Making 243 out of 308's

[budlight]

Using a 243 full length they seem to come out pretty good, but do I need to use a neck outer trimmer because the brass will be to thick? My couple of test loads seemed to feed in and out fine.

 

[Jud96]

Using a 243 full length they seem to come out pretty good, but do I need to use a neck outer trimmer because the brass will be to thick? My couple of test loads seemed to feed in and out fine.

You need to inside neck ream! You have to much brass in the neck causing to much tension which leads to extreme pressures. My Dad knew a guy who did this exact thing and the primer completely denigrated and the flash hole in the case was the same size as the entire primer pocket! This is very dangerous, thank God the guy had a Model 7 Remington or he would have probably been dead without those 3 rings of steel.

 

[budlight]

You need to inside neck ream! You have to much brass in the neck causing to much tension which leads to extreme pressures. My Dad knew a guy who did this exact thing and the primer completely denigrated and the flash hole in the case was the same size as the entire primer pocket! This is very dangerous, thank God the guy had a Model 7 Remington or he would have probably been dead without those 3 rings of steel.

I will buy an inside neck reamer then. Thank you

 

[LoneTraveler]

I have made 243 Win. Brass out of 308 Win. years ago. It does make the neck thick. I would suggest use an outside neck turner. It makes the neck thickness uniform all around the neck. You also have a tool that by changing the mandrel you can use it on other caliber of cases. If you have a 308 Win. also I would suggest NOT doing it. I quit necking down 308 Win. to 243 Win. and junked the converted brass I had. When a 308 Win. rifle joined my stable as a work rifle, (Wildlife Officers Carry Rifle) I quit, 308 Win. head stamp became to important to me selecting the right ammo to carry in my job.

 

[Dgd6mm]

I know how hard .243 brass can be to find. If this is what you want to do, do it you will be fine as long as youlightbulb turn the necks. I inside neck ream my 6mm Competition Match brass. My best group to date is a 2" ten shot group at 800 yards that scored a 99 4x on a 600 yard IBS target. They say out side turning yields more accurate ammo but I'm not sure about that. Many target shooters years ago inside neck reamed and shot small groups.gun)

 

[Bill Johnson]

The problem with inside reaming is that to be accurate, many times you need a custom die that holds your case and neck perfectly so the reamer doesn't just follow the hole. Inside reaming, at that time, was to ensure the inside of the neck was perfectly in line with the case, flash hole and throat. It works great but is expensive and time consuming. It also requires specific reamers depending on when you ream: after shooting or after forming/sizing. Inside reaming also leaves the neck walls uneven/inconsistant so, for best accuracy, you ofter end up outside turning anyway.

These days, most benchrest shooters outside neck turn. This results in consistant neck wall thickness, something inside reaming doesn't do. Sinclair recommends outside turning over inside reaming although they have the tooling for both methods.

The only possible drawback to outside turning is the so-called inside "doughnut". This occurs because, during the reforming process to a smaller caliber, material flows in to the shoulder and neck. Outside turning does not remove that extra material from the neck-shoulder junction. Sometimes, then, a ring of brass forms on the inside of that cunction.

The way I avoid that is first, anneal the neck and shoulder. Then I run a full size decapping pin through it. I say full size because many of mine are reduced from factory diameter .001 or 2. Them use the K&M sizing mandrel which is designed to precisely size the inside on the neck to fit their neck turning tool. These two operations move some of that extra material from the neck/shoulder cunction up into the neck. Then neck turn so necks walls are a consistant .012 to .014.

Turning necks on factory cases to .012 generally removes material from about 75% of the neck as neck wall thickness generally varies from .012 to .018. In your case, you'll probably remove material completely around the neck is you turn to achieve that .012 to .014 wall thickness.

 

[Dgd6mm]

what Bill posted is definitely the best way for benchrest shooting. also the correct way for inside neck reaming.

 

[Clark]

On 9-12-2003 I took my 1938 Turkish Mauser with 243 Addams and Bennet bull barrel to the range in Puyallup. I had trued the action, forged over the bolt handle, drilled and tapped, and put a scope on it. I necked down some LC 308 brass to 243.

Group 11) With 38 gr 4895 and PMC 243 brass, the bolt was already sticky.
Group 12) With 100 gr bullets and 40 gr of bulk 4895 that acts more like H322, and necked down some LC 308 brass to 243, there was real trouble. It sometimes took 100 pounds of force on the bolt handle knob to close the action on the ammo. I had to pound the bolt open each time. The last one I had to pound out the brass.

I quit, went home and took a pic of group 12 brass. Within a few months the pic had been downloaded 10,000 times.

Years went by, and I got better at gunsmithing and reloading.
I was doing real measurement on the chamber neck and the loaded necks, and found out that 308 necked down to 260 will be trouble ~ 20% of the cases. The factory chamber neck is 0.299" at the rear and 0.298" at the front. Chambers from my SAAMI 260 reamer are about the same. 308 brass when necked down will interfere in a way that cause erratic reactions [although tiny compared to the 243] about 20% of the time with RP or Win brass. With Lapua brass all the necks must be turned.