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Old May 22, 2009, 10:08 PM   #1
mrmuffy
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Bullet "jump" from chamber to rifling.

I keep reading that bullet seating depth is very important.
Even a difference of a few thousandths of an inch can affect accuracy.
Can someone explain WHY the distance a bullet jumps from the chamber to the barrel rifling is such an accuracy concern?
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Old May 22, 2009, 10:18 PM   #2
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I'm a fan of long throats. My first rifle was a Sauer 7mm Wby, which has a freebore. My 6.5-06 is throated for 140 Hornadys and my .338-06 is throated for 250 Noslers. They are both MOA and I can get additional velocity. A 140 Hornady goes 2935 and a 200 Hornady goes 2810. I had a Model 700 7mmRM that was short throated and plain misery.
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Old May 22, 2009, 10:18 PM   #3
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in short , the shorter the jump the less time the bullet has to getout of kilter when it hits the rifling.
the heavier bullets have longer surfaces thus cutting down or eliminating the "jump" altogether.
try to imagine an under sized cast boolit rattling down the cyl. in a revolver& when it hits the forcing cone tilted it`ll never ride the rifling true & be a flyer!!
hope this helps even if a little.

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Old May 22, 2009, 10:38 PM   #4
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I have read about the subject but it's been a long while while back, so I don't have the facts. I expect others that contribute that will give you better info.

As for guesses: During the "jump" some pressure will be released around/bypassing the bullet. The load and speed of the bullet would effect how fast it's moving when it does impact the rifling. Might cause increased deformity of the bullet. How long the jump from case to rifling is would also likely increase or decrease any of these problems. Both of these could detract from bullet accuracy.

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Old May 22, 2009, 11:15 PM   #5
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This is one of the reasons the often-called-ugly Savage Barrel nut design is so darn accurate
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Old May 22, 2009, 11:29 PM   #6
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If you are talking about bullet jump to rifling in pistols, it is not a big issue. Seating depth in pistol cases is critical. If the seating depth is to great, i.e. if you cecrease the volume of the case by seating the bullet deeper, then you have pressure increases.

In rifles, there are some magic numbers that will produce better accuracy in some weapons, while in others it seems to make no difference.

Read this thread:
http://www.thefiringline.com/forums/...d.php?t=332789

Then read this one.
http://www.thefiringline.com/forums/...d.php?t=289021
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Old May 23, 2009, 08:14 AM   #7
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As I understand it the bullet jumps slightly to the rifling after cartridge ignition. That's simply how things work.
I follow that the brass itself cannot exceed a certain length and that the bullet must be seated far enough into the brass so that the overall finished length of the round is within published safety specs.
Am I correct in assuming that there are considerable differences between actual chamber, throat and rifling measurements in the same caliber from one rifle and/or brand to the next?
-The point being that every combination of these measurements per each rifle will have a sweet spot that can be found by regulating the bullet seat depth?
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Old May 23, 2009, 08:58 AM   #8
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It's my understanding that free bore can increase velocity due to reducing initial friction, but there are exceptions to that rule where the earlier the friction is applied the greater the pressure build up and shorter time (relative to bullet location) to peak pressure. Barrel length plays a role.

As I understand, the earlier to engage rifling the greater the accuracy potential; but as above there are so many factors that contribute to accuracy that counter examples are aplenty as noted with the Weatherby above.

I'd imagine you'd find more benchresters who have the bullet seated right on top of the rifling than the opposite.

