Case neck/chamber clearance

[Char-Gar]

It seems the common wisdom is the case neck needs to be a tad smaller than the chamber so the case can expand a bit on firing and release the bullet.

However, in my rapidly failing memory bank is some stuff about target rifles with chambers so tight, case necks didn't have to be resized to load the case again.

It would seem to my non-scientific mind that is a bullet was held fast by the nose in the rifling and the case neck was held fast, by being tight in the chamber would give the bullet a good straight start into the barrel. This would be good thing accuracy wise. Of course the chamber needs to be straight with the bore and all of factors condusive to accuracy.

It would also seem the pressure needed shove the bullet out of the "unexpanded" neck would be almost nothing compared to what it takes to shove it down the barrel.

So, the question for the cogniscent is: What is the deal and case necks needing to expanding a mite to release the bullet.?

I loaded some of the new "Fatter 30" bullets up for a trial in the 30-40 Krag. Having discovered that bigger bullets shoot better than smaller bullets in my Krags, I sized these .314. The is a slight feel to the bolt handle rotating down which is not the bullet engraving, but the case neck in the chamber.

[Bass Ackward]

So, the question for the cogniscent is: What is the deal and case necks needing to expanding a mite to release the bullet.?

I loaded some of the new "Fatter 30" bullets up for a trial in the 30-40 Krag. Having discovered that bigger bullets shoot better than smaller bullets in my Krags, I sized these .314. The is a slight feel to the bolt handle rotating down which is not the bullet engraving, but the case neck in the chamber.


Chargar,

The simple answer is that you may have to turn your neck a little to reach your intended senario. That's the safety answer like telling you to start slow and work up.

In your senario, a bullet seated into the lands will likely have no neck tension on it anyway as the neck will expand out before the bullet starts moving. But this is theory as opposed to your real world. You are likely going to size down your lead bullet if things are that tight. The brass is harder than the bullet. Is this unsafe with your load? Who knows. Can it be unsafe with the wrong load or charge? Definately.

Would I care if I were in your shoes? Depends. If I was shooting a 700, 70 or some modern strong bolt and using low velocity cast, probably not. Being a Krag, I wouldn't want to risk it for the value of the gun if not from a safety standpoint. Because that is the feel with a clean chamber neck at this time. Or is it? What happens when you get a little dirt buildup if you don't clean often and it hardens? Would you notice the increase tension or just not think about it because it was safe last month? So the safe answer is not to risk it.

If you are asking what is the ideal condition? Well the tightest a reamer manufacturer will grind a custom reamer for you based on your loaded round is .002 and they would want to quiz you to ensure you understood what that meant before they did it. And then they still might fudge it out to .005 just to be safe unless they really know you.

[44man]

I believe the reason for .002" clearance is so the chamber will not squeeze the brass of a tight case into the bullet when chambering. Increasing the tension on a hot load can make the pressure jump. I will never believe that the neck brass expands open before the bullet leaves. There is no way the pressure can act on the neck when the bullet is in the way.
If the brass just touches the neck walls and is not forced into the bullet, I see no problem.
Until someone can climb into the case to see if the entire case expands all at once, leaving the bullet hanging free, I will not change my mind.
Good case tension is important for accuracy in most guns but the bullet must be free to move. Holding the brass tight against the bullet with the chamber wall can be dangerous.
Then there is the BPCR in which the brass is not sized and boolits are just inserted by hand. My most accurate loads have been with the boolit loose enough to wiggle back and forth. Goes to show the type of powder is influenced more with different amounts of tension then any other factor. A very slow powder that must burn for most of the barrel length needs more tension at the start. But the brass must not be held tight to the bullet.
With lighter loads and cast boolits there is most likely more leeway here as there is less friction and the boolit is soft enough to size itself to move.

[felix]

Remember, all boolits obturate, and therefore move metal from the center of boolit to the edges of same. If this occurs extensively while the boolit is still within the neck area, then we could have a tight neck problem. It appears we can generalize that 0.002 total clearance is quite safe for most loads. Suspected loads would be those having faster powders with softer boolits, i.e., faster than 2400 and softer than air cooled WW when the clearance is approaching 0.001 total. Turning necks and/or harder boolits with the same powder charge will loosen things up into the safety range. ... felix

An 0.001 clearance is what I use in the bench gun. But, I must clean the neck area out at least with a dry brush after the gun sits for more than say 10 minutes. If I don't the "groups" open up considerably. An 0.002 neck clearance probably won't be detected as faulty until maybe a day later instead of only 10 minutes. We are not talking about a tight throat here, which always should be extremely close fitted. ... felix

[44man]

Good way of putting it! I think .001 to .002" is the way to go.
With cast and boolit expansion from the pressure, the lead is still fluid enough flow out but a hard jacketed bullet held tight and bumped up from pressure could be a big problem with less then the recommended clearance.

