I need load recommendations for COW forming 338-378WBY.

A buddy has a P17 Enfield chambered in 338-378WBY and I necked up some new 30-378WBY cases to use in it but mine are .025" shorter at the shoulder than his fired cases.

He's fired factory WBY ammo in his rifle so I could load as is but I'd like to maximize case life and not start the cases off with a stretch.

Thanks

Do the cases from the factory ammo he fired show signs of excessive stretching?

No,They all look fine.
I'm a little nervous with a non factory chamber but the factory ammo fired well.
I'm wishing he'd saved a single cartridge of the factory stuff for comparison but I have none.

The problem with case stretching usually occurs with rimless cases fired in a slightly longer chamber. The case gets slammed forward by the firing pin, expands against the case wall, which unless there's considerable oil in the chamber, holds the case body while the case head gets pushed back against the bolt face. As a result the case stretches between the body and thicker head.

In a properly cut belted chamber, with decent brass (and Weatherby brass is usually good) the belt keeps the case from being shoved very far forward. As a result, the head is held firmly while the case body expands to fit the chamber. The only way the case will stretch between body and head is if the belt portion of the chamber is sloppily cut, or the belts on the brass are too short.

If a problem exists between the chamber and belt, then the COW method will also stretch brass, because if enough powder is used to expand the case fully the same thing happens.

I would try one round and see if there's any evidence of case over-stretching. But if your friend had been firing factory ammo with no problems then there shouldn't be with yours.

I have seen new factory 300 win mag 20 thou short (Nosler and Winchester). I would seat the bullets to touch and use moderate loads. Bet they shoot just fine.

17 gr of Unique will bump them out

Thank you Sir!

I needed some help to reason this puzzle out.
I'll load a single round and will go from there.

I'd neck them up to 35 caliber first, then size them back down just enough to create a false shoulder to headspace on.

Even with the belt wouldn't aalf's suggestion be pretty good for minimizing case stretch?

With a proper crush fit, there'd be zero case stretch concerns.

I've fireformed thousands of Dasher cases blowing the shoulder forward a hundred thou with a 40 degree shoulder, using a false shoulder with nary a problem.

I don't believe there is that much 'shoulder' created when you stretch the neck to create it. I've tried with with COW both ways and in the end there was not much difference in case life. What is 'short' for the shoulder is very common for even factory belted cases and I've seen no difference in case life with and without creating the false should. Blow them out both ways and see for yourself. I'll bet you'll be fine doing it the 'standard' way.

Thanks AALF, It makes sense to capture the brass with the false shoulder and expand the brass shoulder forward to fit the chamber.

I'll try both, some load and shoot and a false shoulder too, so I can see If either makes a difference in this rifle.

If brass were cheaper I wouldn't hesitate but this stuff is a little steep. Link here

Thanks for the replies!

I would be tempted to just load and shoot after verifying the belt was functioning as a rim for head space. Any additional forming or false shoulder is just working the brass more.

Only issue is that you may need to adjust your loads after the initial firing as capacity will be increased slightly.

A big cartridge like this is a good candidate for reduced practice loads. A middling load with 160s or 180s would be impressive on a rock chuck.

Which was my suggestion as well. If the belted cases headspace OK in the chamber, then they won't stretch.

One other note: Something I've seen happen more than occasionally when using a seated-out bullet or false shoulder to"hold" headspace is that heavy-striking firing pins (like those on most military rifles, including the 1917) often generate so much momentum they'll still push the case too far into the chamber. I've often had to remove the extractor-ball assembly from sizing dies to get a firm enough neck-grip on seated-out bullets, or neck the case up a lot more than normally suggested when creating a false shoulder.

Didn't mean to be redundant. Since you mentioned oily chambers is it advisable to fire form the first load in an oily chamber assuming it is not a max load? Some say this reduces stretching and forms better than a dry chamber.

I left Hornaday One wax on some 280 AI cases when doing COW forming and it seemed to work. Had a couple of miss fires that fired the second time due to case slippage. This was bulletless so no jamming.

Had a 35 on an 8mm Rem mag case. Had no dies so used 358 Norma neck die with 375 H&H cases. Just ran them in the die to neck them down, then COW'd them in the new chamber.

