super accurate rifle sort of from the bottom up. Never quite satisfied with each iteration in the process.

Now I'd like to make a big jump up and work at it from the top down in terms of effort necessary to own and manage the cartridge combo selected. In a couple months I wll have gained experience with turning necks for a tighter necked version of my favored 7mm Dakota. I am now having it rebarelled as previously discussed. Then, by using a bushing die I hope to better control neck/bullet tension and reduce velocity spreads...and reduce vertical dispersion at 1k ranges.

If I am comfortable with the marginal payback/effort result I will then proceed with a faster version of a 7mm cartridge. In choosing which cartridge to use, case prep and therefor case life (effort not cost) then gets to be a bigger factor. If my tight-necked 7mm Dakota justifies turning necks then I think the Lapua brass will be my choice for the "Big Seven". Sounds like you are getting only a few firings from the Rem brass?

Plus, as you said, there is the "fun" factor to consider.

 

[Guest]

Len, with the lapua case I think you could get better than 3600 fps with the C176. You may need to go to a 9.5 or 10 twist.

 

[STL_Shooter]

This is quite interesting.

I wonder how, for equivalent barrel lengths, with powders currently available, a 176 grain 7mm bullet could be pushed 150 fps faster than a 178 grain 30 cal from the same Lapua case?

I mean, conventional ballistic wisdom says powders have to get slower, or charge weights have to get smaller for equal pressure performance. Or barrels have to get longer (maybe significantly so) for the same weight bullet in a thinner tube to go as fast, let alone significantly faster. Interesting...

 

[Len Backus]

STL

I agree. I would expect the 7mm version of a Lapua improved cartridge to be slower than the 30 version by 100 to 200 fps all else being the same in the barrel and load. Pressure goes up as the size of the pipe goes down.

I forgot what you said you could get with the 178 AMax?

 

[speedbump]

Len,

Yes, I've very few firings per case in 7mmRUM with anything near max pressures. The primer pockets are horribly stretchy. Never had a pierced primer or split neck - just those

primer pockets!

With the Lapua case volume vs. 284 bore diameter, powder burn speed will be an experiment in itself for you. I ate up a SIGNIFICANT percentage of my barrel life just gathering max data when there was virtually none available. I hope you stumble onto one earlier in the process.

I fully understand the 'interesting' aspect of your quest - it's one reason why I jumped on the 7RUM before it was available (besides the incredible ballistic potential). You'll see a trajectory of a 22-250 out to 400-500, then it'll smoke almost anything, INCLUDING a 50BMG to 1,000.

I asked Santa to bring some Lapua, Norma, or even Winchester brass in RUM. I don't think even the jolly one can pull that off!

 

[STL_Shooter]

Len,

Max pressure was reached at 3450 over the Ohler with 95.5g of RL-25 with the 178 A-Max...

Another interesting thing we should probably start noting is the chronograph brand - my ProChrono regularly reads 75-125 fps faster than the Ohler... That kind of thing makes a big difference when decision time comes, of course!