Home Forums Hunting & Shooting Ask The Gunwriters COW in a large case?

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!