Neck Only vs. Full Length Sized Cases: Differences When Chambered then Fired?

[Bart B.]

Please post your thoughts, opinion, beliefs, observations on how a rimless bottleneck round positions itself in the chamber with the bolt closed then what happens when the firing pin strikes it. For each part of the round (head, primer, body, shoulder, neck and bullet), state what differences there are in what happens between a neck only and full length sized case.

[Nnobby45]

The firing pin hits the cartridge and drives it as far forward as it will go before it stops on the shoulder. When the cartridge is fired, the case expands under pressure tight against the chamber walls and makes a seal. That is, all but the thicker brass near the head. It can't expand to grip the chamber wall so it stretches backwards and slams into the bolt face keeping the primer pocket sealed. Then the brass contracts so you can extract it from the chamber.

Go with FL sizing. Size only enough for easy chambering in your rifle. If you neck size only, the brass will lose it's elasticity after a few firings, and not want to contract after firing, and extraction will be difficult. You'll have to FL size then, anyway.

I emphasize sizing only enough so it chambers easily for your rifle. If you size more than necessary, you're setting the shoulder back more, which allows the cartridge to be driven farther forward when struck by the firing pin, which means the thick brass at the rear has that much farther to stretch before it's stopped by the bolt face.

Constant stretching and sizing decreases case life and can cause a case head seperation when the brass gets too thin after all that stretching (the brass flows forward during the sizing operation and when you see the neck growing in length, that's the brass from the rear of the case I was taling about).

Neck sizing is tempting, but as stated, you'll have to full length resize eventually, and you'll have to set the shoulder back even farther do to the loss of elasticity in the brass.

There are some who seem to be confused about what FL sizing is. If you run the case into the die until there is shoulder contact resulting in only a slight amount of set back, then that's still FL sizing. It's a matter of degree of FL sizing (shoulder set back).

If you run your case up into the FL sizer only enough to size part of the neck, without touching the shoulder, than that's using your FL sizer to neck size. If you do this, you still need to lube your cases.

Better to make contact with the shoulder for mimimum setback, since the body of the case also benefits from the FL sizing process, though the shoulder is the critical area.

[FrankenMauser]

This subject seems simple, with different people offering different opinions.

However, it can get extremely complicated. (If you let it.)

Examples of complications: Powder charge, reduced charge, powder burn rate, loose/tight primer pocket, headspace dimensions, polished chamber or rough chamber, lube in chamber, case construction, brass case or steel case, case coating (if steel), free-bore, lead bullet or jacketed, tapered or straight throat, tight chamber or 'factory' chamber, competition dies or regular, real FL size or just a 'shoulder-bump' FL size, etc....

I am currently being distracted by the female counterpart. I'll try to finish this post later.....

[butta9999]

Nobby has it pretty well covered. I prefer to neck size 2-3 times because the case has been fire formed after the first firing to my chamber.

One thing with neck sizing is less work on your brass especially if you use a collet die. With neck sizing though the case will only fit the rifle which it has been fired from.

It is also recommended to FL size if hunting dangerous game to be sure you dont get a bullet jamming at the wrong time.

In saying that i always chamber every reload after i do a batch to be sure everything is ok.

Nobby is spot on when saying you should bump the shoulder back when FL sizing. If you try and neck size or partial full size in a FL die you will get problems.

Your case gets longer before it resizes so if you try to partial size you will find the case hard to chamber.

And be sure to lube every case in a FL die i have had 3 stuck cases, two in .222 and one in .300wm. Lack of concentration was the cause.

With a Collet neck die there is no need to lube and that is a blessing. I have never looked back since using these dies.

Good luck....

[Bart B.]

Here's some more help regarding my original post.....

Once chambered and the bolt closed, does each type of fired, then resized case rest in the same position?

Is that position such that gravity pulls it down so the case body, shoulder and neck are against the chamber's bottom at those same points with its head against the bolt face?

Are there any dimensional changes to the chambered case before it fires (dimpling of the primer excluded)?

[F. Guffey]

It is like waking up in a new world every morning, PSI is equal in all directions, the extractor has more influence on the static lay of the ammo in the chamber than gravity, partial neck sizing is the best of all worlds, the unsized portion of the the neck will center the case in the chamber, adjusting the die, press and shell holder with a feeler gage would make all issues 'non-issues'.

F. Guffey

[wncchester]

Seems such an expansive question as this should be answered by a good advanced internal ballistics book, not the web. Even if we have the knowledge, it's asking for a LOT of writing just to avoid research by the questioner. ??

[Bart B.]

I'll cover what happens when the round's chambered. The following is based on my observations, tests and measurements (OTM's). If your OTM's differ from mine, please post and explain them.