Quote:
If the seating depth is to great, i.e. if you cecrease the volume of the case by seating the bullet deeper, then you have pressure increases.
And with the same situation with the bullet encountering the rifling immediately the rifling friction can cause a pressure increase as well.
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Old May 23, 2009, 09:10 AM   #9
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Am I correct in assuming that there are considerable differences between actual chamber, throat and rifling measurements in the same caliber from one rifle and/or brand to the next?
Yes. If you fire a factory round and eject the case it will essentially be fireformed to that specific rifle. If you take that case and attempt to load it in a different rifle it may not chamber. That's completely normal and it proves chamber dimensions are different from one rifle to the next. Also, if your rifle has throat erosion, it affects the issue also. You don't need to worry about it with new rifles. If you buy used, have it inspected by a competent gunsmith. If it's been cared for and it's chambered in a cartridge that's easy on the barrel (i.e. .308Win), you shouldn't have this concern. The measurements may be different from each rifle, but they're all made (well, should be) to SAAMI specs. In other words, you can take any common factory made ammo and use it in any rifle and it will fire safely.
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Old May 23, 2009, 01:50 PM   #10
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We need to have some terms defined for common use.

The space ahead of a rifle's chamber just before the rifling starts is called the "leade," or throat, or "freebore" and is really the bore of the barrel with the rifling reamed away. The leade varies considerably in length and in the angle at which the rifling is cut, depending on the different factories, cartridge designers, and barrel and gun makers.

I have come to believe that most high production rifle makers us a leade that will be about 1/32nd of an inch (0.030”) from where the longest heaviest bullets per caliber are seated. I have seen some weapons with the freebore which allows almost half an inch of bullet travel before it contacts the rifling firmly.

General wisdom has told us that the “ideal distance”, from where the bullet is seated to where the bullet makes contact with the rifling, should be a gap of about 0.030”. This allows the pressure to build very smoothly and steadily even as the bullet hits the rifling. Pressure remains safe (normal) throughout the powder burn, and the velocity obtained is "wxyz" feet/second.

Seating the bullet deeper into the case, like most factory ammo, allows more travel before it hits the rifling, and permits the bullet to get a good running start. Powder gases have more room in which to rapidly expand without resistance, and the pressure thus never reaches the "normal" level nor does the velocity; as compared with the same powder charge at 0.030 off.

The saying that the “bullet must be seated at least one diameter in the case” is primarily for hunting rounds. If you have less than one diameter in the case, the bullet can easily be pushed off center line with the bore as it is being chambered. For target use, it is not a problem when feeding cartridges one at a time, provided they have been protected from distortion in the trip to the range.

When the bullet is not concentric (does not align with the bore). There is little chance of it going sideways down the barrel as some people may joke, but it will rattle back and forth in the leade, and can enter the rifling askew by a very small margin, enough to cause the bullet to wobble in flight. The same thing happens when you seat the bullet too far from the rifling. A longer barrel will not corret the wobble.

When the bullet is seated to touch the rifling, it does not move when the pressure is low; and not getting a run at the rifling as did the other bullets, it takes a greater increase in pressure to overcome the friction of the rifling and begin to move down the barrel. The expanding gases have less room than they should have at this time in their burn, and the pressure rise is both rapid and excessive. Velocity is higher than the Short and normal length cartridges, but could be at dangerous pressure if the powder load is at or near max.

Many rifles deliver their best groups when bullets are seated just touching the rifling. Seating bullets into the lands can be done quite safely if the reloader will reduce his charge by a few grains. The lighter load will still produce the "normal" velocity and pressure.

Barrels with several thousand rounds through them have been noted to fall off in the accuracy, using their standard loads. Hot gases, unburned powder, and incompletely burned powder particles, from the all the shots fired through the barrel, erode the throat and thus increase the distance a bullet must travel before contacting the rifling. By loading longer bullets and seating them farther out so they'll be closer to or touch the rifling, and adjusting the powder charge, can improve accuracy again.

