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Anyone own a 7mm 300 Win Mag?

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Unread 04-27-2011, 06:40 PM
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Join Date: Aug 2009
Location: California
Posts: 134
Re: Anyone own a 7mm 300 Win Mag?

I'm almost 100% positive that this is a pressure problem but don't know if it's because of too much powder or an enlarged factory chamber. Barrel was a factory unfired take off and could CONCEIVABLY have been overbored, dunno and no way of finding out. Current chamber cast indicates .5165 just above the belt. Here's what my smith had to say about it:

Rick, I have a couple of thoughts on the issues you are having. Sound like way too much pressureÖthis could be caused by too much powder or a little headspace. Since I did the head spacing I will need to assume it is an overload issue. I would agree w/ him. He is VERY meticuous and has built several rifles for myself and friends. I SERIOUSLY doubt any mistake lies w/ him. When you do the initial fireforming I would make the following suggestions:
1) I would neck size the cases as you have been but I would leave a bit of unsized .30 cal neck showing so that you must forcefully chamber the cases. This insures no head space and a perfectly fireformed case.
2) I would use a very very mild load and consider the fireforming process to be a necessary step but rounds not to be used to gauge accuracy or hunting. I have used a small charge of pistol powder to fireform cases it doesnít take much of an explosion to get the job done. To give you some idea of how little of a bang it takes if you insert a case with fresh primer and no powder it will somewhat fireform the case. Of course the bullet will lodge in the barrel and you would have to knock it out but the point being that you donít even need to be at a minimum load to fireform. I suggest min load or up to 10% under the min load for fireforming.
The expansion of the brass to about .517 shows excessive pressure. If you get more than .001 of permanent deformation of the case above the belt you have too much pressure. This was my understanding as well. .005-006 is way too much! The SAMI spec for cartridges in the area just above the belt is .513. You have cast your chamber and find it to be .5165 this is a bit larger than I would like to see. The reamer I used to cut your chamber was brand new and measures .510 just ahead of the belt. The reamer probably did not touch the factory chamber in this area when I did my re-chambering. So, IF the original chamber was oversized, then the only alternative is to set it back, correct? Usually the case jumps out a couple of thousands and then rebounds back to original dimensions when fired. So with a chamber as large as .5165 your brass is jumping out to the chamber wall and staying there. Way too much pressure. What should happen is the brass springs out but then returns to its original .510 (in this case). Since the chamber is a bit pot bellied the cases might spring back to .511 -.512 but to stay at .5165 means you had pressure in the range of 70,000 + thus resulting in the brass flowing and attaining permanent deformation. Not a good situation.

I suggest you fireform some cases as I suggest above using reduced loads. Measure the case above the belt it should be the original .510 but not top exceed .511-.512. If the cases are not at least that diameter or less stop and call me. If they are .511-.512 I then suggest you reload these cases in increments starting at a minimum load. Fire the cases and then remeasure the area above the belt when you see the belt expand .001 above the dimension you had before firing you have arrived at your maximum load.

I'm off for a weeks vacation and won't get to do any more testing for a couple of weeks. I MIGHT take the gun w/ me and let smithy set it back IF that's what he recommends. Thanks for the input all! If I don't answer for a few days, please know I'm on a beach somewhere w/ a scantily clad wife and an adult beverage in my hand! lol

Originally Posted by SidTheKidd View Post
Just measured the brass and it was was about .512 just above the belt. This was brass that has been fireformed in my rifle. Most were right there.
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Unread 04-28-2011, 10:31 PM
Bronze Member
Join Date: Mar 2006
Location: Salida, California
Posts: 87
Re: Anyone own a 7mm 300 Win Mag?

Gentlemen, I have an Interarms mark X 7mm mag factory rifle. The only thing not factory is it is glass bedded in a McMillan stock. It usually shoots 3/4 MOA with 162g.

Is this be a good donor rifle for a 7- 300wm or 7mm practical? Would I be able to just rebarrel? Would it feed OK and would the new loaded rounds fit in the magazine. I want to shoot 180g Bergers.

I have a 280AI that shoot 168g Bergerís at 2875 around 1/2 MOA on a Howa 1500 SS and a 26" Pac Nor barrel. So I would only be interested in more performance than the 280AI and 7mm mag shooting heavier bullets.

This caliber sounds really good.

Thanks for your help.
When discussing caliber, Dead is dead and itís not worth arguing about.
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Unread 04-29-2011, 08:06 AM
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Join Date: Feb 2007
Location: Townsend, Montana.
Posts: 8,644
Re: Anyone own a 7mm 300 Win Mag?

clamb4, I do not know the mag box length of the interarms x so I cant say about the ammo fitting or feeding. Another factor would be how much freebore was in the reamer you used, how deep you ended up seating the bullets and so on. But if you are looking for a 7mm that will send the 180's at 3100 plus and do it in good brass the 7mm-300 is a great choice.

