What Cartridge Dimension Determines Bullets' Jump to Rifling?

[Bart B.]

Rimmed cartridges that headspace on their rim's front edge; front of case rim to Bullet to Rifling Contact Diameter (hereafter termed BRCD). That's somewhere between the barrel's actual bore diameter and the bullet's actual diameter. All barrel chambers for a given cartridge do not have the same throat angle so that also effects where a given bullet's BRCD is.

Rimless bottleneck cartridge cases headspacing on their shoulder; case shoulder to BRCD, like the 5.56 and 7.62 NATO.

Rimless straight or slightly tapered cases headspacing on their mouths; case mouth to BRCD, 9mm Luger or .45 ACP for example.

Belted cartridges headspacing on their belt's front edge; case belt front edge to BRCD, .300 Win Mag and others with the H&H style belt.

For any bottleneck case sized so it headspaces on its shoulder, it's their case shoulder to BRCD. Such cases have a small spread in case head space; case head to shoulder reference. A few thousandths spread is normal. And the case shoulder gets set back a thousandth or more from firing pin impact as the round fires; how much depends on case shoulder angle and surface area as well as friction to the chamber shoulder plus firing pin impact force.

The measurement from case head to bullet tip of a loaded round is meaningless as the bullet tip touches nothing in the rifle. There's a small spread in bullet shape dimensions from the bullet tip to the BRCD due to tolerances in its forming operations. The case head is rarely, if ever, hard against the breech face when the round fires. Only when the case headspacing dimension is equal to, or a bit greater than chamber headspace, will the case head be against the breech face when the round fires. That's because the firing pin drives the case full forward against the point in the chamber that the case stops against.

An exception is when bullets are seated shallow enough so they stop against the chamber throat (angled part of the rifling from freebore diameter to bore diameter) and they're gripped tight enough by the case neck to prevent them being seated deeper by firing pin impact. Such rounds will have their head firmly against the breech face when fired.

Comments welcomed.

 

[jmorris]

The measurement from case head to bullet tip of a loaded round is meaningless as the bullet tip touches nothing in the rifle. The case head is rarely, if ever, hard against the breech face when the round fires.

Unless the round needs to fit in a magazine, OAL is not as important in a single shot though.

Some of my rifles/bolt action pistols, using bottle neck cases, only get neck sized and some also prefer the bullet to be seated into the lands. In either case the camming action of the bolt is used to chamber the round and the case is hard against the bolt face.

 

[Bart B.]

In my tests with .308 Win cases neck only sized and primed but without powder and bullets seated long to seat a bit deeper when against the rifling, their shoulders were set back a couple thousandths from firing pin impact. And bullets got seated a bit more deeper in the case necks from firing pin impact.

 

[jmorris]

I would have expected the bullet to be shoved into the barrel and the primer to have backed out of the pocket a bit.

 

[fguffey]

Sequence of events: I have killer firing pins, on most of what I shoot the firing pin crushed the primer before the case, bullet and powder know their little buddy the primer got hit. I have no problem accepting the things I can not change. I size my cases to reduce case travel.

Then there are factors that can not be measured with a micrometer or feeler gage. I want to know the distance to the rifling from the bolt face. For me it is a simple matter of drilling the flash hole/primer pocket out to a diameter that accommodates a cleaning rod. With a case that has an enlarged primer pocket and a neck with all the bullet hold I can get I can determine the distance from the bolt face to the rifling if I understand the length of the chamber from the shoulder to the bolt face.

Then there are factors that cause the barrel to move before the bullet leaves, I am the fan of reducing that also.

F. Guffey

 

[jmorris]

I wonder if case movement was what Arndt Kriegeskorte was concerned with when he invented the electronic ignition system (of standard primers) for his Kricotronic rifles.

I suppose it would be easier to get ahold of a Remington electronX rifle to test the difference but they don't use standard primers like the Krico did.

 

[fguffey]

At about that time we would have to start over. Someone is going to have to decide what happens before and again after.

Again, I have fired cases with shoulders that never made it to the shoulder of the chamber. I was setting at a red light minding my own business when the vehicle behind me drove me through the intersection. His front and rear bumpers were destroyed, my front bumper did not get a scratch.

F. Guffey

 

[jmorris]

At about that time we would have to start over. Someone is going to have to decide what happens before and again after

You could certainly get into a vast array of variables. Maybe using an adjustable firing pin spring so one could adjust the force imparted into the primer and an adjustment in firing pin length to record the effect it has as well.

