Necking Down 300 Win Mag brass

tractorkid · September 26, 2009, 02:01 PM

Hi Everybody
New to the forum here and still learning my way around, question for you guys. I have probably 150 once fired 300 win mag cases, got them from a friend when he traded in his rifle. I don't have a 300 win mag but I do have a 7MM remington mag, I measured up the cases and everything looks the same except the 300 is a bit longer, has anyone trimmed and necked down 300 win mag brass to use in a 7MM Rem Mag?? I thought I would pick a few brains here and see how easy that it to do? Figured it would save my trying to buy 7MM brass for a while.

Thanks

BOb

**Mike Irwin** · September 26, 2009, 02:34 PM

7mm Mag is a bit shorter, with the shoulder farther back.

You'd have to run the cases into a 7mm Mag. die to move the shoulder.

You could trim to length before or after, that shouldn't be a problem.

One thing you'd have to make sure of is that, with the bullet seated, the outside neck diameter isn't excessive.

If it is, you'll have to ream the necks to provide proper chamber clearance.

Shoney · September 28, 2009, 11:57 AM

tractorkid

WELCOME TO TFL!!!

Mke Irwin's post is right on the mark. However, it may be very easy for the novice reloader to pass right over the weight of his comment:

Quote:

One thing you'd have to make sure of is that, with the bullet seated, the outside neck diameter isn't excessive.

If it is, you'll have to ream the necks to provide proper chamber clearance.

The important significance is that the neck will be much thicker than a normal 7Mag's standart thickness as the neck is sized smaller. The resulting cartridge may fit snugly in the chamber and fire normally. However, the little bit of extra thickness may squeeze down on the bullet enough to raise pressure levels, some pressure spikes may be dangerously high.

I was at a range where a very experienced reloader had necked down 300WM to 7Mag, and loaded them up to his normal load, which was just below max for his Browning semi-auto. The first cartridge in the chamber had to be pushed into battery. It KB'd blowing the magazine out the bottom. Fortunately he received no significant injuries.

Take care and Good Shooting!

**Mike Irwin** · September 28, 2009, 12:01 PM

Exactly. If the chamber pinches the case neck against the bullet and, in essence, locks it into place, it won't release as pressure rises.

That creates a nasty little scenario that happens with progressive burning propellants (which modern smokeless is...)

The higher the pressure, the more efficiently the propellant burns, which increases chamber pressure, which makes the powder burn more efficiently, which increases chamber pressure, and so forth and so on.

HiBC · September 29, 2009, 02:11 AM

Agreed with what has been said.Merely necking down a few thousandths is not so big a deal,but setting the shoulder back squeezes down a lot of material.

I have found that even once fired brass is often hardened enough to not form successfully

You might find someone here will PM you because they would really like to have some 300 win mag brass.Then you could get some 7 mag brass.Do a search for "once fired cartridge brass" and you may find a deal.There are outfits that sell it.

As PT Barnum once said,"There is an a$$ for every seat!"

Sevens · September 29, 2009, 07:20 AM

Bob, I can do a swap for some of your .300 WM as I have a small handful of 7mm Mag to trade for it. I'm quite sure I don't have 150 pieces, but if you'd like to trade for what I do have, send me a PM. (I'm thinking it's like 30-some pieces)

GeauxTide · September 29, 2009, 07:55 AM

The shoulder must be set back .156 and the case length is .12 shorter. I wouldn't attempt this.

**Mike Irwin** · September 29, 2009, 11:35 AM

There's no real problem with doing it.

It just requires attention to details.

Some years ago I turned 50 .30-06 cases, at 63mm long, into 50 .300 Savage cases, 51 mm long.

For that drastic a change it required a form die.

I didn't have a form die for .300 Savage, but I did have one for 7.65x53 Argentine, so I first ran the .30-06 cases into that and rough trimmed them.

Then I ran them into a .300 Savage die with the expander stem removed.

Then I outside reamed the case necks to bring them down to spec, then I annealed the case mouths.

I then trimmed to length, ran them through the .300 Savage die with the expander die back in to do the final forming.

End result was 50 very usable .300 Savage cases.

In order to make that drastic a change in the case, you need a good, sturdy press, preferably one with compound linkage to increase the force. I wouldn't want to try it with a Lee cast aluminum turret press.

I used an enormous Hollywood Turret Press, all cast iron and weighing about 200 pounds, from the 1950s. It didn't have compound linkage, but a piece of iron pipe slipped over the handle worked wonders.

Pathfinder45 · September 29, 2009, 02:44 PM

.....If it was .338 Winchester or .264 Winchester I'd say OK 'cuz they're more alike. Just trade 'em off at the gun show to one of the brass traders. Keep it simple; life's short enough.