[Bass Ackward]

Don't forget that the neck also elongates which is why you trim .010 off maximum length. And that is with a SAAMI neck that may have .020 clearance. Restrict flow in one direction and you increase it in another. Case design (shape) and pressure ranges can affect this considerably.

While I like a tight neck, that is only so I can have room to clean up my brass for concentricity purposes. I would not recommend .002 for the vast majority of folks unless they understood what they had. Especially in calibers where full house loads can be utilized with cast.

44,

Testing with neck tension in my rifles show that neck tension is a non factor for either accuracy or velocity if seated into the lands. So the only explanation I can offer is that it lets go totally. And breech seating is often looked at as the most accuracte method for launching cast. And there is usually done without sizing a case. In some cases, the bullet isn't even in a case. So there is no neck tension at all.

[44man]

Very true but a lot of guys are hunters and have rifles that will not take a long seated bullet that can reach the rifling and still cycle through the magazine. The bullet also has to be held so it does not move in the magazine under recoil. There are just too many variables involved to make a blanket statement. For strictly target or in a single shot what you say is correct. However, we must address the whole instead of the few. Sometimes it is very hard to do! This is a curse, trying to reach all types of shooters and reach a happy medium. What you know and what I know sometimes will not apply to others. It is what makes all of our posts a truely great experience but everyone has to do what they must for what they shoot and how they shoot.
I will never argue about what is best because it isn't in all cases, so even though we agree on a lot, we can both be wrong a lot of times. But your input, my input and everyone elses input is very important. Let us hope that each shooter can adapt and use what he needs.

[Char-Gar]

OK Guys... I am learning here...now for the question of application. How to you determine clearance. I know you mic the neck of a loaded round to get that number, but how do you determin how much clearance there is in the chamber neck. I assume you mic a fired case from the same lot of brass...but it would seem to me the brass would spring back a mite after the initial expansion. Therefore, measuring the fired case might not give the true number.

Case in point...ammo in point. The loaded ammo measures .001 smaller than the fired case. So what is the clearance... .001 or some other number. Inquiring minds want to know!

Oh yes.. on more investigation, the slight resistance upon final bolt closing as noted in the original post is indeed the nose engraving on the lands. I opened the bolt and droped a round into the chamber and it slipped in easy as you please, so the neck of the loaded round slips easily into the chamber neck.

[StarMetal]

Charger,

One way is with a chamber cast, either cerosafe or the soft lead pound method. Other then that a fired case will give you a close measurement, as you know the brass shrinks back a small amount. Then you load as fat bullet that will leave enough clearance for the mouth to expand and release it.

Joe

[44man]

Yes, the fired case springs back, just how much depends on the caliber, brass hardness, etc. Normally, if you can slip a new bullet into a fired case you have plenty of clearance. All factory rifles are chambered with enough. The only chambers you must watch are tight bench rest ones that are cut to a minimum.
I neck turn a lot of brass but never remove more then it takes to even out the thickness. The cutter will not touch one side much at all, there will be a spot untouched. I don't want to thin the whole neck, it is not a good idea with a factory rifle.
The only way to see what the chamber is, is with a casting. Not really worth the trouble in 99% of guns.
Now if you are forming cases from some other caliber, you have to be careful and measure everthing. Reducing to a smaller caliber will thicken the neck and they must be reamed or turned. Not an issue with factory brass.
I don't see why you are even thinking about it! Just load for the most accurate and stop worrying. The 30-40 should have a large chamber unless it is custom. Try fitting a boolit in a fired case. If dropping right into the chamber, you are also good to go.

[StarMetal]

Charger,

Another real simple way is take a fired case from said rifle, and then just keep slipping the bullets into the mouth till you find the fattest one that will just slip in. Use that one. You will have to make sure the throat will take it too though. Also if you use a fatter bullet you don't want to size the neck down as much as you did before either. A fatter bullet will fit tighter and the tighter neck will damage it.

Joe

[grumpy one]

Unless you have very recently neck-annealed the brass there will be quite a lot of springback - at least .001 on diameter. I use the slug-pounding method, which is a bit of a hassle and needs to be done carefully or you might do damage.