I guess my thought above was with a live primer, but you couldn't use a spent primer for accurate testing as it has already been "crushed". Would have to remove the anvil from a live primer, then remove the priming compound and replace the anvil to have any meaningful data. Would also need to try out different brands of primers because they are all not the same hardness.

You also couldn't use the same case over and over because of work hardening but couldn't really use altogether different cases. Or fired more times than others since last being annealed. As cases harden I have found they can get quite "springy", so the actual deflection was more than where it returned to.

Then again not sure how much quantifiable data you would get out of such tests for the work it would take to perform them.

Also have the influence on various extractor designs as well, like a case I found at the range one day where a 9mm round had been fired out of a .40 caliber barrel, so it wasn't headspaced on anything in particular. Guess that would throw in rim thickness as another variable.

Maybe a modified plunger style ejector using a strain gauge of sorts could be used to measure "real time" what happens.

 

[Metal God]

Bart :

Does a bullet need to be seated to run the firing pin setting back the shoulder test . If you just use a sized case with primer , no powder or bullet . Will you get the same shoulder set back ?

I remember running some test that revealed the natural operation of the AR-15 loading a round set the shoulders back a couple thousandths . Have you found this to be true with other auto loaders ?

 

[Bart B.]

Bullets don't need to be seated in the case to see how much case shoulders are set back.

A primer killed with water can also be used to see the effects of case shoulder setback. No primer detonation issues whatsoever.

Semiautos typically set case shoulders back a little bit. I've measured .30-06 and 7.62 Garands with dead primers in dummy rounds and seen a couple thousandths shoulder setback.

Hatcher reported some .30-06 semi and full auto weapons having up to .007" shoulder setback from both extractors slamming the case into the chamber full forward plus the firing pin's impact adding to whatever chambering setback caused.

Electronic triggers for conventional primers were invented to have minimal rifle movement caused by mechanical triggers coming against their stops. Electric primers were something else.

http://www.remington.com/products/f...special-runs/centerfire/model-700-etronx.aspx

All these variables effect how far the case shoulder's set back upon firing pin impact:

* Firing pin spring load force. Most bolt action centerfire rifles' springs are rated at about 25 to 30 pounds. Handgun springs are much less and that's why handgun primed cases typically back out their primers in primed cases.

* Firing pin weight; the old kinetic energy thing. Rifle springs pushing their heavier firing pins smack the dickens out of rifle primers.

* How deep the primer gets dented. A rifle with its firing pin stopped by the bolt shoulder with its tip protruding from the bolt face .060" will dent a primer deeper than one with only .045".

* Primer cup hardness.

* Friction between case shoulder and chamber shoulder. Nickel plated .308 Win. cases have more shoulder setback than plain brass ones.

If there's enough room in a chamber for a loaded case shoulder to stop against the extractor claw, that claw will hold the case in place to fire the round and its shoulder will get blown forward. All firearms and their cartridge cases properly built are such that the case stops against its headspece point in the chamber before its rim touches the extractor lip.

I once chambered 42 rounds of 7.62 NATO ammo in a long range match to shoot 22 bullets out the barrel. 20 of them had no powder in them; just 190's seated to 2.850" OAL in new M118 primed cases. All primers were still in their original positions. Not enough pressure built up in the empty, bulleted cases to push them back out. Their shoulders were set back a couple thousandths from firing pin impact as compared to unfired rounds of the same lot. The original 172-gr. bullets werre pulled, new powder put in then the Sierra 190's seated; they had much less grip by the case mouth than the 172's did.

One reason the favorite benchrest cases have a large shoulder area and angle is to minimize shoulder setback by firing pins. Much more consistent primer flash with such cases; muzzle velocity spread is smaller.

Over a hundred years ago, H&H learned their experiments with the rimless versions of rimmed large caliber double rifle cartridges set their shoulders back too far in bolt guns increasing head clearance past safe limits. So they put a wide belt on the case a bit larger than case diameter to prevent shoulder setback as well as let the cases function good from staggered box magazines. Rimmed cases don't work well from such magazines. That was the birth of the H&H belted magnum cartridges.

 

[fguffey]

If you just use a sized case with primer , no powder or bullet . Will you get the same shoulder set back ?

What would be the point? I have killer firing pins, the case is setting there without the weight of the bullet and powder, my firing pins like resistance to moving.

