308 necked down to 243

[JackSprat]

I use 308 brass in my 7mm08,and it sizes so easy that you cant even feel it as it goes thru the die.It is just a hair short,but not enough to matter to me..I picked up a 243 today,and intend to order some dies,I still have half of the 308 brass I ordered for my 7mm-08,and if it will neck down as easily as to 243 as it did for the 7mm-08 I'll use it,if not I will order,or buy some actual 243 brass..Have any of you necked 308 down that far,and if so does it end up too short,or is there any other problems I may run into?

 

[Dog Soldier]

It is more important that you ream the I.D. of the necks. The .308 necks are too thick after sizing down to a 6MM. When the loaded round is chambered the pressure rise becomes dynamic before the .234 bullet is released. Always anneal the .308 brass before sizing down.

 

[FROGO207]

Necking down to 243 (6MM) from 308 (especially military brass) may cause necks to become thicker and thus require turning to get proper neck tension and allow the brass to enter the chamber all the way. If you try it compare neck wall thickness with a piece of regular 243 brass. It might just be me but I would like to have a different head stamp on my brass so that along with neck diameter they are plainly distinguishable from the other. You should be able to find some reasonably priced 243 brass in the reloading components area of the trading post on this site as well.

Dog soldier beat me to it. +1 on the annealing as well.

 

[243winxb]

Maximun loaded rounds neck diameter is .276" Sizing down LC 308 brass produced hard chambering.

My 243 Fl die could not push the shoulder back the extra .002" that was needed.

Size down in steps. 7mm-08 without the expander , then 243 win may help.

 

[kingmt]

I size straight from 308 to 243 with no problems. I've never done any LC brass but the ones I have done have worked great with no neck turning needed. There hasn't been any signs of pressure or anything else odd. The cases end up just a little short but still plenty to hold the bullet. I try to only use free brass & I don't find much 243 anymore but I do find 308. Funny thing is I had found several 243 brass is what made me decide to buy this caliber. I've been missing for it since day one & the two 243s that are in my house have never seen factory ammo.

 

[Dog Soldier]

This is an interesting Link for serious re-loaders to study.

http://www.rifleshootermag.com/reloading/ammo-reloading-tips/

 

[Clark]

With Win or RP brass, I can neck down from 308 to 260 and have less than 0.001" clearance around the loaded neck. Most can be snuck through, but 10% need to be thrown out, so it is not a good process. With 243, forget it. I tried that in 2003. The resulting picture of trouble was downloaded 20,000 times in the first year.

 

[farmerboy78]

I resized .308 to 6.5 Creedmoor with ease. Had to neck turn though, I use a k&m neck turner.

 

[homatok]

As noted above, downsizing to a smaller caliber MAY leave you with a neck that is too thick. Then again, it MAY NOT. If the neck is too thick it can cause a (sometimes catastrophic) spike in pressure as the bullet is held back from exiting the case. Here is a seat of the pants check:
Measure the inside diameter of a factory round case fired in your firearm with the crimp remnants removed. This should be done on a new or once fired case that used a full powered load. Your maximum, safe bullet diameter will be 0.0005” to 0.0010” smaller than your measurement. That is a rough estimate of what the chamber will allow without the potential of spiking pressure.

Another way is to measure an unfired factory round and then make a dummy round using a sized-down donor case with the bullet you plan to use seated in it. As long as the dummy round chambers with no resistance, and (at the neck) measures the same or less than the factory round, you are good to go. It is best to use a micrometre to get your measurements because a caliper is really not accurate enough.

 

[kingmt]

Calipers aren't a great tool when you need an exact measurement .001"or less on the ID but they would be fine for what you propose. You don't really need the measurement. You just need to know the bullet is smaller than the hole once formed to the hole.

If you try forming a case I'd use 8gn of Bullseye, Red Dot, or something in that burn rate. Put a tiny toilet paper wad on it then a pinch of corn mill (enough to fill 1/8" up the case) & another paper wad of you like. Firing this up in the air will do a good job of forming the case. If a bullet will slide in this case by hand then you have enough room.

Another way would be to size the case & load or dummy round it & smoke the case with a sharpy. Chamber it to see if there is a interference fit. If there is it'll rub most of the marker off. If it doesn't you should be good.

 

[Clark]

I have an off the shelf SAAMI 243 reamer with some taper in the neck. The ammo can have some taper. This really gets to be a 3 dimensional problem. If you really know what you are doing, with 6mmPPC and have a an extra chamber neck cut on a piece of the barrel cut with the same lubricant at the cut at the same lathe speed [yes that affects inside diameter], then you can shoot with tolerances in the tens of thousands of neck clearance. But if you are some putz like me making hunting ammo........ you want real clearance of more than 0.001". I can push pin gauges into a chamber an map the neck inside diameter.
And don't forget the brass being too long. Sizing brass makes it longer. Firing makes it shorter. Long brass can pinch the bullet and jack up the pressure too.
Every year I built rifles in new cartridge and kill big game. Every time case forming is required, there is lots of trouble that needs to get straightened out. If you have the books on brass conversion by Donnelly, Howell, and others.... the joke on instructions should be "then a miracle happens".

 

[JackSprat]

I guess my best bet would be to order some actual 243 brass..Thanks everyone

 

[kingmt]

Yelp. Leave that 308 brass at the range. I'll be by to pick it up.