But first, note that there's no such thing as a perfectly round chamber or fired case. They are all out of round by a thousandth or so, cases more so than chambers. If you have good tools to measure out of round (dial indicators on V blocks for cases and star gages for chambers, not outside micrometers nor hole mics) you can see what they really are. So there's no such thing as a "perfect fit" between case and chamber.

And second, fired cases from bolt action rifles are always smaller in outside dimensions than the chamber they were fired in. Not much smaller, but a thousandth or two is about all for max, safe loads. I've measured proof loads (blue pills) and they're smaller, too, but not as much.

Once the bolt's closed on a rimless bottle neck round, the bolt's extractor pushes the back of the case off center opposite the extractor's place on the bolt head. The back of the case doesn't rest on the chamber bottom but instead gets pushed against the chamber wall at the case pressure ring's location. Mauser style claw extractors push the case straight to the side. Sliding ones in the bolt face, such as the post-'64 Model 70's, push the case up. You can easily see where your bolt's extractor pushes the back of a chambered round.

Spring loaded plunger ejectors in bolt faces push the round forward until its shoulder centers in the chamber shoulder. So the front of the case body is typically clear of the bottom of the chamber. Centering the case shoulder in the chamber shoulder also centers the case neck in the chamber neck with a couple thousandths clearance around it. Even partial neck sized cases have clearance; go measure the chamber neck and case neck diameters and you'll see how much there is. If the action has a Mauser style ejector external from the bolt, it won't touch the case at all and the round could be anywhere between the bolt face and chamber shoulder, but it's back end is still pressed off center by the extractor and contacts the chamber wall opposite the extractor.

Depending on where the egg-shaped case fits the egg-shaped chamber, the clearance around the case will vary. Full-length sized cases will have a few thousandths more clearance than neck-only sized ones. If the bolt face has been squared with the chamber axis, it will have about the same clearance all around the fairly square case head; if not squared, the clearance will vary depending on how the round's oriented in the chamber and how much out of square the case head and bolt face is..

So here the chambered round fits; back end's against the chamber wall at the case pressure ring. The front's centered in the chamber at the shoulder if pushed there by the in-line ejector or just sitting there a bit below center if an external ejector's used.

Now the trigger sear releases the firing pin and it headed for the primer. It'll travel a fourth to a third of an inch in a few thousandths of a second and strike the primer with 25 to 30 pounds of force. Rimless bottle neck round such as the .30-06 weigh about 1 ounce.

My next post will explain what happens when the firing pin smacks the primer. If you well understand what's above, you can probably figure this out yourselves.

[Bart B.]

Chapter 2.....

Along comes the firing pin driven hard against the primer with 20 to 30 pounds of near instant force. If the loaded rimless bottleneck round’s shoulder isn’t already seated fully in the chamber shoulder, the firing pin’s force will certainly do that. There’s enough clearance on the extractor to allow this to happen. If everything from the shoulder forward is very straight, this also centers the case neck in the chamber neck with the bullet in that centered neck well centered on the bore axis with a couple thousandths clearance between the case neck and chamber neck. Even if there’s a slight difference between case shoulder angle and that of the chamber shoulder; both are cone-shaped and centering is assured because the chamber shoulder is larger in diameter. This much energy striking the back end of the cartridge may well move it off contact with the chamber wall where the extractor’s pushing it. Or, it may make the case contact the chamber wall harder. I suspect it depends on where the bolt head’s positioned in the receiver ring and its reaction to extractor forces pressing the case off center.

Firing pin force sets the case shoulder back a few thousandths of an inch as it dimples the primer cup crushing the priming compound detonating it like a hammer on a match head. And the neck gets a tiny bit longer, too. Now the case head is further away from the bolt face by the same amount the shoulder was set back. This is the only time the loaded round is in the most critical position for firing. Any variable in how the bullet’s positioned at this time effects accuracy. It doesn’t matter how much clearance there will always be around the case neck or body relative to the chamber; as long as it’s always about the same amount, that’s all that’s needed. The case head will be a few thousandths off the bolt face. There will be some small, very small angle between the chamber axis and the cartridge axis because of how much the extractor pushes the back of the case off center. But it’s very repeatable from shot to shot so it’s not important. Both neck-only and full-length sized cases will be, for all practical purposes, fit the chamber the same way. The exception is full-length sized ones will have their back end a tiny bit further off center and the neck-only sized ones may not have their case necks holding the bullet as perfectly aligned with the chamber. If bullets are seated out enough to set back a few thousandths when pushed into the lands when chambered, this will help a tiny bit with their alignment to bore center providing the case necks are well aligned and centered with the shoulder.

