Lemme make sure I have this correct, neck only sizing AR brass?

Nope. AR’s only close the bolt with the force of the recoil buffer spring. Neck sized only brass will be a snug fit in the chamber it was fired it. You need more force than a spring will reliably provide.

Neck sizing is for bolt guns only. Unless you enjoy clearing jams. Sticky jams.

I personally wouldn't do it, but if it's your only AR, trying it with loading up a couple and chambering them both fast (bolt slamming it in) and slow (riding the bolt down, and maybe hitting the forward assist if needed) and if they chamber and fire, you might be OK. If they were to ever get into another AR, it'd be a crap shoot (possibly literally) as to whether it wouldn't KABOOM. Not worth the infinitesimal gain in accuracy you might get.
If the Camp Perry bunch did it with their Tubb 'space gun' AR's, then it might be worth looking into. Slamfire, what say you on this?

Generally, if neck sizing for a bolt rifle, it is most reliable if fired in the same rifle. Not all chambers are created equal.

Generally, besides semi-auto rifles, folks do not neck size for lever action rifles. Lever actions do not have sufficient mechanical advantage to chamber over size ammunition.

You have to bump the shoulder back at least a few thousandths... for reliable chambering.

In general, you don’t want to neck-size only for any rifle action. Doing so for bolt action rifles was a short term fad based on theory which has since been disproven.

Beyond the disadvantage for all actions, neck-sizing-only comes with inherent mechanical risks for levers, pumps, and semi-autos. With a few firings, the case expansion will typically be sufficient to cause excess resistance to chambering, and these other actions simply don’t have the mechanical advantage available to close on an oversized case.

So neck-sizing-only is a bad move, and exceptionally bad in AR’s.

Same number of strokes on the press. Just use a FL die instead of neck-sizing die.

Charlie98 said:

You have to bump the shoulder back at least a few thousandths... for reliable chambering.

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Now I haven’t loaded for bottle neck cartridges that head space on the shoulders for a decade or more, and if wrong I know I’ll be taken to task here, but pushing the shoulders back a few thousands sounds like a recipe for case head separation.
Full length resizing should involve the case body and neck diameter only, or so I thought.

Charlie98 said:

You have to bump the shoulder back at least a few thousandths... for reliable chambering.

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That right there is a good reply. Just bump the shoulder back a little for reliable clambering and you will still get the benefits of using the brass fired in your chamber.

Typetwelve said:

Got it...and completely noted. Thanks for the replies, I'll stick to full size only.

In all honesty, I mas mostly wanting to do it because it is far less work and I was being lazy...

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If you want to save time use an RCBS X die. After the initial trim you usually don't need to trim the case again.
https://www.midwayusa.com/product/1012280346?pid=194698

whughett said:

Now I haven’t loaded for bottle neck cartridges that head space on the shoulders for a decade or more, and if wrong I know I’ll be taken to task here, but pushing the shoulders back a few thousands sounds like a recipe for case head separation.
Full length resizing should involve the case body and neck diameter only, or so I thought.

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All full-length sizing moves the shoulder. They’re saying to move it only a little.

whughett said:

Now I haven’t loaded for bottle neck cartridges that head space on the shoulders for a decade or more, and if wrong I know I’ll be taken to task here, but pushing the shoulders back a few thousands sounds like a recipe for case head separation.

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Consider yourself “taken to task,” as your statement is not correct.

Varminterror said:

In general, you don’t want to neck-size only for any rifle action. Doing so for bolt action rifles was a short term fad based on theory which has since been disproven.

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what he said.

I stand corrected. As I mentioned it’s been a while. These days it’s all hand guns.

Gentlemen,

ANY cartridge case that's resized enough will fail at some time. It could be a split at the neck/mouth or a head separation. When a case is resized to a smaller diameter, the brass has to go somewhere. It's not going to go to the web or case head where the brass is thicker but will go toward the front of the case. That means during each resizing, the brass gets thinner above the web of the case. If there is a big difference between chamber diameter and die diameter, more thinning will occur with each resizing and a head separation will occur sooner.

Head separations are more likely to occur with high pressure cartridges; either straight wall or bottleneck. Splits around the mouth are more likely to occur with low pressure cartridges.

There are some very compulsive individuals that have dies made that size to pretty much exactly the chamber size of their rifles. They can get far better case life than most of us. I am not that compulsive.

Grumulkin said:

ANY cartridge case that's resized enough will fail at some time.

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Most of the time we can eliminate case head separations entirely, and annealing can almost entirely eliminate neck/shoulder splits. Such the remaining failure mode for the last ~20,000 pieces of bottleneck cartridge brass I have worn out has been case head expansion/loose primer pockets.

Typetwelve said:

Thanks for the replies, I'll stick to full size only.

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+1

(Gas Guns and FL Resizing go together like peas & carrots)

^^^ You forgot the butter!

whughett said:

Now I haven’t loaded for bottle neck cartridges that head space on the shoulders for a decade or more, and if wrong I know I’ll be taken to task here, but pushing the shoulders back a few thousands sounds like a recipe for case head separation.
Full length resizing should involve the case body and neck diameter only, or so I thought.

Click to expand...

Bottleneck cases in a semi-auto rifle... particularly something with a violent extraction cycle like the M1a... are an expendable commodity. Truthfully, I've never experienced a case head separation in 5.56mm, I usually lose my brass in the weeds before I have a chance to load it more than 2 or 3 times, but I have experienced it in 7.62mm with my M1a. The primary concern is reliable cycling and feeding, and full-length resizing, even if it's just bumping the shoulder back, gets you there. Brass life is what it is.

Charlie98 said:

Bottleneck cases in a semi-auto rifle... particularly something with a violent extraction cycle like the M1a... are an expendable commodity. Truthfully, I've never experienced a case head separation in 5.56mm, I usually lose my brass in the weeds before I have a chance to load it more than 2 or 3 times, but I have experienced it in 7.62mm with my M1a. The primary concern is reliable cycling and feeding, and full-length resizing, even if it's just bumping the shoulder back, gets you there. Brass life is what it is.

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I started reloading in the 70’s, hand loading 30/30 for me and the kids. Then 358 30/06 308 243 358 and some 7.62x39.
Dies were carefully set on the Lyman T Mag according to instructions, brass was lubed in a big stamp pad. Care was taken not to oversize or move the shoulders back. Case mouths were carefully check for thickness and brass trimmed as needed. Cases were checked for signs of stretching using a sharpened paper clip inserted and pulled up the sides near the web for any signs of the separation that would start there.
All this for hunting rounds for me and the sons. Never had any problems. Except learning early on that most auto feeding rifles required a small base die. The 308 sure did. As mentioned haven’t loaded a bottle neck cartridge since around 1991. Boys all grown and gone and I lost interest in hunting except for the annual week trip to Maine.
Always tip toed around changing head space.

I load for 3 different AR's, 3 different .308's (an M1a, a Savage bolt gun, and a Savage 99 lever-action,) and a .30-06 Garand, among other rifles. I don't use SB dies for anything, reasonable resizing with standard dies has always worked well for me... although I understand some rifles have different chambers and require SB sizing. Even in the .308's, they each get their own brass (for various reasons) and even for the bolt gun I bump the shoulder. I have to on the M1a, and it is prudent to on the lever-action... the 99 does not have the mechanical leverage to ram an overlength case into the chamber and lock up.

Different strokes for different folks, I guess... that would be press strokes.