Big Seven Case Capacities

[Austin]

Not a problem. I have not chrono'd any of the 162's or 168's just yet. I have only worked up a couple loads that are safe and will chrono after the stock work is done. The problem with the hang-fire was this: I started out low on the 870 at around 80grs. This falls around 70% of the case capacity. At these levels, I had consistent hang fires. After working up to about 97.5grs., there was a noticable decrease in hang fire. I tried two different primers. The CCI250, and the Fed 215. Both offered no improvement. As I worked up past the 85% capacity mark, hangfires were fewer and further between. As I neared 88% (around 102grs.), hang-fires pretty much went away. Another thing that helped somewhat on the lighter charges was to warm them by keeping them inside my jacket pocket next to my body rather than out on the bench where freezing cold air is blowing right on them. But, this did not make THAT much difference. So, between 102.5 and 105grs is pretty much the limit. I pushed at 168gr SMK with 109.0grs H870 and began to see extractor/ejector marks on head and the primers began to crater at this pressure. As for the magazine issue, I did have not put one in as of yet. I do know this though: The cartridge will NOT fit in a blind magazine without some work. I designed it pretty much as a single shot rifle. I considered adding a magazine, which could be easily done, but I really never have liked magazines. Just one more thing for me to lose! Anyhow, it seems to be a promising cartridge and it has been a definite challenge and not an inexpensive undertaking either. I've tackled a number of wildcats, but this one would have to take the cake thus far. It's much different than working with a wildcat for which there is absolutely no data out available. What I wouldn't give for a copper crusher! Hope this answered your questions!

TH

 

[Austin]

One thing I noticed you had mentioned earlier about neck-turning. You didn't have experience? It's pretty easy and something you may want to consider. At this time, I use a .313 Titanium Nit. neck die for this case. The neck tolerance runs at .003 (neck of chamber dia. = .3195 and I have to turn the neck down to an even .316 OD after bullet seating. I also have a full-length set which is built from the same reamer used to cut the chamber. This has been a big plus also and i'm glad I invested the money in it. The reamer used to cut this chamber at this present time is yielding a .321 chamber neck which gives you more room to play with your bullet pull. I've found that what helps more in this rifle is to leave about .010 of the neck unsized at the base to better help in cartridge alignment and to seat the bullet right up to the lands. At this point, neck turning is much less of an issue as far as accuracy goes. Many a benchrest shooter I know would have to call me on this one! But, for my needs, and for all of my purposes, I have never really seen neck turning to make that big of a diff unless the reamer you are using is one which is set to cut a match-tolerance neck. Now, I HAVE seen neck sizing help some in the smaller 6mm bore in my 6PPC, but the only reason I turned the necks is because that chamber neck so danged tight! I had to shave .002 - .003 of the brass just to be able to chamber the things, but that is part of the territory with those match chambers. It was a good learning experience though and I DID learn where neck-turning is appropriate and necessary. It seems so far that the cartridge generates quite a considerable amount of pressure even with the slow burners like H870. I have been keeping bullet pull VERY low and seated WAY OUT in the lands. As I said, on some bullets, I have only been seating them about .191" into the case. That's very little and bullet pull is low, but it seems to keep pressures down. So, neck turning will not likely help here even though the chamber is fairly tight. That's why I said that I am learning more and more about the importance of the chamber neck and designing a barrel and action around a certain weight bullet. The next time I undertake this cartridge, it will be from a different reamer with a neck suited especially for the weight of bullet I am looking to shoot. As it stands, this reamer was built for a the larger bullets (168 and 175gr), thus, the neck is a little longer to accomadate these bullets without having too much seated into the case. It was also designed with a 1:12 twist bbl in mind. Though, it has been used succesfully with a gain twist bbl. in the past and has seen velocities exceeding 3600fps with these larger bullets. Given these details, it seems logical to pressume that one could run the 130 and 140 grainers at around the 4000 fps mark in a bbl of this length and twist rate. Just my two cents for what it's worth!

 

[Len Backus]

Thanks, Austin.

And...as of last week I now have some experience turning necks. Not that complicated with good equipment.

My barrel will be throated for the long 176 and 180 grain vld bullets.

 

[Austin]

Let me know what you find when you start testing. You're the only one out there that I have been able to find that is attempting this. You, as I, are very obviously a .284 cal lover being that you are willing to undertake this project!! I'm very interested in any info you may find. I will make sure and give you all the data that I come up with so that you can at least have a good starting point! I will post everything on my website and send you a link to it when I get the database finished. Best of luck!

TH

BTW: You're absolutely right about neck-turning! There's nothing to it! Learning how and where to appropriately apply it was the hardest part, and that wasn't even all that difficult! Peace!

 

[Austin]

Len: Couple more questions for ya:

1) I read that you are planning on using a 34" bbl in this project, but what twist rate are you going to use?

2) Will you be using a muzzle brake?

3) What kind of scale were you using to weigh charges when you were getting vertical stringing at 1k yds? I have had the exact same problem in the past and I switched to a digital scale which made the problem all but disappear.

4) On your switch bbl rifle, what kind of problems (if any) have you noticed when it comes to accuracy? I was considering making this 7/.338 a switch bbl so I can shoot .338 Lapua Mag as well and I am concerned about accuracy. Seems like it would throw the blueprint and true centerline of the action off when you switched bbl's, but, hopefully I am wrong in assuming such and it won't cause me a problem.

I'm very interested in your datum here as I would REALLY like to see just how far the 7mm envelope can truly be pushed.

TH

 

[Len Backus]

1) 9

2)yes

3)digital

4) I doubt the switch barrel setup affects accuracy but maybe some of the benchrest guys can chime in on this question.

5) While I have my own ideas on reamer design, I'd enjoy seeing the drawing for your reamer.

[ 01-12-2004: Message edited by: Len Backus ]

 

[Austin]

Thanks Len. Whereas, I don't have an actual "drawing" per se, I can give you the specs. I will dig them all up and post for you to check out in the next day or so. I will say this: the shoulder is 38 degrees and max trim length is 3.720". I will also include a pic of the loaded case for your perusal if I can figger out how to scan it in since my dig camera is somewhere in a cab (or pawn shop!) in Cancun! Peace bro!

TH

 

[Len Backus]

Thanks, email coming your way.