What Cartridge Dimension Determines Bullets' Jump to Rifling?

[Walkalong]

Tight necks do nothing to center case necks and bullets in the bore.

We still disagree here. It certainly helps IMO.

A round that is crush fit is going to have a near impossible task of centering in the chamber so the bullet is as dead straight as possible, simply because the brass is jammed, and held, crooked by the bolt face. How can it not be since it is turning as it squeezes up against the case head jamming the shoulder into the chambers shoulder. Or as you point out, the extractor has it pushed over, and then the crush fit locks it crooked. I could be wrong of course, but no one crush fits rounds in Benchrest, nor in High Power as far as I know.

 

[gamestalker]

Bart, as I continue threading the die down, the brass gets squeezed down smaller, which pushes the shoulder up. And while continuing to thread the resizing down, at some point the shoulder begins to make contact with the shoulder in the die, and then as I continue threading the die down, I eventually reach shoulder bump that falls into acceptable tolerances. anything from .0015" to .000" is my target bump.

So I guess the answer to your question, yes, during the adjustment process of the resizing die, the bolt would have to be forced into battery, if, I didn't have a method to measure head to datum off the shoulder to determine acceptable tolerance. But without a method to measure where I want the shoulders, one would have to manually find the appropriate amount of adjustment to get an acceptable head to datum line shoulder tolerance, or cartridge head space to chamber fit. But I have all my chambers documented, so it's a relatively simple process for me.

GS

 

[Bart B.]

GS, read post #7 in the following:

http://www.thehighroad.org/showthread.php?t=785352

. . . and you'll see how you can measure a fired case headspace then again after FL sizing it to see how much you bumped its shoulder back.

Inside diameter of the bushing is not important as long as it contacts the case shoulder somewhere between the body and neck. The actual reading on the caliper's not important, just the difference between after firing and again after full length sizing it.

Walkalong,

Rimless bottleneck cases are pushed forward into the chamber shoulder by the firing pin when fired. If the bolt's got an in line ejector, that pushes the case forward into the chamber, centering it there, before the firing pin does. It's easy with a thousandth or two head clearance.

I measured how a case fits in .308 Win. chambers with its back end pushed off center about .001" to .002". If the case diameter at the pressure ring's .470" and chamber diameter at that point is .472", the most it can be moved off center is .001". The tip of a bullet in a round that's 2.810" long is about 2.510" forward of that. With the case pivoting at its shoulder 1.330" forward of the pressure ring, its tip will be 1.180" forward of the shoulder. That tip is going to be about 11% less off center than the case is at its pressure ring; 0.00089". Same as a bullet's runout that much relative to the case shoulder. And opposite the direction the back end's off center.

It's the same for every round chambered. Do your own measurements or calculations using SAAMI chamber and case drawings.

http://www.saami.org/PubResources/CC_Drawings/Rifle/223 Remington.pdf

You can also use a Wilson case gauge whose chamber is SAAMI spec. I was loaned the chamber end off a worn out .308 Win. barrel cut off and faced at the chamber mouth. Also, the SAAMI drawings can be used to calculate from:

http://www.saami.org/PubResources/CC_Drawings/Rifle/308 Winchester.pdf

It's easy to see the case mouth off center at the chamber mouth if its shoulder's a couple thousandths off the chamber shoulder as the case body rests at the bottom of the chamber. As soon as it's gently pushed forward a couple thousandths against the chamber shoulder, the case mouth rises up then centers in the chamber mouth because the case shoulder is now centered in the chamber shoulder. Case mouth never touched the case mouth at all. Chamber neck diameter was .344", case neck diameter was .336"; .004" clearance all the way around when case fully chambered. Seated bullets follow the case mouth.

Then I put a .243 Win case in that chamber. As predicted, its case mouth centered perfectly in the .308 chamber; its case neck is perfectly centered on the case shoulder, just like that of a .308 Win. case. There was a lot more clearance from its neck to the chamber neck, but still centered. Its shoulder and body closely matched that of the chamber, so why not?

