Thread: Necking down 308 to 7mm08

Originally Posted by RimfireNZ

Thanks P38. So from what I've read and seen on YouTube about annealing its to release the tension in the brass and make it less brittle? That about right? Also apparently air and water cooling can have pretty different results. Based on this thread I don't think it would be needed but I'll do more reading and see.

Annealing returns the brass grain structure back to its condition prior to being work hardened. Taking .308 brass and sizing it down to 7mm08 you have to plastically deform the neck i.e. deform it past the point it elastically returns. You can also plastically deform your case neck in firing it (case neck expands to the chamber neck diameter then contracts a little) and then sizing it back to be reloaded again (neck diameter is reduced to allow seating of the projectile with some tension). This plastic deformation work hardens the material, too much work hardening and the material becomes brittle and will easily break. Think of it like a paper clip that you bend a few times until it breaks... this is work hardening.

Plastic deformation deforms the material grain structure. The brass grain structure has to stretch and elongate to allow for the deformation, too much of this and the grain structure will crack. Heating the material up above its annealing temperature (but below its melting temperature) will allow the grain structure to re-crystallize (return to its pre-deformed structure).

Air cooled vs water cooled. Water cooling rapidly cools the brass compared to air cooling. The more rapid the cooling the harder the material ends up... and the less ductility it has.

Personally I resize .308 brass down to .260 and I have not found the need to anneal it yet. That said the chamber in my .260 is tighter than the SAAMI spec and I have to neck turn my re-sized brass. As a result I run very close tolerances between my chamber neck diameter and my loaded brass neck diameter. In running such close tolerances the neck on my brass does not go through much deformation either when it is fired or when I neck size it to be reloaded. Some of my brass has been reloaded nearly 10 times now and I have yet to throw one out.

Originally Posted by Gillie

Annealing returns the brass grain structure back to its condition prior to being work hardened. Taking .308 brass and sizing it down to 7mm08 you have to plastically deform the neck i.e. deform it past the point it elastically returns. You can also plastically deform your case neck in firing it (case neck expands to the chamber neck diameter then contracts a little) and then sizing it back to be reloaded again (neck diameter is reduced to allow seating of the projectile with some tension). This plastic deformation work hardens the material, too much work hardening and the material becomes brittle and will easily break. Think of it like a paper clip that you bend a few times until it breaks... this is work hardening.

Plastic deformation deforms the material grain structure. The brass grain structure has to stretch and elongate to allow for the deformation, too much of this and the grain structure will crack. Heating the material up above its annealing temperature (but below its melting temperature) will allow the grain structure to re-crystallize (return to its pre-deformed structure).

Air cooled vs water cooled. Water cooling rapidly cools the brass compared to air cooling. The more rapid the cooling the harder the material ends up... and the less ductility it has.

Personally I resize .308 brass down to .260 and I have not found the need to anneal it yet. That said the chamber in my .260 is tighter than the SAAMI spec and I have to neck turn my re-sized brass. As a result I run very close tolerances between my chamber neck diameter and my loaded brass neck diameter. In running such close tolerances the neck on my brass does not go through much deformation either when it is fired or when I neck size it to be reloaded. Some of my brass has been reloaded nearly 10 times now and I have yet to throw one out.

Great explanation thanks Gillie.
So if I wanted to get the most out of my brass, I'd anneal the cases AFTER being sized down to 7mm, then air cool them (so they're not as brittle as water cooling), then bob's my uncle (after all the safety checks)?

Originally Posted by RimfireNZ

So if I wanted to get the most out of my brass, I'd anneal the cases AFTER being sized down to 7mm, then air cool them (so they're not as brittle as water cooling), then bob's my uncle (after all the safety checks)?

Yep, that would be my plan. I would check the case neck diameter and case length after you anneal them.

It is marginal whether you need to anneal when necking 308 down to 7mm. I would skip that stage. Maybe anneal after 5 reload cycles. If you are getting no split necks (and you shouldn't) and good accuracy (consistant neck tension) all is good. Will depend more upon the life of the brass after you start using it - how sloppy is you chamber at the neck, how much work you do to the brass when reloading it. Also water quenching brass increases the annealing, the opposite to quenching steel.

Check your neck wall thickness before and after sizing down. I have found when necking up 6.5-284 brass to 284W that even though I have lubed well the process has introduced a wee bit of neck thickness variation. (If for hunting use probably not an issue.) Didn't matter though as I needed to neck turn for my tight chamber. Also if you neck turn your cases be frugal on how much you remove as this can also create extra brass movement in the chamber when fired and the ultimate need to anneal. Long time past I necked down 30-06 brass to 308W and I didn't bother annealing, only neck turned. I probably should have annealed though as I was necking the section of 06 brass that would have been the hardest in the case. Never experienced any problems in the life of the cases after that though. Re the sense of necking down brass to something that is commercially already available - heh that is the joy of reloading. Go for it, neck away.

Originally Posted by RimfireNZ

then air cool them (so they're not as brittle as water cooling), then bob's my uncle (after all the safety checks)?

Other way round, quench them to make the brass soft. If it was just air cool then you would never need to do it as that would be the same as firing and letting the brass cool naturally.

Originally Posted by Wirehunt

Other way round, quench them to make the brass soft. If it was just air cool then you would never need to do it as that would be the same as firing and letting the brass cool naturally.

Been doing a bit of reading and yep, brass would be better annealed using water cooling (as zimmer pointed out this is the opposite to steel and what i was basing my previous post on). Of course still wouldn't be bothered using water as i wouldn't have the patience to wait for them to dry

But firing the case in your rifle is not the same as air cooling after annealing. Annealing you will get the neck of the case much hotter than just firing it in your rifle - well in all my rifles at least as i have never seen a case come out of one of my rifles glowing hot! Hot enough i didn't enjoy it going down the back of my shirt? Absolutely! But i haven't seen one glowing yet!

here's where I ended up.
1 box of LAPUA 308 all run through a LEE f/l die with LEE lube in a LEE hand press.
Loaded,FED 210 43 grn V540 139 SST.


7mm08 SAAMI spec .3150

Loaded round LAPUA .3125

loaded 7mm08 FED .3105

SAAMI chamber spec neck at shoulder .3170 Neck at throat .3160

Fired neckdown case throat .3160

Make of it what you will

call me lazy but if it was me id sell the 308 brass and buy a heap of 7-08 brass!

Originally Posted by savage270

call me lazy but if it was me id sell the 308 brass and buy a heap of 7-08 brass!

Good thinking Batman.

Theres always an easier soultion wating in the wings.

Necking down 308 brass to 7-08 is a simple process but it still leaves one issue that concerns me.

No matter what calibre the case ends up as, it will always be head stamped 308.

Originally Posted by savage270

call me lazy but if it was me id sell the 308 brass and buy a heap of 7-08 brass!

Yeah that's an option, but I'm wanting to add another reloading skill to my toolkit.

When you come to do it do a normal re-size first with 308 die (assuming you have one) but with expander button removed. Put good chamfer on outside of neck, plenty of lube and size to 7mm. The effort required in the press is not great. Again you shouldn't have to anneal particularly only necking down one transition. Different if you were taking it through an intermediate calibre ie 2 sizing steps, which is what needs to be done with some creations. And I am not sure but necking up is probably more devastating for the crystalline structure than down, and up would definitely take the brass through its plastic limit. I have no proof of that gem though.