Why does case mouth bell when necked down? Fix?

[BK7saum]

I have necked down some Norma 300 SAUM brass to 7mm. The case mouth has belled a few thousandths over the rest of the neck. Why does this happen? Is the case mouth work hardened from the tumbling/polishing more than the rest of the neck?

Do I need to anneal for the case mouth and neck to be the same diameter?

I necked down the brass in two steps. First through a 7mm SAUM body down to reduce diameter by .007 or .008, then used the Forster size die without decapping pin and expander ball to reduce to final diameter. But the cases have a significant bell to the case mouth.

Just wanting to know why this is occurring and how to fix the problem so the the mouth and neck are the same diameter/size.

Thanks, BLK7

 

[BK7saum]

Re: Why does case mouth bell when necked down? Fix?

Anyone? Was hoping someone might be up as late as I was last night to give me some input on how to size the cases without ending up with the bell at the mouth, since these need to be neck turned.

Well, maybe today. Thanks for looking

BLK7

 

[Furtaker_.223]

Re: Why does case mouth bell when necked down? Fix?

What die are you using? If it is a bushing style you need to take it down to the shoulder.

 

[Victor N TN]

Re: Why does case mouth bell when necked down? Fix?

Try firing the brass a couple of times and see if it doesn't go away.

One thing to remember is that when you squeeze the neck down the extra brass is still on the case neck. Personally I cast and measure the chamber necks so I know what the outside diameter of a loaded round can max at. You MIGHT have to turn the case necks.

The extra working of the brass can make it harder. Annealing is a good idea.

 

[BK7saum]

Re: Why does case mouth bell when necked down? Fix?

This is a tight neck chamber. Reamer is .315. Loaded factory rounds measure .3145-.315 and will chamber. I bought a box of factory to see if I could shoot them. I have shot most of the box after pulling the bullets and turning the necks to .3125 loaded diameter.

I have since annealed the 300 brass and necked some down and the bell on the case mouth is still there. But when I expand up with the expander mandrel, most or all of the bell disappears. I was just surprised to see it in the first place and did not expand back up. I think the bell was about .002 or .003"

Other than my expander mandrel being a little smaller, everything will work out. I just need to order a new E28 from sinclair. I took .0006" off my first one when I was working with RP brass and now it is too small for the necked down norma brass. I don't know why that makes a difference, but it does. The expanded RP brass is very light snug fit on the turning mandrel and the sized expanded 300 to 7mm brass will not fit on the turning mandrel without some serious pressure getting it there.

 

[alf]

Re: Why does case mouth bell when necked down? Fix?

What's your overall case length after necking down? Maybe it's too long? 300 brass is .020 shorter, so it shouldn't be an issue...

Your mandrel doesn't fit cause you took out the expander ball assembly. I've necked 300 SAUM down to 7 for a better fit in a factory Sendero in one pass in a regular RCBS FL die with plenty of lube with no problems. I neck turned, than annealed, after finding dimensions with a chamber cast.

 

[BK7saum]

Re: Why does case mouth bell when necked down? Fix?

I haven't checked the case length with a mic, but it isn't noticeably longer than the 7mm factory that I have checked. Also if it was too long, shouln't it crimp instead of bell.

After sizing, I expanded with the E28 sinclair expander mandrel (which I took .0006" off of when I was working with RP brass). The expander ball assembly was taken out so that I could expand up with an expander mandrel (which takes the place of the expander ball although adds a step to the process).

I have since ordered a new E28 from sinclair which will fix the current problem I'm having.

Thanks.