As there's no perfectly round cases nor chambers, a perfect fit of one to the other will never happen; a small clearance is better than an irregular one. Case necks are much straighter on case shoulders when the sizing die keep their body axis well aligned with their neck axis. Neck only sizing dies don't do that; full length sizing ones do. One piece dies are a tiny bit better than those using bushings. Bushings don't center perfectly on the case body axis with full length bushing dies.

 

[fguffey]

I am the fan of having nothing between the case and chamber but air, I do not want a lot of air, just a little. I understand air is fluid, I understand air flows and I understand air can be compressed. I do not know what others go through when they pull the trigger, ME? When I pull the trigger things happen very fast.

Then there is the primer, I have no clue what happens when other shooters pull the trigger, as for me? When I pull the trigger the firing pin hits the primer, the primer is setting still. Back to the part where I have killer firing pins. My firing pins put a dent in the primer before the primer starts to move. Then I want to go back to that part about things happening very fast when I pull the trigger.

Then there is that part about the extractor, I can only assume there are no shooters on this forum that shoot a Post 1964 Winchester Model 70 push feed rifle. Then there is the Remington with the ring around the ring around the ring.

F. Guffey

 

[gamestalker]

I'm not sure you are understanding me Bart. I first push the shoulders up while resizing until they make solid contact with shoulder in the chamber, case head making hard contact against the bolt face. That is zero, I document that number for that chamber.

Another method I've used. I run a piece of brass that's been fired from that chamber and run it through the FL die. Then I shim the bolt face starting with .0005" shim. The bolt face and chamber must be spic and span clean, free of anything that could alter the result. I chamber a piece of brass, if it chambers freely, I continue adding different shims in .0005" increments until I can physically feel resistance, thus indicating the brass is fitting to zero. I measure the piece of brass from head to datum line shoulder and add the shim measurement to that number. I now know what my head to datum line measurement to zero is for that chamber. This is a bit more cumbersome, but it works well for me.

I use a DIY tool to measure head to datum line shoulder, which has been very darn accurate, more accurate than is likely necessary I'm sure. But hey, I like to gets things right the first time. It stinks to get out to the range and then discover you have loaded cartridges that don't fit the chamber.

GS

 

[Bart B.]

GS, I think I understand what you're saying. But to me, it's way too complicated and has too many steps. But do what you feel best with.

I think it's much simpler to just measure fired case headspace (case head to some shoulder reference), full length size the case, then measure that sized case headspace again. If the sized number is .001" to .002" less than what it was in the fired condition, the die's set in the press OK. If not, adjust the die up or down in tiny increments testing a few cases with each die setting. Those labels linked to in an earlier post make adjusting die height .002" at a time so darned easy.

There'll be a couple thousandths spread in sized case headspace due to variables in press spring and case lube type/application. Not a problem at all. It's not important to me to know the exact measurements in inches or whatever for either case or chamber headspace. Just the difference between fired and resized case headspace numbers on the gauge used to measure them.

Having measured several fired cases' headspace for different cartridges, then measured the rifle's chamber headspace with adjustable headspace gauges, one thing stood out. Chamber headspace for rimless bottleneck cartridges is typically .001" greater than that of a max load fired case headspace.

I think it's best to strip the bolt when checking sized case fit in the chamber. That way, there's no interference in bolt parts that may mask the feel of it going into battery on a chambered case. Because cartridge brass is springy, it's common that some cases will have .001" spring in shoulder setback that's barely felt closing a complete bolt on one. A 'smith educated me years ago about such measuring techniques and said the bolt should be stripped and let the weight of the bolt handle drop down closing the bolt on the case; no human touching the bolt handle at all. That way, the exact same force will be applied to the case each time it's measured. Otherwise, a .002" spread in measurements with the same setup is likely. That's why headspace gauges are made with solid steel; they don't compress nearly as much as hollow brass cases do.

