necking down to 7mm

Depends.

Personally i don't like trying to form on cases that have been shot more than once.

New or once fired, lube, and run through your 7mm-08 F/L die.

Don't forget to trim afterwards!
When you neck down, the neck gets longer.
 
If you have an annealer it wouldn’t hurt to anneal them first, but as long as you lube them good you should be fine without annealing. I’ve necked down .308 to 6.5 and as long as I lubed them good and just slowly ran them through a die in short increments I maybe had one in twenty that would have a crease in the neck. If I ever try that again I’ll anneal them first, I didn’t have an annealer the last time I tried this.
 
I once accidentally ran a 45 Auto case into a 44 Special/Mag carbide sizing die. I didn't even notice by the feel that I had done this, so it was quite a surprise when I lowered the ram to see my new wildcat case sitting in the shell holder. Small reforming steps of 20 or 30 thousandths like this are not a problem.
 
The only neckdown I have done was .45-70 to .33 WCF and .40-65. No problems if done in easy stages. I would hope .30 to 7mm no worse.

Trivia

I once accidentally ran a 45 Auto case into a 44 Special/Mag carbide sizing die. I didn't even notice by the feel that I had done this, so it was quite a surprise when I lowered the ram to see my new wildcat case sitting in the shell holder.
Click to expand...

There was once some interest in cheap .44 plinkers made this way.
 
You're going from .30 down to .28". Its not a big deal. Lube the brass for normal resizing and all should be well.

Yes, you do want to check for length when finished. Also would be a good idea to check the case neck wall thickness. It PROBABLY won't change enough to matter. BUT, check it, to be sure.

"Once fired" .308 brass can be anything, from commercial sporting ammo shot once through a bolt gun to GI brass fired from machine guns. It makes a difference in the amount of case prep you might need to do.
 
do you have to turn the necks downas well when going from 308 to 7-08
Click to expand...

Have to? I don't know, I've never done that particular case forming, but my guess would be it depends on what .308 brass you're starting with, and how thick its neck is. (and if you are moving the case shoulder) Which is why you should measure, to be sure. Neck turning MAY be needed, I can't say. Measuring the formed case neck thickness will tell you.

And, speaking of moving the shoulder, take a close look at the dimensions for the 7-08 and the .308 cases.

Overall length, the 7mm-08 case is slightly longer (0.02"), the case shoulder is slightly longer (0.04"), but the case neck is slightly shorter than the 308. Both cases are the same from the base to the bottom of the shoulder, but then the dimensions differ.

Like I said, I've never done it, but going by the case drawings, what I'm thinking is, that running a .308 case into a 7mm-08 sizer will squeeze down the nect, BUT, since the 7-08 case neck is shorter and the shoulder longer than the .308, I THINK it will leave a .308 size "ring" at the base of the new 7mm case neck.

IF this is what happens, will it chamber in your 7mm-08? I don't know. If it chambers, I would expect it to fireform and be ok.

I have formed a number of cases from other cases, but not that particular combination, so I can't say for sure. You'll need to find someone who has done it (or do it yourself) to be sure of all the little quirks that might apply.
 
I have made a lot of .260 Rem from LC .308. I recommend a light annealing first but I have done it without annealing. Occasionally a shoulder will accordion if it is over annealed. I also anneal afterwards.
 
Calculation by constant cross-sectional area suggests the neck brass would grow in thickness by 0.001". However, in the actual forming, a portion of the brass moves into increased length rather than thickness, so the actual change in thickness would be expected to be less than that. It's not really going to be a significant difference. If you want to, you can outside-turn the necks down 0.001" before sizing them down. This will reduce both the thickening and the lengthening, but, more usefully, will result in more uniform neck thickness in the finished cases.
 
Have to turn the necks. No.
Not in my experience.
The neck is essentially staying where it was on the parent case. Just going down in diameter.

If you had said 7mm-08 from 30-06, that's a whole different story!

I've formed 257 Roberts from 7X57 Mauser. Simple one step operation in a f/l die.
Again, necking down requires trimming.

The only case i've had to do more than that was 6.5mm Rem Mag from 7mm Rem Mag.
2 annealing steps & inside neck reaming involved.
The 6.5 is considerably shorter tham the 7mm case.
 
Not worth it IMO

I tried this and it was difficult with 7.62 brass.
Can it be done? Yes. Should you do it? I decided I didn't want to, and bought new Starline 7mm-08 brass.
 
Just a FYI I did try lubing the 308's and running thru my 7-08 sizing die, did not work out well, seemed to be putting way to much stress on my press and the shoulder on the 308 was starting to bulge..Ill stick with store bought 7-08.
 
While late on this there was a time maybe mid 90s when I could not find 7mm-08 Remington brass but had a mountain of .308 Win brass. While yes in theory the case mouth should grow in thickness I never noticed it. All I did was ran the lubricated brass through a 7mm-08 RCBS sizing die and trimmed. They all worked just fine till I got some 7mm-08 brass.

Ron
 
Another approach would be to get Lee Collet Die to size just the neck down first, then run it through the full-length resizing die.
 
Hmmmm...I've necked down 375 H&H to 7mm STW. I almost forgot I have a BLR in 7mm--08 (it's so pretty I keep it under wraps most of the time) and usually use starline brass for it but have used .308 in a pinch but try to avoid NATO LC brass simply cause it's beefier and often has a crimped primer pocket.
 
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