Neck Tension for Semi Auto

[gomeybear]

Whats a good neck tension for semi auto 300 blackout? I feel like I might be on the looser end - Im getting .002 approx.

[gomeybear]

Wizards, dellet, 20x11, bangbangping! Need yalls take!

[Regaj]

Whats a good neck tension for semi auto 300 blackout? I feel like I might be on the looser end - Im getting .002 approx.

So, are you having a problem with your current neck tension? You mentioned a couple weeks ago you that wanted to shave a thousandth off your expander ball. Did you do that?

There's no one "right" neck tension amount. The bullet you're using and its seating depth significantly affect how much bullet pull exists. So does the type and condition of your brass (elasticity and grip changes as a case work hardens). Moly or other types of bullet coating can dramatically reduce bullet pull. As can any residual lubrication on the inside of the case neck - whether that be a deliberate use, like graphite powder; or unintentional, as in the spray lubricants, intended for the outside of the case, that are much in vogue these days. Even the carbon on the inside of the neck from a previous firing can act as a lubricant. Crimps, excessively or improperly applied, can actually reduce neck tension. And compressed loads exert some amount of force on the base of the bullet... and in the presence of one or more of those other issues can move the bullet.

All that said, and with everything else in good shape... 0.002" would generally be considered adequate. If you're still wanting to fool with it (neck tension) I'd remove the expander ball from my sizing die and go to mandrel-based neck sizing.

[dellet]

Just enough to keep the bullet from moving, either banging around in the magazine, or when chambering.

In addition to the things Regaj mentioned, the three biggest issues with not being able to give a concrete number for this cartridge are
extreme bullet weight variations,
combine that with compressed loads,
overgassed operating systems creating extreme bolt speeds.

If you crimp, neck tension becomes a non issue.

Anything over .004" is generally pointless but may damage bullets with softer jackets or cast.

[gomeybear]

Just enough to keep the bullet from moving, either banging around in the magazine, or when chambering.

In addition to the things Regaj mentioned, the three biggest issues with not being able to give a concrete number for this cartridge are
extreme bullet weight variations,
combine that with compressed loads,
overgassed operating systems creating extreme bolt speeds.

If you crimp, neck tension becomes a non issue.

Anything over .004" is generally pointless but may damage bullets with softer jackets or cast.

I do get a bit of forward movement on 125 tnt speer bullets. Usually .008” to .01” COAL increase after measuring a bullet thats been chambered. Thoughts?

[dellet]

Just enough to keep the bullet from moving, either banging around in the magazine, or when chambering.

In addition to the things Regaj mentioned, the three biggest issues with not being able to give a concrete number for this cartridge are
extreme bullet weight variations,
combine that with compressed loads,
overgassed operating systems creating extreme bolt speeds.

If you crimp, neck tension becomes a non issue.

Anything over .004" is generally pointless but may damage bullets with softer jackets or cast.

I do get a bit of forward movement on 125 tnt speer bullets. Usually .008” to .01” COAL increase after measuring a bullet thats been chambered. Thoughts?

Where is it happening, chamber for certain?

Does it happen with other bullets?

First thing I would do is sow down the bolt speed and maybe lighten the buffer, depending on what you are using.

A smaller diameter expander may help depending on the brass.

If I was to crimp it would likely be with a Lee factory crimp die.

I would get the rifle where i wanted it first before playing with the load, ad this is a big part of the reason that if I am serious about accuracy I go to carbine gas.

[gomeybear]

Just enough to keep the bullet from moving, either banging around in the magazine, or when chambering.

In addition to the things Regaj mentioned, the three biggest issues with not being able to give a concrete number for this cartridge are
extreme bullet weight variations,
combine that with compressed loads,
overgassed operating systems creating extreme bolt speeds.

If you crimp, neck tension becomes a non issue.

Anything over .004" is generally pointless but may damage bullets with softer jackets or cast.

I do get a bit of forward movement on 125 tnt speer bullets. Usually .008” to .01” COAL increase after measuring a bullet thats been chambered. Thoughts?

Where is it happening, chamber for certain?

Does it happen with other bullets?

First thing I would do is sow down the bolt speed and maybe lighten the buffer, depending on what you are using.

A smaller diameter expander may help depending on the brass.

If I was to crimp it would likely be with a Lee factory crimp die.

I would get the rifle where i wanted it first before playing with the load, ad this is a big part of the reason that if I am serious about accuracy I go to carbine gas.

It’s happening when I chamber the round. I extract very slowly so there isn’t additional pull on the bullet by the chamber.

[dellet]

It’s happening when I chamber the round. I extract very slowly so there isn’t additional pull on the bullet by the chamber.

So basically it’s acting like an inertia puller. You have two choices, decrease the amount of hammer force, or increase the hold on the bullet.

One will also effect accuracy, the other may effect reliability of cycling.

.004” of neck diameter difference is a pretty normal number for semi auto. I think it’s pretty well established that more than that does not increase the amount of grip or hold the case neck will have on the bullet. That’s why I say more than that is pointless.

You could reduce the diameter of your sizing stem to get more than .002”. That’s probably the easiest thing to try..

Reducing bolt force will involve cutting back the gas and or reducing buffer weight. My preference is to tune the rifle first then the load.

Not sure if that helps, but it’s my thinking.

[rebel]

The only other thing I might add is that if you decide to crimp, like with a Lee factory crimp die, just kiss the brass with the collet. As dellet said, much more than .004 and you begin to deform the bullet. I remember a thread many years ago where I guy posted a pic of his rounds for ,I believe, was a completely unrelated problem. The brass was noticeably dug into the bullet with a heavy roll crimp.
You don't need much to hold it in place.

[gomeybear]

Id be wanting to avoid crimping. With that being the case, should I try to bump up tension .001 from my approx. .002 that Im currently getting?