Setting neck tension with a mandrel

[avidflyer]

working with a new batch of palma brass and wanting to set neck tension via a mandrel. i've got a 306 mandrel and a 307.5 mandrel figuring about .001 for spring back on new brass. pressure ring measures 308.5 (30cal bullet) With spring back looks like the 307.5 mandrel will give it about .002 neck tension I think the 306 would be too tight. anyone have an idea of what spring back on 308 would be, is 1K a good number

 

[Richard Coody]

Great idea

You will just have to experiment. Your target will tell you when you get it right.

 

[Ned Ludd]

I open up .308 case necks on virgin Lapua Palma brass with Sinclair's oversized expander mandrel, which is approximately .307" diameter. Bullets seated directly into these brass necks would be way too loose, IMO. I then re-ssize using an FL die with a .336" bushing, which tightens the neck up a smidge more. However, it is enough to make a noticeable difference. Neck tension of the fully prepped brass is ~.002", so I'm guessing the necks might have .001" or even slightly less neck tension if treated only with the .307" mandrel and not re-sized afterward. All you really need to do to figure out which of your mandrels to use is to simply measure the neck outside diameter before/after seating bullets in cases prepped with the different mandrels. Whichever mandrel gives difference of about .002" is where I'd start. Hope that helps.

 

[avidflyer]

was mainly wondering about spring back of fresh new brass. Agree Greg that 3075 might be a little loose (of sorts) I just need to make up a couple dummy rounds

 

[KMart]

The neck will spring back various amounts depending on the size of the mandrel and how much you size the neck down. I was doing some trial and error the other day on a 6 BR necks. I was using a .242 mandrel.
Sized some necks down with a 266 bushing(neck measured .266) and expanded with the .242 mandrel. Necks measured close to .2667.
Then sized necks with 264 bushing(neck measured .264) and expanded with the 242 mandrel. Necks measured .266.
You have to test the necks with your tools to get the results you are looking for.

 

[BoydAllen]

The IDs of expanded necks will get slightly smaller over time. This is basic neck turning experience. Dwell time (time that the full diameter is in the neck) will also affect the result.

 

[ireload2]

You might try a long tapered mandrel with a total taper that gives you a full range of grip from .000 to about .006 grip over 2.0" if your case is long enough. This in effect will give you a one size fits all "adjustable diameter" mandrel.

 

['Freak]

The IDs of expanded necks will get slightly smaller over time. This is basic neck turning experience. Dwell time (time that the full diameter is in the neck) will also affect the result.

Is the opposite true (ID gets slightly larger over time) of necks sized down but not expanded?

 

[BoydAllen]

Is the opposite true (ID gets slightly larger over time) of necks sized down but not expanded?

I don't know, but I do know that if you size a case and wait two weeks it will take more force to seat a bullet in it than it takes to seat one in a case that was sized right before seating. This may have nothing to do with ID.

 

[avidflyer]

I don't know, but I do know that if you size a case and wait two weeks it will take more force to seat a bullet in it than it takes to seat one in a case that was sized right before seating. This may have nothing to do with ID.

I've noticed that but thought that was more spring back. The case was tightening back up. Been doing a lot of studying up on brass and metal working and seems a lot of people wont size a large batch of brass and let is set. (say you did a lot of 400 pieces) they only size and anneal what there going to load.

 

[BoydAllen]

It is kind of funny that with all this discussion that none of us has tested this. If we are dealing with turned necks and sizing with or without expanding, if changes in bullet seating effort are really caused by changes in ID, then something easier to measure, neck ODs, should show it. Of course measuring this reliably would require careful technique since the 40 TPI of a typical micrometer has a lot of leverage when it comes to measuring a thin walled brass tube. a lock and drag through technique would probably be best.