I believe the diameter of the loaded round is well worth thinking about in any case where the final diameter of the sized bullet is larger than nominal bore diameter, because the loaded cast bullet round will have a larger neck diameter than a factory round. In some cases, notably my extremely loosely bored 30-30, the difference is considerable. I've found that if I make up a 30-30 dummy round with an unsized bullet (which means about 0.313 diameter, and bigger across the parting line), it has a neck diameter of 0.331, increasing to 0.334 at the parting line. This compares with the SAAMI spec of 0.332, and a factory load at 0.328. Is this a problem? I slugged the chamber neck, and found it is 0.3437, so there is still nearly 0.010 of clearance, and hence no problem. At the moment I'm sizing my bullets to 0.312, which is barely sufficient for my 0.3116 groove diameter. So, at some point I'm going to experiment with 0.313 sizing, and I'm not going to have to wonder whether it is safe to do so.

My point is that because we sometimes size bullets well above nominal bore sizes, measuring your neck, throat and leade dimensions can be much more important to cast bullet shooters than it is to j bullet shooters.

Geoff

[Char-Gar]

Joe... I took a Starrett hole gage and stuck it in the fired case. I came out with an ID of .314 which is the size of the bullet I am using. I would rather use .313, but I dont have that size. My sizers dies go .309, , .310, .311, .312, 314, .315.. no .313. I have used .314 bullets in this rifle before and got outstanding acccuracy and not funky pressure signs. I guess I need to get Buckshot to make me a .313 as I don't trust what I buy, my .314 says .313, my .312 says .311, and the rest of them tell the truth. I just hate to spend money for a Lyman or RCBS and not know what i am getting.

I don't worry about crushing the bullet, as my M die expanding plug tapers (in .001 steps) from .307 to .317. I run it into the case until the mouth will accept the gas check and seat the bullets. This leave a nice easy internal cone. With this expander the ID is always right for the bullet.

[versifier]

Felix,
How often do you need to trim your BR cases? And how long do they last (how many firings)? I feel that the less room there is for brass to expand, the less it moves and stretches, and the less sizing it needs. Neck sized brass in my rifles seldom requires trimming, especially compared to cases that need FL sizing to be shot in different rifles of the same caliber. Now, I don't shoot hot loads, and I believe this puts even less stress on the brass, and it is not unusual for me to get ten firings (or more) from a neck sized case. If this is true as observed, then logically it should be even more true with the extremely tight tolerences of a BR chamber/case fit. But I don't have a BR rifle, nor do I know anyone around here who does.

Now, how does this relate to the thread? The only time the neck is a serious issue is when it has lengthened to the point where it enters the throat and actually obstructs the passage of the bullet. It seems to me that as long as there is no actual obstruction, the tolerance can be extremely tight. I would not have thought it could be as tight as .001, but I was fascinated by your observed differences between .001 and .002. So I got to thinking and pretty soon was asking myself: If there is nowhere for the case to expand, then there's really no way it can expand, and that would lead me to the conclusion that the tighter the tolerance, the less the brass is worked in reloading, and the less it would lengthen with repeated firings. Is this true in the real world?

[StarMetal]

Charger another way to measure that neck area is flare the mouth of the case pretty much, not real real drastic, then chamber that empty case and let the neck portion of the chamber size that flare down. Then extract and measure where the flare was and what it sized it down too.

Joe

[felix]

Versifier, if the boolit/bullet cannot move, the brass/boolit/bullet is going to attempt to expand. If the brass cannot expand sideways, and then foward when necessary but cannot, then the boolit/bullet obturates more than usual causing the log jam, and finally an explosion of sorts occurs. Unlike a SEE, this kind of explosion would be gentle in comparision, and back through the primer area. In a nutshell, a tight neck set-up must allow the brass to move. Brass enlongation would prove there is not enough clearance. Odds are good that most throats are long enough to not crimp the case, so that idea can be ignored for a shot or two. Case crimping won't occur with folks checking cases after each firing anyway. Case length can be checked every time the neck gets thicker via a donut. A rod can be made to enter the case neck for checking donut growth, and this technique is preferred over checking for case length alone. In other words, case length will grow only after the brass has moved from the body area of the case and into the neck area. After a load is found that shoots good, that load should be stuck with. Increasing the load, the brass will begin to flow. The flow is less for a sharp shouldered case, it appears, but not really. The donut grows faster. The idea is to hold the charge, or increase the neck clearance until there is no more flow. ... felix

[versifier]

Gotcha Felix. Now that makes sense and I understand it. Nowhere else to go, brass flows into the neck and forms donut. Ignored donut = blown primer (or worse). There goes another perfectly good logical theory. [smilie=1:
I'm still curious, though. How many loadings can you get with your br cases?

[Bass Ackward]

I'm still curious, though. How many loadings can you get with your br cases?


