Big Seven Case Capacities

[Brent]

Len, I shot the 178 A-Max in my 30/338 Lapua IMP one time so far, 9 shots. The Oehler 43 picked up two shots as light was fading which were 3365 and 3404 fps. That was with 95gr RL25. I imagine 95.5gr would get me a solid 3400, or maybe a tad bit more. I could get 3450 - 3500 without a problem, pressure is still pretty low at 95gr. You run H870 or something and you might get 3600 with the 7mm if the barrel was long enough. I'd guess 3400-3500 in a long barrel for accuracy, and hope for better.

A side note:
Dad used 7 WSM data for the 140 load development on his 6.5 WSM, and even starting loads were way to high, obviously from the step down in barrel I.D. making more pressure, and the 6.5 bullets longer bearing surface.

 

[Len Backus]

Brent

I know that Mr. Bifurcated's barrel length is 32 inches, what is yours? I am planning a 34 inch barrel this time. STL's is 32 and if we are ever seen together I want to have a "bigger one" than he does. Size does matter, you know. By the way, you should see the shape of the brake on the end of his thing. Its diameter is larger than the end of the barrel and its shape is like a.....no, I can't say in public!


The Hogdon online reloading data indicates that the 300 Rum gains about 200 fps when going from a 220 caliber bullet down to a 190 grain (same pressure). That's in a short barrel. I don't know if a longer 32 inch one would show a larger or smaller difference. The Hogdon data shows about a 200 fps difference between 175 grain bullet velocities 7mm vs 30 cal.

So that would suggest adding 200 FPS to STL's 3,300 with the 210 grain bullet when working up a 180 grain load. And then drop 200 fps when switching to the 7 RUM pipe size. Resulting in 3,300 fps for a 7 Lapua with 32 inch barrel and the 176 grain Cauterucio bullet. Add about 40 to 50 fps for my 34 inch barrel for a total of 3,350 fps. So, 3,400 may be pushing it a little.

Incidentally, Hogdon doesn't sell H870 anymore and they warn that any H870 being sold today by others may not match Hogdon's data for H870.

STL

That's interesting about chronograph brand velocity reading differences. I know Oehler is supposed to be the most consistent but I never would have expected that much difference.

[ 12-14-2003: Message edited by: Len Backus ]

 

[dwm]

For what it's worth I measured 3300+ FPS with a 168 SMK and 100 grains of H50BMG with a 7MM RUM and a 28" barrel. Pretty sure that I could have gone faster ...

Add 8 grains more bullet and 6 inches to the barrel ...

I've never had a primer pocket loosen in 7MM RUM Remington brass so I must not operate anywhere near the pressures some do.

I get bad head stamps and bolts that are quite hard to lift from time to time.

I have had a case separate after 8 or 9 firings ...

Doug

 

[dbhostler]

dwm,

So you have found H50BMG to work best with the heavier 7s in your Ultra? What twist are we talking? I too have had no problems with brass, SB must really be pushing them out the bbl.

db

 

[Len Backus]

db

You said your case water capacity is 113 grains. Could you please describe your method of measuring.

STL

Can you tell us the capacity of your improved Lapua case.

Anyone else...Wayne, Brent ?????

[ 12-14-2003: Message edited by: Len Backus ]

 

[Guest]

Len,
I measured the water weight capacity of one of my 300 WAMBOMAG cases and it was 116.1 grains. This was with a primer installed upsidedown and filled to the top of the neck, level not rounded. I added a drop of dishsoap to about a cup of water, don't know why, just heard it helps.

 

[dwm]

dbhostler

I have an 11 twist barrel.

168 SMK shoot fine, 168 Bergers keyholed but I didn't try running them real fast, they may have stablized.

I was not really interested in shooting 180 grain bullets when I bought the barrel. I thought 168 SMK and 162 SST/AMAX would do fine for my intended use.

I was concerned with barrel/throat errosion and wanted the 160+ grainers to go real fast. So I went for the 11 twist.