When the primer finally makes enough fire, the powder starts burning, building pressure, and expanding the case against the chamber walls. First the neck and shoulder expand as they’ve got the thinnest walls, then the front of the case body and while pushing the case head back stretching the body along with it and pulling some of the case neck out of the chamber neck. The case head finally stops against the bolt face with considerable pressure in PSI which makes the case head about the same angle of the bolt face. Meanwhile, the body stops expanding against the chamber body walls with the same pressure. As both the case and chamber are not perfectly round, each round fired will have its case body first start pressing against the chamber walls at different places for each shot. Pressure starts to drop after the bullet’s about a third of the way down the barrel and goes to zero quickly after its left the muzzle. As cartridge brass has elasticity, it now shrinks the case away from the chamber walls a couple thousandths and there it sits until the bolt’s opened. It probably wasn’t oriented the same as it was on its first firing and its out of round points may well be at different places than after its first firing.

(Note: This case head space shortening can be seen by firing a 10 to 15 percent reduced load then noting how far the primer’s backed out stopping against the bolt face or using a case head space gage before and after the firing. There wasn’t enough pressure to push the back half of the case against the bolt face. But enough pressure existed to push the primer part way out of its pocket.)

From these situations, it should be easy to see than the only significant physical difference between neck-only and full-length sized cases when they’re fired is back at their pressure ring. Neck-only sized cases will have a body diameter a couple thousandths of an inch larger than a full-length sized one. Either one has their case shoulders’ centered just at the chamber shoulder when fired. Their back ends will be pushed off center by the extractor about half their difference in diameters at the pressure ring. Full-length sized case with its smaller diameter will be off center by only a thousandth of an inch; maybe two at the most. At the bullet’s tip, it’ll be tilted off center by about one-third that amount at the most; not enough to make a difference which doesn’t matter much any because whatever angle it is at will typically repeated for each shot.

I’ve noticed one interesting difference between neck-only and full-length sized cases in one very important area. Remember how the case shoulder centers the front of the round in the chamber, especially the neck holding the bullet? Measuring sized cases for neck runout showed me that full-length sized cases can have straighter necks than neck-only sized ones. It’s best measured and seen if you support the case only at its pressure ring and mid point on the shoulder with the dial indicator 1/16th inch back from the case mouth. Which makes sense to me as neck-only sizes the neck on a case whose body is not supported by anything to keep it in alignment with the neck during the sizing operation.

Lapping a full-length die’s neck out so it’s 2 to 3 thousandths smaller than a loaded round’s neck diameter eliminates the use of an expander ball (IMHO, more correctly called a neck-bending ball). Decap your fired cases in a separate die then clean them before they’re lubed and sized. Setting the fired case shoulder back only a couple thousandths with the case body held tight in alignment with its neck tends to keep the neck straighter and better oriented with the shoulder. I’m convinced that when the full-length sized round fires with its shoulder hard and perfectly seated against the chamber shoulder, its straighter neck lets the bullet enter the rifling with less distortion therefore shooting more accurate. I’m also convinced this is why fired cases properly full-length sized in dies with lapped out necks typically shoot more accurate than neck-only or partial-neck sized fired cases.

[F. Guffey]

If Bart B. won't, I will, thanks to everyone in the choir for your effort, the question was misleading, I thought someone wanted to know, I never thought someone would introduce an answer with a question, Nnobody 45, thanks you for the time and effort.

FrankenMauser, I agree, as to the last part, my time could have been better spent doing something else also.

Butta999, thank you for the support of Nnobody 45 and and your time and effort.

Wncchester, hind sight, did you see this coming.

Bart B. What did you measure? What tools did you use to get your measurements? Nothing is really round? Nothing really fits? A few thousands difference is a guess, not a measurement and as someone said there is full length sizing, neck sizing, partial neck sizing, neck sizing with partial body sizing 'with one die', that one die is a good investment, protect my investment in dies, I preserve the integrity of the die, grinding a die or shell holder is not necessary, an investment in a basic feeler gage allows a hand loader to duplicate any technique gained by grinding, in the past RCBS has marked special dies with an 'X' or 'SP', I have purchased dies that have been altered for reasons unknown to anyone but the person that altered them.

F. Guffey

[Bart B.]

F. Guffey asks:

Bart B. What did you measure? What tools did you use to get your measurements? Nothing is really round? Nothing really fits?

I used a 90 degree V block with a dial indicator on top of cases rolled in it. Chambers were measured with a star gage as well as chamber casts rolled in the same V block.

[butta9999]

No problem guffey

[Bart B.]

Thanks to all contributors for you varied inputs. I often seek this info from folks just to see what current trends and opinions are.