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Unread 04-29-2011, 12:17 PM
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Join Date: Mar 2006
Location: Salida, California
Posts: 87
Re: Anyone own a 7mm 300 Win Mag?

I take it that I measure the inside of the mag box and get a reamer to cut the chamber say .010Ē longer than the length of the mag box measurement. Then I would be able to set OAL from .010 from the lands on out by seating the bullet deeper in the case until I found a sweet spot.

I assume that even if the bullet is being seated deeper into the case than normal (if the mag box is shorter than some others, have not checked yet) the MV would still be enough that it would be worth doing. After all the mag box is made for 7mm mag.

This rifle has been setting in my safe for a long time maybe 15 or 20 years, which is before Berger bullets. I hunted with it for about 5 years and killed a lot of deer with it.

From reading here it sounds like keeping the 25 deg shoulder would make it easier to get dies and form brass.

I did some numbers on JBM ballistics software 3500í elevation and 65deg temperature.

7mm/300wm 180g Berger at 3125ísec at 1000 yards is -18.1 MOA, 10 cross wind 4.2 MOA, and velocity is 1980í sec.

My 338 RUM at 2900í mv with a 300g SMK @ 1000 yards is -20.4 MOA, 10 cross wind 4.0 MOA, and velocity is 1916í sec.

The only real advantage of the RUM is ft/lbs of energy 2445 compared to 1989 for the 7mm bullet. And the added frontal area of the 338 bullet kills stuff.

The 7mm/300wm is sounding better all the time.
When discussing caliber, Dead is dead and itís not worth arguing about.
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Unread 05-01-2011, 08:31 PM
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Join Date: Jan 2006
Location: Saskatchewan, Canada
Posts: 27
Re: Anyone own a 7mm 300 Win Mag?

Hey Guys:

Funny I should find this post.......I have been too busy latey to check all the hunting / shooting sites...and then I find this thread!!!

I thought of this cartrige a while back (also very close to the 7mm Mashburn Mag of yesteryear) and wanted to get STW velocities but not have to deal with finding brass (Canada), or seating the bullets too far into the case.

I just recieved my Rem 700 in H-S stock in a 7/300 Win Mag. My reamer is simply a .300 win mag necked down to 7mm. with no throat and a floating pilot. I retained the 25 deg shoulder so I can body size in a .300 win Mag dies and neck size in a 7mm mag die........a cheap man's 3 die set!!!

I finally took it to the range and fire formed some loads and broke the Shilen barrel in. I went with a #5 conour and made the gun so I could carry it all day and shoot deer at any sane range.

I started with H1000 and the 162 A-max, found that I could get to 3350 fps in a 27" barrel before pressure signs and heavy bolt lift, but the accuracy node was around 3250 and the gun will hold .5 MOA or better at this velocity........the best group so far went 3 rounds into .073" x .095" !!!!!!!

I then tried Retumbo and some 180 Bergers and the Max velocity is around 3175 fps.....any more and the loads are too hot in my gun. Those loads held around 1/2" - 5/8" MOA.

I have more work to do with this gun and cartridge but so far its a keeper!!! I will be giving it a go at the local F-Class rifle range this summer and it should give those 6mm....6.5 mmm boys a run for their money!!! Then hunting season!!!

I will try and keep up with this post and update when I can!!! Any info that the guys can share will be great!!

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Unread 05-02-2011, 06:43 PM
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Join Date: Aug 2008
Location: E. TN
Posts: 87
Re: Anyone own a 7mm 300 Win Mag?

I have a new, never used 7mm-300WM reamer I'll let go for $100.
-Tres @ WildernessMeans.com

Check my website for articles, training info, etc!

NOTE: I'm a machinist, gunsmith, writer, and instructor of many outdoor topics looking for gainful employment in any geographical cool place to live. Resumes Available.

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Unread 06-04-2011, 08:54 PM
Junior Member
Join Date: Jun 2011
Location: Cottonwood Arizona
Posts: 1
Re: Anyone own a 7mm 300 Win Mag?

I finished my 7mm/300 win mag a couple of months ago, and I finally broke down and bought the Quickloads program. Saves me a lot of frustration when developing and building a new round.

When I was fire forming my first set of brass, I started out low pressures but with enough powder for a case fill that I didn't worry about an air gap bomb.

Myself, I really hate doing the old style of "cream of wheat" fire forming.

My latest goof before trying the quickloads program, was trying 64 grains of Varget on the 7mm-300 win Mag and sure enough I blew out primers and flattened a couple right off the bat.

This quickloads is expensive, but well worth the money when building a new round. IMHO..

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