Versi,

Lot of issues there. Chamber dimensions and die set-ups. But mostly case design and pressure tell the tale. That was why PO Ackley had the straight sided case and sharp shoulder theme. It was to prevent brass flow forward. That is how he could get away with some of the pressures he ran in his testing. His stuff is way hot by today's standards. A long tapering case throws thrust back against the bolt face. Once it can retreat no further, everything flows forward. His cartridge designs almost eliminated bolt thrust and extended brass life.

That's why some case designs need trimming or neck turning more often. But the sharper the shoulder, the less the brass can flow beyond the start of the neck ala the donut. Brass under excess pressure for which it was designed will try to .... flow somewhere.

[9.3X62AL]

Chargar--

I like that idea of your tapered expander plug--a lot. With all the 30/31 calibers I feed and herd, such a device would be right fine to have around.

Approaching this question from another angle......I recently had Buckshot make up M-die spuds at .457" (for .459" boolits) and at .459" (for .461" boolits) in the 45-70. The RCBS expander in the die set mic's @.454", and the 459 boolits were getting distended and/or shaving sidewall metal a bit during boolit seating. Fired cases show free-fall with .459" boolits, just the barest hint of touch with .461". This is with R-P cases, W-W are microscopically tighter with my lots of brass.

[45 2.1]

Approaching this question from another angle......I recently had Buckshot make up M-die spuds at .457" (for .459" boolits) and at .459" (for .461" boolits) in the 45-70. The RCBS expander in the die set mic's @.454", and the 459 boolits were getting distended and/or shaving sidewall metal a bit during boolit seating. Fired cases show free-fall with .459" boolits, just the barest hint of touch with .461". This is with R-P cases, W-W are microscopically tighter with my lots of brass.

If you neck size that brass to just below where the boolit base is about 0.001" under boolit diameter for the I.D. you only need to flair lightly. This will give you a better aligned case and better accuracy to boot.

[mainiac]

Versifier, if I may, I can give some info on br cases. My old heavy varmit class gu8n in 6ppc, originally came with 10 pieces of the old sako .220 russian brass,and i shot these cases for about 2 years,and loaded them 140 + times apiece! Always shot this gun hot also,3400 f.p.s. (66 grain bullet) In a tight chambered gun, the brass just doesnt expand. My brass is cut for .2613 loaded round,and the barrells i have are exact .262 neck diameter. I never size this brass,exept on occation i have to bump there shoulders back(when i have to stand up to open the bolt) Fast foward 12-15 years,and now we have lapua brass and it is as exellent as the old sako. Between the ppc,s and my variuos .30 cal wildcats, i never have to trim brass,as it just doesnt grow. The only way to ruin brass in a tight chambered br gun, is to pour the fuel,and expand primer pockets! Hard to win matches when you have to hold finger on primer as it goes in the chamber (so it dont fall out!) Thats what i had to put up with in the early 90,s when there wasnt any brass to feed my guns. I could go on and on, about br, but i realize that thats not what this site is about. thanks,mark

[felix]

Mark, there is one thing to add that could be pertinent here. It takes a true BR action (oversized, overlocked, etc.) to continually fire these so called hot loads without bending the action into an out-of-spec state. These actions were first built to hold heavy barrels without distorting either, like straight tubes 1.2 inches in diameter. The added advantage was the extra strength afforded for hot loads, where 80K cup could be considered to be on the high side. The downside is the destruction of cases over a few times reloaded, but when money is on the table, who cares about the cost of brass. There are typically two kinds of competitiors, those who wait for a certain condition and then "machine gun" all the rounds with the hope the condition does not change. The faster the bullet hits the target, the less time in the air for that wild uplift (usually) to affect it. These folks rely on the hot-rodded ammo. The other kind of competitor is a true weatherman, and shoots somewhat slower ammo because of his skill in reading the unknowns. Usually, the latter type of person wins the day's match, whereas the machinegunner might win one or two relays. ... felix

[9.3X62AL]

45-2.1--

That's the other half of the equation, sir. I just started with a new lot of R-P brass, as well as a like number of R-P that is getting a little long in the tooth. The neck sizer (Lyman) was employed with the older brass, and will be used on the new brass after its first firing. I had odd lots of several different boolit designs--ranging from 325 grain RCBS to some 500 grain RN's for test firing, and I might attach a scope for grins and jollies. All this is basically R&D for a MM design for the #1 that needs a little fatter boolit. I'm leaning toward 400 grains and gas check and a 450 grain PB in one block, if Dan can squeeze 4 cavities this size into one block. A 2-banger would be fine, really--a #1 ain't belt-fed, and so far it hasn't gone full-auto on me. Drat these mainstream moldmakers and their skinny d--n cavities, anyway.