 

[fguffey]

I think it's much simpler to just measure fired case headspace (case head to some shoulder reference), full length size the case, then measure that sized case headspace again.

And I always wonder how a reloader full length sizes a case without full length sizing. There has to be something missing like the die is designed to return the case to minimum length also know as full length sizing.

In the perfect world my 30/06 chambers are go-gage length when measured from the shoulder to the bolt face. My cases are .005" shorter from the shoulder of the case to the case head. Again, my dies are designed to return the case to minimum length. Then there are those that grind the top of the shell holder to reduce shell holder deck height and or the bottom of the die.

After that it gets more complicated and beyond 'typically'. Typically the case increases in resistance to sizing, remember the one about firing a case 4 time then start over by full length sizing. My cases increase in resistance to sizing, when that happens I am forced to increase the presses ability to overcome the additional resistance to sizing.

I measure the length of the chamber from the shoulder to the bolt face first. I want to cut down on case travel, again, I have found not all case shoulders make it to the shoulder of the chamber. By knowing the length of the chamber before firing I can off set the length of the chamber with the length of the case.

F. Guffey

 

[Walkalong]

I understand what GS is saying, the brass expanded to fill the chamber, then retracted back so it could extract easily. That is what makes brass so suitable for cases.

So, the case has some head clearance built in after firing (Unless you have just hammered it several times with heavy loads and it will be very close), and we move it back even more when we bump it.

I still do like Bart does for Bolt guns, I just bump the shoulder a bit more and call it good. If I bump the shoulder an average of .001 or .002, I know there is more head clearance than that, but it is still small enough to suit me, has given excellent case life and accuracy, so I do it that way, which is simple. GS wants a tighter fit, which is cool. Or did I read that wrong?

For autos I either use a case gauge to set up the sizer if I know the headspace is good on the rifle, or bump the shoulders an average of .003, which has been working for me in 300 BLK.

Whatever works.

 

[gamestalker]

No, you are dead on Walkalong, I go for as close to .000" brass to chamber tolerance as possible. But I don't trust fired brass to find my chamber head to shoulder dimension, it can be unreliable. I've had shoulders get pushed back excessively when fired, as much as .005".

And I also don't like neck sizing much, as it doesn't bring the web back into spec, which creates some unwanted resistance when extracting / bolt lift, which can create some difficulty when reading pressures.

GS

 

[Bart B.]

GS, it's my opinion as well as data from a few tests I've run, anytime case headspace is shorter after firing than before, the load's reduced too much and its peak pressure isn't enough to push the back half of the case against the bolt face.

All bottleneck cases headspacing on their shoulder get their shoulderx set back a bit from firing pin impact. Normal peak pressure pushes the back half back to its limit while the front half's pressed hard against the chamber wall. I've had both .308 Win and .30-06 cases do that when loads are more than 10% below max. Others have mentioned observing this same thing. It's why many loading data sources say not to go below minimum. And full length sizing them does indeed squeeze their shoulders forward.

Now that you've mentioned shoulder setback, i understand why you want a snug fit of the case in the chamber. Thanks for mentioning the rest of the issues involved.

 

[fguffey]

I've had shoulders get pushed back excessively when fired, as much as .005".

I would like for someone to explain how that could happen.

F. Guffey

I am sure GS was not talking about reduce loads, again, I have fired cases that had shoulders that never made it to the shoulder of the chamber. Those are my favorite, no case stretch between the case body and case head.

 

[fguffey]

Unread July 29, 2015, 05:21 PM #30
gamestalker
Member

Join Date: September 10, 2008
Location: SW Arizona
Posts: 9,295 Curator, I form all my brass by jamming into the lands, thus ensuring the brass will get properly formed on the first firing. But I don't run reduced loads, not max charges either, but slightly above mid table, and with a slow burning powder for that specific cartridge and bullet weight.

My concern and reasoning for not using a light load when jammed hard, is that the bullet may be delayed for one mil second too long, thus increasing the chance of experiencing a high pressure spike.

