Cornmeal forming bad on a new barrel??

[Len Backus]

Interesting thread. I will be forming .284 wildcat cases from .338 Lapua cases. Do you think the extra amount of neck size reduction (over that of .338 down to .308) will require any changes in method?

[ 01-24-2004: Message edited by: Len Backus ]

 

[tomahawk #1]

Len

I would turn necks before fireforming.
I make my 338 tomahawk cases from 375 ultra mag brass and turn before fireforming and cases come out perfect

Tomahawk#1

 

[Len Backus]

Tomahawk

You're probably right but please explain why. My 7 Lapua's reamer's neck dimension will be .3150. I know from the experience with my 7mm Dakota that if I take off about one thou in turning that particular brand of brass, then my loaded round will measure .3113 for about .0037 total clearance.

With the 7 Lapua Improved project, in forming the neck down further, as stated above, will I get thicker necks and therefor have to experiment with a few unturned necks in fireforming before I then go ahead and turn a bunch of brass in advance of fireforming?

 

[tomahawk #1]

Len

I would try a few as you stated but i think you will gain neck thickness when you take them down to 7mm

Tomahawk#1

 

[Len Backus]

Yes, I thought so. I assumed I'd have to take more off if I want that same .3113 loaded dimension.

 

[tomahawk #1]

Len

It would be interesting to know how much neck thickness you gained going from 338 to 7mm.

Tomahawk#1

 

[Brent]

Len,

<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR>Interesting thread. I will be forming .284 wildcat cases from .338 Lapua cases. Do you think the extra amount of neck size reduction (over that of .338 down to .308) will require any changes in method?<HR></BLOCKQUOTE>[

What's your neck length on this one going to be?

Have you go another die for this one, or where you planning on using the 30/338 LI die and smaller bushings?

Is this one just a necked down version of your 30/338 Lapua Imp, with the neck a tad shorter from necking it down smaller than 30?

 

[Len Backus]

Brent

Don't know yet. Haven't finalized the reamer design.

 

[MAX]

I don't know the truth of it but have read advice against using corn meal. Maybe in Dr. Howell's Blue book? Wherever it came from the caution was that it may not flow thru bottleneck cases and result in overpressure scenarios. Rick Jamison wrote an article some time back that was critical about COW due to substantial erosion from fireform rounds. I'm sure I cannot explain it, but one of life's facts is that soft materials can and do cause wear/erosion on harder materials. Why else would diamond blades wear? Or carbide grow dull?

I like Bounty Hunter's approach.

 

[Brent]

Len,
If you go with a longer neck than that of the 338 Lapua, turning the necks before you fire form them will yield less than satisfactory results IMHO, especially if you have a tight neck chamber. Inconsistant neck thickness the length of the neck is what you'll likely end up with.

The 338 Lapua shoulder ends up being IN the neck on your new cartridge, and leaves a smaller outside diameter "ring" at the new junction on top of this, so the neck looks like a wave when looking down into it. No mandrel will remove it, and it has to be hammered out by pressure. IF you're LUCKY, you might be able to turn and ream the high spots after running them through the sizer die and end up consistant, IF the low spots inside and out turn out to be higher than the amount you're cutting inside and out.

Clear as mud?

I would go with a .375" neck length on the 7mm, .400" seems about perfect for the 30, and maybe .425" on the 338's.

The throat errosion on my 30-338 here is so little it's almost too good to be true, but fire forming with COW after FL sizing the virgin stuff, I would consider necessary for a good neck turning job.

Moving the shoulder forward will leave you with a short neck, and I believe throat errosion will be greatly accelerated, but you WILL be able to turn the stuff and not have to form with COW first. I would might go this route on a 338, 416 or something larger having this case volume, but no way I'd do it on a 30 or smaller with what I've learned now... One reason I don't particularly care for the 300 and smaller Ultra Mags, short necks...

My 2 cents.