There are factors, be sure to include all the factors.

Then there is the sped of the bolt closing, who knows how fast that bolt is traveling when it slams the case into the chamber?

F. Guffey

 

[jmorris]

I once chambered 42 rounds of 7.62 NATO ammo in a long range match to shoot 22 bullets out the barrel. 20 of them had no powder in them; just 190's seated to 2.850" OAL in new M118 primed cases. All primers were still in their original positions. Not enough pressure built up in the empty, bulleted cases to push them back out. Their shoulders were set back a couple thousandths from firing pin impact as compared to unfired rounds of the same lot. The original 172-gr. bullets werre pulled, new powder put in then the Sierra 190's seated; they had much less grip by the case mouth than the 172's did.

I am not sure if I follow, 20 squib loads fired and they didn't move the bullet or primer?

How much would you change neck tension if you just seated bullet "A" then pulled it and seated bullet "B"?

 

[Bart B.]

I am not sure if I follow, 20 squib loads fired and they didn't move the bullet or primer?

No measurable bullet or primer displacement. Several people at the match didn't believe that either when I showed them the misfired rounds. But the facts stood out. Dispelled a myth, for sure.

Handgun primers in my S&W 19 and Browing Hi Power always back out of primed bulleted cases without powder; I too, have seated bullets in such powderless cases

How much would you change neck tension if you just seated bullet "A" then pulled it and seated bullet "B"?

The original M118 7.62 match bullets (lot LC66 12064) needed about 30 pounds of release force to pull those .3086" diameter, 172-gr FMJBT bullets from the case neck. The black asphalt sealer about .0015" thick between bullet and neck glued them in very well. When the .3082" diameter Sierra 190 HPMK's were seated in the case mouths, there wasn't much grip by the sealer even though it had thickened a bit from the bullet being pulled and the case neck probably shrunk back a little. My guess was about 10 to 15 pounds of release force was needed as I operated my RCBS Jr. press pulling them. There was no rifling marks of the four lands in the barrel on the misfired rounds. Bullet jump to the lands in was about .010" as I remember. That happened back some year around 1971; I don't remember all the details now.
__________________

 

[jmorris]

No measurable bullet or primer displacement. Several people at the match didn't believe that either when I showed them the misfired rounds. But the facts stood out. Dispelled a myth, for sure.

I wouldn't have bet on that one, a firing pin that has more energy than the primer does in a pressure vessel. Mythbuster indeed.

 

[Bart B.]

I don't know how primer energy could be compared to that of a firing pin moving 1/4" in .0025 seconds. Here's what I learned about primer's backing out with reduced loads.

Starting out with 44 grains of IMR4064 in new, unfired Federal nickel plated cases primed with Fed 210 primers under 165-gr. bullets. Then reducing the charge in 1% amounts to 15% below that maximum charge; 37.4 grains.

When charge weights got to 40 or so grains, primers were backed out of their pockets about .001" barely visible. Case shoulders were set back about .001" from when new,

At 38 grains, primers were sticking out .005" or thereabouts and case shoulders were set back about .004".

Shot one with 37.4 grains and primer was backed out .006" or so; shoulder set back .005" to .006".

That test convinced me that excessive load reduction is not good. Don't reduce loads enough to let primers end up out past the case head. Also seemed to prove that all those cases had their shoulders set back from firing pin impact but only those with more than about 40 grains of powder fully expanded the case to the chamber limits such that the protruding primer was pushed back into its pocket as pressure reached its peak.

Learned all about that stuff when I bought one of A.R. Martin's .308 Win. case headspace gauges; long before RCBS introduced their Precision Mic. A.R. was a tool and die maker in Sierra Bullets machine shop making bullet forming dies as well as that gauge to measure fired case shoulder setback in their loading room reloading cases to test their bullets with.

 

[jmorris]

Shot one with 37.4 grains and primer was backed out .006" or so; shoulder set back .005" to .006".

So is your thought that the primer backed out because of the low charge and the shoulder set back was due to the firing pin and low powder charge not expanding the case enough to fit the chamber?

I don't know how primer energy could be compared to that of a firing pin moving 1/4" in .0025 seconds.

I suppose you would need to know the weight of the firing pin and use the 8.33 fps number you have. That energy number would be how much force you would need to impart into a primer to set the shoulder "X" inches back and could be converted to in/lbs.