GS

I was correct in my assumption about gamestalker and reduce loads.

F. Guffey

 

[gamestalker]

I surmise that when initially forming brass, that by not jamming to keep the case head held hard against the bolt face, and even with a typical pressure charge, that the case head is elevated off the bolt face at ignition, do in part to both primer impact, and extractor button pushing it away. So when that happens, I think that when the case expands and seizes against the chamber wall, the shoulder only gets pushed up part way, or only as far as it can be, and when pressures subside, the shoulder retracts, which is a normal occurrence / function with brass, head to shoulder datum line measurement is either diminished, or unchanged, rather than fully formed to chamber head to shoulder dimension.

I've seen this more commonly with cases that always head space on the shoulder, but I've also seen it happen with belted brass also.

Just my thoughts and observations of the steps I've incorporated that have successfully attained desired results, and those that didn't.

GS

 

[Walkalong]

I have always fire formed with medium to heavy neck tension, near full loads, and the bullet jammed into the lands.

Some fire form with cream of wheat and no bullet. Some fire form with a bullet but a light load.

 

[Bart B.]

GS,

First, please read post #15 in this thread; thanks. Then finish reading this post.

All rimless bottleneck rounds (dimensioned correctly for the chamber; a little head clearance) have their case shoulder hard against the chamber shoulder when the primer fires. The case may be pushed there by an ejector beforehand. It stays there while the round fires and expands the case to chamber limits. Shoulder brass is drawn into the body area and neck brass drawn back into the shoulder area as the case head is pressed against the bolt face. After the bullet leaves the barrel, the case shrinks back from the chamber dimensions.

Case length is a few thousandths shorter after it fires. It grows that much back plus near another thousandth when full length sized. It's about a thousandth longer the next time it's fired.

Belted bottleneck cases sized to headspace on their shoulder do the same thing. Including having their shoulders set back a bit when the firing pin smacks their primer. I've measured several.

People who don't think rimless bottleneck case shoulders don't set back from firing pin impact need to find then research the reason H&H put the first belt on their .400/375 Belted Nitro Express to fix the problem of shoulder setback upon firing causing excessive head clearance.

All cases sized correctly for their chambers they shoot in will never have their rim touch the extractor catch because they'll stop against the chamber headspace reference; shoulder, belt step or rim step. There's plenty of clearance built into the rifle's design to let that happen. It's a decades old myth that extractors old case heads against bolt faces. Go measure your own stuff and find out.

 

[Metal God]

I have fired cases that had shoulders that never made it to the shoulder of the chamber. Those are my favorite, no case stretch between the case body and case head.

Any reloader that understands what's going on when the cartridge is fired would understand why there is no case stretch if the shoulder is off the chambers shoulder .152" when fired . There is a perfectly good reason why . I'd be glad to explain that for you if you don't understand why .

 

[fguffey]

Any reloader that understands what's going on when the cartridge is fired would understand why there is no case stretch if the shoulder is of the chambers shounlder .152 when fired

Any reloader? You should start over and read what is being said. It is being suggested the firing pin is driving the case forward to the shoulder of the chamber. If that is what is happening there is nothing you can do to prevent the case from stretching between the case head and case body. Because the case head is not against the bolt face.

Any reloader that understands what's going on when the cartridge is fired would understand why there is no case stretch

?

F. Guffey

 

[Metal God]

If you have a huge amount of head clearance like when forming a wild cat where the head of the case is touching the bolt face and you have many many thousandths of space between the case shoulder and the chambers shoulder . When the firing pin hits the primer the extractor stops the case from moving fully forward and contacting the chambers shoulder .

When this happens , If you didn't have enough pressure to blow that short shoulder forward enough to get to the chambers shoulder . You would have never even come close to having the pressure needed to stretch the body of the case . The pressure is going to deform/stretch the case in the path of least resistance . That is going to be the shoulder area because the shoulder just lays down creating more case body to match the longer length of the chamber rather then the body actually stretching to match the longer length of the chamber .

