Enlarging tight necks

[Chiquita]

I have sized my .308 necks to .334 I think they are too tight. without firing the cases, I assume if I use a .306 or .3055 mandrel it will be expanded enough so I can resize to .336. Correct?

Fired neck = .3445
- wall thickness = .0290
Delta = .3055

Or I need to fire them in order to expand the necks.

They are Starline and on the 4th firing.

Thank you.

 

[Chiquita]

I checked my current mandrel and it is .3065. So I guess I ned to enlarge using .308 or .309 mandrel to account for spring back.

 

[MikeMcCasland]

Edit: You can expand with a .309 and then size back down. It probably won't be the exact same as if you'd fired the brass and sized it using the ideal (~.335/.336/.337) bushing, but you'll end up with less tension than you've currently got.

I would probably anneal after you expanded with the .309 mandrel.

 

[Chiquita]

I would probably anneal after you expanded with the .309 mandrel.

Curious why?

 

[MikeMcCasland]

Curious why?

I would basically treat it the same as if I had fired it, thus would anneal.

 

[Chiquita]

I would basically treat it the same as if I had fired it, thus would anneal.

I see.

Thanks.

 

[Ned Ludd]

If you go solely by bullet diameter (0.308") rather than neck wall thickness, a mandrel that is .001" under bullet diameter (i.e. .307") will give you about .0015" neck tension (interference fit) in a neck that has been sized down slightly smaller prior to the mandrel. This is due to spring-back, which means the neck closes back up about .0005" more after the mandrel is withdrawn. A mandrel that is .002" under bullet diameter will give you approximately .0025" neck tension, and so forth. If you're after ~.002" neck tension, the mandrel size you want is likely to be .3065", but actual measurements with a few different mandrel sizes will give you the exact value.

I would suggest simply measuring OD of the necks before/after seating a bullet with a few cases and determining the average in order to quantify neck tension (interference fit). Although you can certainly do it using neck wall thickness, it is a more tricky measurement to make accurately without the proper micrometer, and adding/subtracting neck wall thickness to bullet diameter and overall diameter does not take spring-back into account. All in all, it's a more complicated way to take the measurement than simply measuring OD before/after bullet seating, and is subject to more sources of possible error.

If you have several different diameter bushings (i.e. .334", .335", .336", .337"), you can simply size a case or two with each one, then measure OD before and after seating a bullet in each one. The difference is your neck tension value. I have had good success running somewhere between .0015" and .002" neck tension in .308 Win. YMMV. Importantly, the test cases will not be lost or ruined; you can simply size the necks back down after seating the bullet and measuring using the appropriate bushing that gives you the desired neck tension as indicated by the differential OD measurements before/after seating bullets. It might not be a bad idea to re-anneal those pieces of brass first as was already suggested above.

 

[D-4297]

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