If one were to go with a barrel that was 34 inches long and get some serious velocity, I think a 10 twist would be fast enough, even for big bullets.

Would be interesting to know what twist the other 7MM RUM/Weatherby users have.

I know that a lot of people will be thinking 9 twist is right for the real big bullets, but I think the real fast twist contributes to the burned out barrel/throat thing. If you can get them to higher velocities they will stablize with a slower twist.

The bigger the bullet and the faster the twist the harder it is to get them spun up and pushed down the barrel. Something to do with overcoming the rotational moment of inertia of the bullet and the added friction due to a larger bearing surface. I think this holds the fire ball closer to the throat for a longer period of time. This is purely a thought exercise and I have no experience or proof to back it up though.

Doug

[ 12-14-2003: Message edited by: dwm ]

 

[dbhostler]

Len,

I just filled the case with water, dumped it into a powder pan and weighed it. After doing this, I realized I should have weighed the case, filled it with water, weighed again and subtracted the difference. I actually doubt, however, that the wet walls would contribute even one grain difference. I can do this again if you would like.

dwm,

My factory bbl has a 1/9.5 twist. Some of the factory loads run up to 160grns, so I think I am safe with the medium heavy bullets. The only drawback is the rifle has a 26" bbl.

db

 

[Len Backus]

db

Yes, please try it again. Could you use Wayne's method with the primer inserted just so both are measured the same?

 

[Guest]

The way I measured the capacity is just like you stated, weigh an empty then add the water, reweigh and subtract the empty weight. I also used a spent primer installed backwards, should have stated that before. I don't know if trying to put in a live primer backwards would be safe or not.
Wayne

 

[Brent]

Len,

My barrel is 30".

After reading Wayne's post with his weighing 116.1gr, I went back to look at mine, mine was also 116.1gr, not sure where I came up with 117gr. Maybe I figured when the necks lengthened back out to 2.720" they would be 117gr. They were all trimmed to 2.690" after fire forming and 116.1gr is wrote right on the side of them.

Wayne's case has a bit more capacity above the shoulder because his has a 35 degree and mine has a 40 degree, mine is a tiny bit larger in diameter below the shoulder though (.570"), which is why I think we have basically the same capacity in the two.

210 Bergers blew holes right through my 3/8" steel plate at 600 yards yesterday, and almost through the 1/2" plate. They're going 3200 fps. I've got a bunch of welding up to do on 'em now!

Going back out to try 800 and 1000 yards today, a few shots before it's dark anyway.

DB,

The water left in the case WILL add about 2-3gr in my experience. Even if you tap it out real well it will always weigh over by this amount. I weigh them first, primer upside down, fire formed, drop of dish soap in the eye drop bottle, top it off until the water is flush to the top. No big bubble on top, just flush so you can barely see the water looking at it across the case mouth from the side.

Tap every bit of water out of the case afterward to see what the difference in dry weight verses wet weight is.

Now there's a difference in the way your rifle is throated for long bullets, which could give an even bigger edge to the shorter fatter Lapua case design, that is if your magazine box doesn't allow the big bullets to be seated out in the Ultra's, it can using the Lapua case. The neck on the Ultra is real short, which also eats up more capacity when seating bullets when a chamber is improperly throated for them. If each is throated properly, the mag box will still limit the Ultra or Weatherby/Rigby case where as the Lapua will gain an edge on it as it won't have to be seated any deeper.

Loaded OAL at land contact for the 30-338 Lapua IMP with:
210 JLK....... 3.818"
210 Berger.... 3.741"
240 SMK....... 3.694"
200 Accubond.. 3.769"
178 A-Max..... 3.701"

I think I'm going to have to end this shooting with Michael, he pretty much wipped my *** with his awesome shooting 308win yesterday! When I shot his 308, I found out his secret! It's a freakin 1 ton trigger pull!!!