Then know the volume and measure PSI reached when you ignite a primer. With the surface area of the bullet/primer and you could convert to in/lbs as well.

Just be the resistance of the case to move at all in any direction vs what it takes to shove the shoulder back.

 

[Bart B.]

My thoughts based on charge weights and case measurements; here's the sequence of events:

1. Firing pin smacks the primer, drives the case hard into chamber shoulder setting case shoulder back a little, the primer fires as the case shoulder's set back its maximum amount.

2. Powder burns, pressure rises, bullet get pushed on it's way out of the case neck, more pressure pushes front half of cartridge case hard against the chamber wall holding it there and at the same time pushes the primer a bit out past the case head. This is as far as things go with maximum reduced loads.

Note: a 30 caliber bullet needing 20 pounds of force against its base to move it forward in the case neck requires about 270 psi chamber pressure. There has to be more than 25 pounds of force from the primer cup to the 25 pound rated spring pushing the firing pin back along with the primer cup.

3. At peak pressure at normal max levels, the case body's back half has stretched back a bit pushing the case head against the breech face which reseats the protruding primer. The case body in front of its pressure ring (about 2/10ths inch in front of the case head) is fully pressed against the chamber limits.

4. As pressure drops and the bullet's on its way, the case shrinks back a thousandth or so from the chamber dimensions.

 

[jmorris]

Would be neat to know what peak pressures are being reached in the various examples as well as in say a "cream of wheat" fire form load using pistol powders, using no bullet.

I know it's a fine line to get it perfect, at least before fire forming with a bullet.

 

[gamestalker]

Regarding bottle neck, belted, and rimless, I've been jamming hard into the lands for a good number of years, probably 20+ years of the 30 some years I've been reloading. Despite unsubstantiated warnings from those who have never tried this, I have my reasons, which are based on a great deal of satisfactory results seating in this manner, I'll elaborate a bit as follows.

I've found that when attempting to fire form with bullets seated off the lands, the shoulders never get consistently formed, thus cartridge head space is all over the place. Some are too tight, some exhibit excessive shoulder displacement, while some will be acceptable.

But when I seat with the bullet jammed enough to maintain constant bolt face to case head contact, the shoulders are consistently formed to an acceptable and desired tolerance.

I've heard time and time again that jamming is a dangerous and risky process. But IMO, there is a right way to do this, and a wrong way. I honestly believe one of the mistakes that causes problem, is either using the wrong powder, or using to low of a powder charge.

Fast burning powders are not the right way to go, they reach peak pressure too quickly concerning the slight delay that jamming creates. I've found that mid table with an appropriate burn rate, or slow burning powder, provides for a good starting charge. BTW, this information is all generalized and doesn't reference a particular cartridge, bullet weight, or powder, which are all pertinent elements of forming brass in this manner.

That said, it goes without saying that this is not something for the novice or beginner.

When I get brass that has not yet been fired from the weapon I'm loading for, if the tolerance is excessive, I partial resize in steps, measuring the distance from case head to datum line off the shoulder until the brass is fitting to within .0015" of zero. Then when I jam the bullets, I get the same consistent desired tolerances after firing. Subsequent resizing is then performed by FL sizing.

I only use this method with all bottle neck bolt actions.

I don't use this method with AL rifles, such as AR's, or really any AL-ing action.

With straight or tapered wall handgun cartridges, I don't do anything special. I seat so the bullet doesn't contact the rifling and fits the magazine. I do trim all handgun brass though, which is something few reloaders do, and brings up another position I have on reloading. Many reloaders claim this is a pointless and unnecessary step, but I've just always approached reloading with respect to SAAMI specification. But a lot of reloaders state that the last time they checked, their AL-ing brass actually gets shorter, not longer, or it doesn't change at all. I haven't ever seem my brass get shorter, but I have clearly seen it exceed SAAMI specification, if not on the first firing, definitely by the second.

Does it matter? I don't know, all I can say is that if it exceeds SAAMI max, it's clearly out of spec, and therefore needs to be brought back into spec.. If I were the type who loads low or mid table, I'm pretty sure it wouldn't make much difference. But I don't, I only use jacketed bullets, slow burning powders, and at the top end of the tables, and if +P data is available, I use it. So it's for this reason that I seek the most consistent process reasonably possible.

GS

 

[Metal God]

When I did my AR BCG slamming home and setting the shoulder back test . I did the test with the complete bolt . I then did the test with the ejector no extractor , then with the extractor no ejector and then a few final test with out either .

