300 blackout and Lee resizing dies neck tension.

I have a neck tension gauge from Ballistic tools and noticed that the Lee resizing die sizes it to the. 308 diameter is the OK? If I use my RCBS small base dies it sizes the neck to 305. Is there any concern I should worry about?

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So the inside diameters are .308 and .305? What are the outside diameters? I’m curious about neck wall thickness, is why I ask. Without enough neck tension, you would have to worry about bullet setback during feeding, and you may not get proper velocities. I myself would like at least .002 neck tension to ensure proper ignition and to hold the bullet in place


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Bullet outside is 308. I'm confused because Lee excellent dies. Bullet won't start in the case after resized with the Lee resizing die, but am getting 308 I. D. with the tool. I'm using Hornaday 150gr Fmjbt as a FYI

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Originally Posted by JeffreyDeGraff

So the inside diameters are .308 and .305? What are the outside diameters? I’m curious about neck wall thickness, is why I ask. Without enough neck tension, you would have to worry about bullet setback during feeding, and you may not get proper velocities. I myself would like at least .002 neck tension to ensure proper ignition and to hold the bullet in place


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What is the outside diameter of the necks of your brass that have .308 and .305 inner diameters?


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Even .305 is just fine. The bullet needs neck tension. If you are loading the 150gr FMJ/BT, it will seat just fine and will go in with pressure.
Those gauges are a bit tough to use, and aren't accurate unless you chamfer the case mouth first.
Seat a few and see what it feels like

I just reloaded about 200 300 AAC. I used Lee Dies. The Neck Tension was tight, I didn't measure it, but the 110 Gr Varmegeddons seated with no issues. Cycled thru my Son's 300 and fired and ejected, no issues.

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Originally Posted by Wes Sage

Even .305 is just fine. The bullet needs neck tension. If you are loading the 150gr FMJ/BT, it will seat just fine and will go in with pressure.
Those gauges are a bit tough to use, and aren't accurate unless you chamfer the case mouth first.
Seat a few and see what it feels like

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That’s what I was thinking. .305 is just about perfect, but the die that only sizes to .308, doesn’t seem like enough to me.


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Gotcha Wes. Thanks for the help.

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Originally Posted by Wes Sage

Even .305 is just fine. The bullet needs neck tension. If you are loading the 150gr FMJ/BT, it will seat just fine and will go in with pressure.
Those gauges are a bit tough to use, and aren't accurate unless you chamfer the case mouth first.
Seat a few and see what it feels like

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So here is a way to measure the neck tension on your bullets. After you size your brass with your lee die, measure the outside diameter of the case neck. Then after you seat your bullet, measure the outside diameter of the neck again. The difference of those two numbers would give you an idea of how much neck tension there is. If it grows by at least .002”, that should be enough neck tension.


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Thanks Jeff as well. Seems correct to me. I'll seat some to see how they feel.

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Originally Posted by JeffreyDeGraff

That’s what I was thinking. .305 is just about perfect, but the die that only sizes to .308, doesn’t seem like enough to me.


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