September 26, 2009, 02:01 PM	#1
tractorkid Junior Member Join Date: September 26, 2009 Posts: 1	Necking Down 300 Win Mag brass Hi Everybody New to the forum here and still learning my way around, question for you guys. I have probably 150 once fired 300 win mag cases, got them from a friend when he traded in his rifle. I don't have a 300 win mag but I do have a 7MM remington mag, I measured up the cases and everything looks the same except the 300 is a bit longer, has anyone trimmed and necked down 300 win mag brass to use in a 7MM Rem Mag?? I thought I would pick a few brains here and see how easy that it to do? Figured it would save my trying to buy 7MM brass for a while. Thanks BOb

September 26, 2009, 02:34 PM	#2
Mike Irwin Staff Join Date: April 13, 2000 Location: Northern Virginia Posts: 41,380	7mm Mag is a bit shorter, with the shoulder farther back. You'd have to run the cases into a 7mm Mag. die to move the shoulder. You could trim to length before or after, that shouldn't be a problem. One thing you'd have to make sure of is that, with the bullet seated, the outside neck diameter isn't excessive. If it is, you'll have to ream the necks to provide proper chamber clearance. __________________ "The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza Baby Jesus cries when the fat redneck doesn't have military-grade firepower.

September 28, 2009, 12:01 PM	#4
Mike Irwin Staff Join Date: April 13, 2000 Location: Northern Virginia Posts: 41,380	Exactly. If the chamber pinches the case neck against the bullet and, in essence, locks it into place, it won't release as pressure rises. That creates a nasty little scenario that happens with progressive burning propellants (which modern smokeless is...) The higher the pressure, the more efficiently the propellant burns, which increases chamber pressure, which makes the powder burn more efficiently, which increases chamber pressure, and so forth and so on. __________________ "The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza Baby Jesus cries when the fat redneck doesn't have military-grade firepower.

September 29, 2009, 07:20 AM	#6
Sevens Senior Member Join Date: July 28, 2007 Location: Ohio Posts: 11,756	Bob, I can do a swap for some of your .300 WM as I have a small handful of 7mm Mag to trade for it. I'm quite sure I don't have 150 pieces, but if you'd like to trade for what I do have, send me a PM. (I'm thinking it's like 30-some pieces) __________________ *Attention Brass rats and other reloaders:* I really need .327 Federal Magnum brass, no lot size too small. Tell me what caliber you need and I'll see what I have to swap. PM me and we'll discuss.

September 29, 2009, 07:55 AM	#7
GeauxTide Senior Member Join Date: April 20, 2009 Location: Helena, AL Posts: 4,423	My .156 The shoulder must be set back .156 and the case length is .12 shorter. I wouldn't attempt this.

September 29, 2009, 02:11 AM	#5
HiBC Senior Member Join Date: November 13, 2006 Posts: 8,283	Agreed with what has been said.Merely necking down a few thousandths is not so big a deal,but setting the shoulder back squeezes down a lot of material. I have found that even once fired brass is often hardened enough to not form successfully You might find someone here will PM you because they would really like to have some 300 win mag brass.Then you could get some 7 mag brass.Do a search for "once fired cartridge brass" and you may find a deal.There are outfits that sell it. As PT Barnum once said,"There is an a$$ for every seat!"

September 29, 2009, 11:35 AM	#8
Mike Irwin Staff Join Date: April 13, 2000 Location: Northern Virginia Posts: 41,380	There's no real problem with doing it. It just requires attention to details. Some years ago I turned 50 .30-06 cases, at 63mm long, into 50 .300 Savage cases, 51 mm long. For that drastic a change it required a form die. I didn't have a form die for .300 Savage, but I did have one for 7.65x53 Argentine, so I first ran the .30-06 cases into that and rough trimmed them. Then I ran them into a .300 Savage die with the expander stem removed. Then I outside reamed the case necks to bring them down to spec, then I annealed the case mouths. I then trimmed to length, ran them through the .300 Savage die with the expander die back in to do the final forming. End result was 50 very usable .300 Savage cases. In order to make that drastic a change in the case, you need a good, sturdy press, preferably one with compound linkage to increase the force. I wouldn't want to try it with a Lee cast aluminum turret press. I used an enormous Hollywood Turret Press, all cast iron and weighing about 200 pounds, from the 1950s. It didn't have compound linkage, but a piece of iron pipe slipped over the handle worked wonders. __________________ "The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza Baby Jesus cries when the fat redneck doesn't have military-grade firepower.

September 29, 2009, 02:44 PM	#9
Pathfinder45 Senior Member Join Date: January 7, 2008 Posts: 3,224	Don't bother with it.... .....If it was .338 Winchester or .264 Winchester I'd say OK 'cuz they're more alike. Just trade 'em off at the gun show to one of the brass traders. Keep it simple; life's short enough.