The results were all the same . I would get an average of .0015 to .002 of shoulder set back . This was done with cases that had about .003 of head clearance . I then repeatedly chambered the same round from the mag to see what would happen . I continued to get the set back until the total amount of set back was around .005 to .006 or .009 of head clearance . Once i got to a total of about .006 of set back it stopped . I never researched a specific reason why it stopped . I had a couple thoughts on it though .

1 ) the fact I had no case wall support I was bulging the area of the case at the body shoulder junction which contacted the chamber walls stopping the set back

2) The head clearance became so great that the bolt was no longer forcing the shoulder as hard up against the chambers shoulder as it was .

 

[Bart B.]

GS says:

When I get brass that has not yet been fired from the weapon I'm loading for, if the tolerance is excessive, I partial resize in steps, measuring the distance from case head to datum line off the shoulder until the brass is fitting to within .0015" of zero.

When you do this, does the bolt bind up a bit and needs to be forced closed so the case is a tight fit between bolt face and chamber shoulder?

 

[fguffey]

1. Firing pin smacks the primer, drives the case hard into chamber shoulder setting case shoulder back a little, the primer fires as the case shoulder's set back its maximum amount.

I said I have shoulders on my cases that never made it to the shoulder of the chamber.

A shooter decide to bring 15 fired cases and 5 cases that did not fire over for examination. The 5 failed to fire rounds had two opportunities to fire by the owner in a new rifle. He allowed every shooter at the range with a 30/06 to have a go-at-it. Each of the 5 had 4 more opportunities to join the ranks of once fired.

When it comes to measuring case dimensions I lack for nothing. The shooter purchased 2 boxes of Remington ammo, I compared the failed to fire ammo with my new in the box R-P ammo. No harm came to the failed to fire ammo even after having been hit with firing pins from at least 4 rifles. His fired cases chambered in my chamber gages, the cases also allowed my bolt to close on my 30/06 chambered rifles, the fired cases fit my Wilson case gages and my new 30/06 chambered barrels.

to finish, we removed the primers looking for a reason for the fire to fire problem, nothing. We then installed the primers back into the case 5 cases that never fired and were never harmed by the firing pin. After installing the primers we chambered the cases in one of my M1917s with killer firing pins and busted the primers one at a time. THEN? We measured the cases for the effect the killer firing pin had on the length of the case from the shoulder to the case head NOTHING! The case did not get longer nor did it get shorter and the primer did not protrude from the case head.

F. Guffey

 

[fguffey]

setting case shoulder back a little

One more time, I have shoulders on cases when fired never make it to the shoulder of the chamber. I have fired cases in chambers that were so huge/long from the shoulder to the bolt face had the shoulder of the case made it to the shoulder of the chamber the case would have come apart.

If my shoulder gets set back I want to know 'by how much'. A little?

F. Guffey

 

[Walkalong]

When you do this, does the bolt bind up a bit and needs to be forced closed so the case is a tight fit between bolt face and chamber shoulder?

A very good question. We want a bit of "head clearance" so the round will not be in a bind and can center its self. Also it is a PIA in a match when bolt closure is not smooth and easy.

A tight fit to the chamber is good, but there needs to be clearance. A tiny bit of neck clearance, side to side body clearance, and of course front to back (Head clearance) clearance.

I know people who want a "crush fit" for good case life, but think it is over kill for getting good case life, and not conducive to extreme accuracy.

I could be wrong of course.

 

[Bart B.]

No rifle I know of has its cartridge centered in the back of the chamber when fired. Most of them are pushed off center by the extractor's pressure against them so they're touching the chamber wall. Yes, the round's a tiny bit crooked in the chamber; not an issue as it's one of the most repeatable parts fit in the system.

Tight necks do nothing to center case necks and bullets in the bore. Even a .243 Winchester round that's perfectly straight will center its neck and bullet dead center in a .308 Winchester chamber and bore when fired. It's the case shoulder centering in the chamber shoulder that centers the front end of the cartridge in the chamber. Case necks are free floating with some amount of neck clearance.

A bit of case head clearance is needed so the bolt face won't bear against the case head and twist it off center at the back end of the chamber when it's closed into battery. That typically offsets the bolt head off to one side that also causes accuracy problems. All moving parts of the rifle have to go back into perfect battery each and every time for best accuracy.