7mm Rem Mag Case Stretch

[Prairie Sasquatch]

I've recently started loading some new brass for my 7 Rem Mag. I've never loaded new brass for this gun. In fact I've loaded very little for it in the past as I bought it for a moose hunt and got a decent load put together and shot a moose. Now I'm trying to work up a longer range load with the 162 ELDX.

New RP brass measures 2.1045" at the shoulder. First firing brass measured 2.1170" at shoulder. Second firing measured 2.1200" at shoulder and still chambers without any resistance.

I have a few questions. Is this excessive stretch for the 7 mag and do I need to worry about case head separation? Second, will the cases settled down now after this second firing and should I just wait until later for any shoulder bump.

I'm not running hot loads. I found a mid range load that shoots well while once firing and the second loading of this required a .3 grain drop in powder charge to maintain same accuracy and velocity.

Any thoughts are appreciated.

 

[Dean2]

That is not excessive stretch and pretty normal for the first couple of firings on new brass. Just make sure when you size the brass you don't bump irt back too much. All my Belted Mag brass gets neck sized because I found FL made the brass grow near the belt and after 5 or 6 firings it didn't chamber well. There is a special die made to fix that but I found it easier to just neck size. I test fit all my hunting ammo, reloads, or factory, before using it anyhow so I know it chambers easily.

 

[Muddyboots]

This is what you want, Peterson 7MM RM Long brass. The headspace will be about 0.003 less than your chamber on average. You get more loads since you are not going thru excessive stretch on first load. Grafs out but sign up and maybe score. I bought same for 300WM and love the headspace new brass measurement. I know you have brass but I would keep as backup once you grab these.

Peterson Brass 7mm Remington Mag Long Unprimed Bulk Box of 250 - Graf & Sons

This is a Bulk package of Peterson Cartridge casings, they will not come in one of their plastic ammo boxes. Made in the U.S.A. Peterson Cartridge Co is committed to producing Match-Grade Brass that enables our customers to get more reloads per casing than industry average. Warning: Only use...

www.grafs.com

 

[MagnumManiac]

The stretch is normal. To size belted mags, you MUST fire it 3 times while neck sizing for those firings…then measure, then adjust your FL Die to bump .0015" and keep it there.
There truly is no need for long brass as long as you don't bump your shoulders back more than the above, .002" is MAX for good brass life.

Cheers.

 

[Muddyboots]

Yes the stretch is completely normal but if you can eliminate it with long brass, why not? Any time there is an opportunity to extend brass life is worth the change IMO.

 

[jimbires]

you can form a false shoulder on the standard length brass , this will help on case stretch .from what I've seen it's to be about .015" is what belted cases stretch .


+1 on the long brass , this will solve this problem .

 

[AZShooter]

This seems to be fairly common with belted magnums. I was amazed how much shorter my friend's chamber was in his Mark X in 300 win mag when compared to my model 70 also in 300 win mag.

Seems to me the solution for this nonsense is to buy a custom reamer made with minimal headspace between the shoulder and the base.

 

[243winxb]

A Guy always neck sized his 7 Rem mag, till it was hard to chamber the rounds. Then fl sized, controlling the shoulder bump. Still hard to dhamber. More bump, no go.

Because of neck sizing, the unsupported area in front of the belt became to large in diameter. Standard fl dies dont size there.

https://www.larrywillis.com/The fix.

 

[MagnumManiac]

A Guy always neck sized his 7 Rem mag, till it was hard to chamber the rounds. Then fl sized, controlling the shoulder bump. Still hard to dhamber. More bump, no go.

Because of neck sizing, the unsupported area in front of the belt became to large in diameter. Standard fl dies dont size there.

https://www.larrywillis.com/The fix.

Actually, you CAN use a body die, it does the same thing…or use a larger cases' die, such as a 375 Weatherby die and size the case just above the belt with nothing else touching.
Anyway, I have never seen a bulge on my cases except for ONE instance and that was with thin webbed brass made by Norma…

Cheers.

 

[victor3ranger]

Yep what everyone else has said

 

[MagnumManiac]

This seems to be fairly common with belted magnums. I was amazed how much shorter my friend's chamber was in his Mark X in 300 win mag when compared to my model 70 also in 300 win mag.

Seems to me the solution for this nonsense is to buy a custom reamer made with minimal headspace between the shoulder and the base.

The A191 reamer, as used by your military, takes care of all of these problems and is what I based my reamer prints on, or just used it as is. Some don't like the throat length, but it is superior mechanically.

Cheers.

 

[cajun]

yeah about 15-16 thou on both 7 mags I've loaded for. I have 350 rem mag that's about 24. I found out the hard way why you should measure your resizing on that one. Lee dies were pushing the shoulder back .016 when following instructions.

 

[Tiny Tim]

A Guy always neck sized his 7 Rem mag, till it was hard to chamber the rounds. Then fl sized, controlling the shoulder bump. Still hard to dhamber. More bump, no go.

Because of neck sizing, the unsupported area in front of the belt became to large in diameter. Standard fl dies dont size there.

https://www.larrywillis.com/The fix.

I guess I'm having a hard time grasping how FL sizing prevents this issue if it's incapable of fixing it once it happens. Seems to me that if that portion is outside of the ability of a standard FL die to size, its irrelevant. Agree about the Willis die as the fix no matter the process that led to the problem.

 

[Dean2]

A Guy always neck sized his 7 Rem mag, till it was hard to chamber the rounds. Then fl sized, controlling the shoulder bump. Still hard to dhamber. More bump, no go.

Because of neck sizing, the unsupported area in front of the belt became to large in diameter. Standard fl dies dont size there.

https://www.larrywillis.com/The fix.

You realise you have this backwards I hope. FL sizing creates the bulge because the brass from squeezing down the body has to go somewhere. Neck sizing could not possibly push brass down to the belt, at most it would cause the necks to get longer and need trimming, but I have never seen that happen on any of my belted mags. As far as bumping the shoulder when needed, all of my neck dies, bushing and standard, are capable of doing that, I don't need to use a FL die to get a shoulder bump. Neck sizing belted cases I get over 20 firings per case, you won't do that FL sizing unless you use the Whidden die.