If you are still at high enough pressures after the NEW case shoulder hits the chambers shoulder and there is still space between the head and bolt face . You then can get case stretch above the web that ultimately could lead to head separation .

Hope this helps

Metal

 

[fguffey]

Any reloader that understands what's going on when the cartridge is fired would understand why there is no case stretch if the shoulder is of the chambers shounlder .152 when fired

"there is no case stretch if the shoulder is of the chambers shounlder .152 when fired" Did you mean of or off? And I will assume you meant shoulder, not shoundler. .152 what? I will assume you meant .152".

If you have a huge amount off head clearance like when forming a wild cat where the head of the case is touching the bolt face and you have many many thousandths of space between the case shoulder and the chambers shoulder . When the firing pin hits the primer the extractor stops the case from moving fully forward and contacting the chambers shoulder .

Any reloader that understands what's going on etc..

Any reloader that understands? A reloader that understands would never get caught up in the 'huge amount of head clearance like etc., etc..

F. Guffey

 

[Metal God]

haha that's all you got , typing errors . If that's all you find wrong with my post then I must assume you know I'm right .

Thanks for pointing them out . I'll go back and fix them .

 

[fguffey]

OK, while you are fixing fix the quote below:

When this happens , If you didn't have enough pressure to blow that short shoulder forward enough to get to the chambers shoulder . You would have never even come close to having the pressure needed to stretch the body of the case

F. Guffey

 

[Bart B.]

MG, you're right.

Head separation typically starts in the case web behind the pressure ring. That's the unsupported area behind the chamber edge so it's the easiest place for metal to stretch. It's also the most work hardened part of the case.

I've never seen a rimless bottleneck case cutaway showing that web thinning forward of the pressure ring's ridge on full length sized ones. If it does happen, I think it's rare.

 

[fguffey]

MG, you're right.

Head separation starts in the case web behind the pressure ring. That's the unsupported area behind the chamber edge so it's the easiest place for metal to stretch. It's also the most work hardened part of the case.

I've never seen a case cutaway showing that web thinning forward of the pressure ring's ridge.

I disagree, what we need to do measure case head separation from the separation to the case head. For those of us that do measure we find the separation is ahead of the edge of the chamber radius. Remember we have case head protrusion and unsupported case head. Then we have case head thickness, I have case heads that are .260" thick and I have case heads that are .200" thick. then we have a couple of very proflic posters that have cases that separate half way between the case head and case shoulder.

F. Guffey

 

[murf]

shoulders don't get setback by firing pin impact. they get setback by 4000+ psi pushing on the case primer pocket base by the explosion of the primer compound.

if the firing pin pushed by a 25# firing pin spring were all it takes to set back a shoulder, lee would have made their lee loaders also set the shoulder back instead of just resizing the neck. i also just finished a little experiment: put a fired 270win case in a case gauge, measured headspace (distance below upper shelf on gauge), wacked the primer with a punch (slightly bigger diameter than firing pin and much deeper hole) and a hammer, measured headspace again. measurement was the same before and after.

anyway, case shoulder setback happens upon firing, so carry on!

murf

 

[Bart B.]

Murf, you did exactly what a friend did years ago trying to disprove my test results. But something in it masked shoulder setback from firing pin impact on the primer. Without knowing the details of both your case and case gauge, I cannot comment.

So I brought him my rifle, some empty bottleneck cases with primers deactivated with drop of motor oil, a case headspace gauge then let him snap the rifle's firing pin on them and measure their before and after case headspace.

They all had shoulder setback from .002" to .005" depending on the case type. Brass ones had the least, nickel plated ones the most. No primer fired; all shoulder setback was the result of firing pin impact on the primer transferring that energy to the case.

Hatcher observed 30-06 rifles setting case shoulders back from just closing their bolts on them:

https://books.google.ca/books?id=yE...OEygHdwIHgDQ#v=onepage&q=hatcher .006&f=false