Must be all of 10lbs anyway. He stacks 'em in there with it though, 3 different bullets too! You don't ever want to be in this guys sights!

 

[dbhostler]

Len,

Fired case, primer seated upsidedown, case neck sized a couple of thou, 393.2gr wet, 278.2gr dry = 115.0gr. Different case than the first however.
New case, primer seated upsidedown, 390.8gr wet, 278.7gr dry = 112.1gr. This is soft water, if that makes a dif.

Hope this helps,

db

 

[Austin]

After coming upon this forum, I found that you are trying do exactly what I have been working on for some time now. I currently have a rifle chambered for a .338 Lap necked down to 7mm. It is based on a Remington action, has a 28" Pac-Nor 1:9" SuperMatch bbl, blueprinted/trued action and bolt face, heavy recoil lug, Sako extractor, bbl is free floated and is glass/pillar bedded into a blue-laminate, tactical-style stock. It has an adjustable butt-plate with a kick-eeze pad installed. It is a hard-core powder burner. The cartridge is called a Big 7 TurnerBurner. It is one of the proprietary cartridges from a local gunsmith here. We decided we would try something new and build the rifle with a 1:9" bbl and take advantage of the 130, 140, and 154gr. bullets (which are perfect for Texas whitetail!). It loves the 140gr Ballistic Silver-Tips from Nosler and it burns about 109.5grs. WC872 with the bullet seat right at the lands. On lighter loads of about 100.0grs and the same bullet, chrono readings were 3685, 3649, and 3693 for three shots. With the heavier bullets, such as the 168gr. Sierra MatchKings, pressure signs become evident at 107.5grs. of H870. This appears to be the limit for this bullet / powder combo and would say the max safe load would be about 102.5 - 105.0 grs. I am currently working on developing more load data for the 162gr A-Max which is running pretty close to the same capactity as the 168 grainers. So far, H870 has had some problems with hang-fires at loads below 100grs. (tried two different primers, so detonation is more suspect since this cartridge seems to prefer charges > 88% with H870) Between 102.5 and 105grs. H870 looks like a good comfortable area for my rifle with a 168gr. Sierra MK and a Fed 215 primer. WC872 has shown VERY promising results without hang-fires also. Will report more on this later.
The cases are Norma .338 Lapua Mag and were made by necking them down in a two step process, fire-forming with a 140gr bullet seated on the lands on top of 17grs. Unique. After initial fire-forming, cases are annealed, and shot with a full load to complete the fire-form process. After this, cases are all trimmed to a length of 2.715", flash holes deburred, primer pocket reamed, and then, culled by weight. Final case capacity is 116.3grs H2O.
I am doing some final work on the stock, so I will be running some 130gr. MatchKings on top of 112grs. WC872 through the chrono some time next week when I finish. I will publish more ballistic/load data as I gather it. I'll let you know my results soon. Maybe some pics also.

One more note: When building this caliber, I have noticed that throat size becomes critical. Many times, with the 130gr. bullets, I am seating only .191" of the bullet in order to make contact with the lands. Usually, I seat right on the lands and drop my powder charges back, but I have had to seat at .005" - 009" off the lands with the 140gr bullets to maintain sufficient bullet pull. With this twist, the throat could stand to be .008 - .010" shorter for the lighter bullets. Have not really shot for accuracy too much yet, but 140gr. Nosler BT's were hitting at under 1/2" mark at 100yds. Seems to be an EXTREMELY flat, accurate shooter so far, just needs more time in development.

So far, COAL for tested bullets is as follows:

All bullets seated to contact with lands.

162gr. Hornady A-Max: 3.818"
168gr. Sierra MatchKing: 3.705"
140gr. Nosler Bal. Tip.: 3.702"

[ 01-07-2004: Message edited by: Austin ]

 

[Len Backus]

Austin

Thanks for all the good info on your project. Do you have any velocities on the heavier bulets you shot...168, 162?

Can you describe your hang-fire situation, please.

Do those long coal's fit in your magazine?