Worked slick and never lost a case. I guess it headspaced on the belt for forming but it sure was easy to do.

Tejano,

It's often good to be redundant on the Campfire! Many people don't read all the posts on a thread after the first few.

Bob,

I also had a .35 Shooting Times Alaskan for a while, and also used .375 cases. But did have .35 STA dies, and fire-formed the cases while working up loads.

With belted cases that headspace correctly in the chamber, on the belt of that particular brass, there's no danger of case stretching. Same deal with rimmed case, like the .22 K-Hornet. The big problem is rimless cases. But occasionally there'll be a belted or rimmed chamber sloppy headspace, and those have to be treated like rimless chambers, using bullets seated into the lands or creating secondary shoulders.

You can say that again.

It would have to be a really big case to put any of my cows in.

I often wonder what a casual web-surfer concludes when reading our posts

Thanks for all your thoughts and replies, I hope for a little range time this weekend and I'll post a report on findings!

Personally, I don't like the COW method. I formed some cases for my 6.5-06 AI with COW just for the convenience of being able to fire the rounds in my garage. Got some case separations, not at the head but at the shoulder of the .25-06 brass I was using (not the new shoulder), all the way around the case.

The virgin brass had the slight crush fit it's supposed to have in an AI chamber and I had a gunsmith double-check the headspace, it was perfect.

Never had any case separations shooting fire-forming loads.

I had the same thing happen with some new .338 Winchester Magnum brass, when it was fired in a standard .338 WM chamber. Turned out that batch of brass was badly annealed.

Have never had anything like it happen with the COW method.

That could've been part of the problem, but once I saw those cases separate I unloaded the rest of the COW rounds and had no separations with the normal FF rounds. Same brass.

Interesting!

What brand of brass?

It was Norma .270 brass.

I guess you can run into all sorts of odd problems. I've never AI'd a standard case but belts work good.

7mm Mashburn Super from 300 Win Mag using COW. No problem.

This is what I did with the .280 RCBS when making brass out of .270 Brass. Up to .308 and back down to .284 to just a snug fit. Worked great. I used a Lee expander in my universal decapping die, as Lee pins have a good taper to them. miles

It stunk up the garage too.

LOL worst part!

Ill bet it did! I tried it in the basement at first....NOT a good idea!

On the other hand, they worked like a charm to chase away those pesky trick-or-treaters last year. The muzzle blast can be seen a half a block away at night....

An update on the 338-378Wby project,

I don't have the rifle in hand and I was running out of time so I gave up the dream of setting a false shoulder with a .358 expander to control where the case would expand with a COW load.

I had to resort to "load and shoot as is" and see if they would form nicely.
The brass is new 30-378 necked up to .338 and they all formed perfectly.
The shoulder pushed forward ~.023" to fit the chamber with no signs of stretching or separating internally.

He's fired factory Wby. ammo in this rifle so I didn't expect any anomalies or pressure spikes with my loads and sure enough, velocities with my loads were within 10fps of loads listed by Barnes, Hodgdon and Nosler.
That velocities were so close to those listed was a surprise.
I have another 40 new cases to form so I'll do some COW loads as first planned just because I'm curious and want to compare.
So far it seems that with new cases, in this rifle, I can load and be done.

Thanks for the thoughts, advice, and reasoning you all offered, I do appreciate it!

Thanks for the follow-up report.

A long time ago I used Cream of Wheat -early 1980's - 7 TCU cases out of military 223 cases.

I've not tried it since. I've improved 270 cases by making them 280 cases. I've also made 350 Rem Mag cases out if 'standard' mag cases. And 7mm-08 cases out of 243 cases. 338 Win Mag out of 7mm Rem Mag cases. I've tried to make 22-250 cases out of 30-06 cases - just because. Plus I've made 256 win mag out of 357 cases. There's other I suppose, only the 7 TCU were required to be reformed. The others lots of cheap brass that I wanted to use in other chamberings.

With belted and rimmed cases it was a snap. Others I used cast bullet loads and double shoulders AND pushing the bullet hard into rifling. It worked ok.