In addition to the seating depth, factors such as primer, bolt alignment, barrel length, tightness of bore, height of the lands, bullet, brass, powder, weather, elevation above sea level, and more all contribute to accuracy.
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Old May 23, 2009, 03:07 PM   #11
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Great post, Shoney. Some may not read all of it due to the length of your post, but they would have missed out on some invaluable information. A costly mistake I say...
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Old May 23, 2009, 03:58 PM   #12
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This is one of the reasons the often-called-ugly Savage Barrel nut design is so darn accurate
Please correct me if I'm wrong, but I'm pretty sure Savage uses the barrel nut as a way of controlling headspace. It really has nothing to do with freebore.
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Old May 23, 2009, 04:25 PM   #13
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Quote:
Please correct me if I'm wrong, but I'm pretty sure Savage uses the barrel nut as a way of controlling headspace. It really has nothing to do with freebore.
Actually, it's simply the means to attach the barrel firmly to the action. It CAN be used to adjust headspace by loosening it and turning the barrel in or out as required.

In my experience, there's no hard and fast rule about just what freebore will do for velocity OR accuracy. Accuracy is generally expressed as group size at any given range.

Most semi-autos are limited as to how far out you can seat a bullet by their box magazines. In the AR platform, you have to live with the mag length restrictions, then deal with accuracy by load development.
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Old May 24, 2009, 09:08 AM   #14
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In the AR platform, you have to live with the mag length restrictions, then deal with accuracy by load development.
Or give up on semi-auto and single feed the rounds.

Relatively common with the heavier (and therefore longer) bullets used for some shooting games and distances.
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Old May 24, 2009, 01:48 PM   #15
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Or give up on semi-auto and single feed the rounds.

Relatively common with the heavier (and therefore longer) bullets used for some shooting games and distances.
Excellent point, for slow fire or long range targets, you can single load longer than mag. length ammo. but for DCM or CMP marksmanship programs, or IPSC 3 gun,you have to be able to use the magazine fed rounds.
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Old May 25, 2009, 07:28 AM   #16
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When Oehler came out with its pressure testing stuff, a friend used it to measure the difference between bullet's seated to just touch the lands and backed off with a .025-inch jump. Those touching the lands were about 2,500 psi higher. Cutting the 45 grain charge in his .308 Win. by about 1/2 grain for loads with land-touching bullets made pressure the same for both loads.

In my own tests, bullets held lightly by the case neck and set back a few thousandths when chambered have lower muzzle velocity spreads than those held tight by case necks and backed off from the lands .020- to .030-inch. Seating bullets out to touch the lands is common for folks shooting long range matches as it does improve accuracy especially when new cases are used.

The man who designed and made Weatherby's first stocks told me Roy W'by. wanted long leades for his magnums so as much powder as possible could be used with heavier bullets. Mr. Weatherby was "obsessed" with high velocity and willingly give up accuracy to get it. At the ranges his rifles were intended to be used at, accuracy was not a bit issue. Roy Weatherby's shop built two single shot 9-lug actions (no magazine cut out; solid bottom) for this stockmaker to use for his 1000 yard target rifles. He used Hart barrels with both standard long Weatherby leades and short ones; the short ones shot better. But his .30-.338 on a Win. 70 action was much more accurate so he quit using the Weatherbys.

Regarding the chamber in front of the case neck:

Throat, the constant diameter part of the barrel in front of the case that's a bit larger than bullet diameter.

Leade, the angled part of the rifling tapering down from the front of the throat to bore diameter.
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Old May 25, 2009, 07:53 AM   #17
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Quote:
I'm a fan of long throats.
+1, AND a gentle taper into the rifling. the big deal about having a little freebore before the bullet hits and engages the rifling (at least as i understand it) is that it helps reduce pressure spikes. when the bullet has a little room to gain momentum before it hits the rifling, there is more area for the powder to burn, giving the gasses more room to expand, thus lowering the peak in pressure. a little momentum will go a long way and a gentle taper (at least in my mind) will help the bullet engage the rifling easier, thus loosing as little of momentum as possible. combine the two, and you have a bullet that is happy, well stabilized and a lower peak pressure. of course all of this will help bullet velocity. and all of it depends on GOOD workmanship. if the barrel is slopilly done, and the bullet gets a chance to engage the rifling crooked, say goodbye to the accuracy. but if the freebore is tight enough to hold the bullet straight and parralel, all should be fine.
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Old May 25, 2009, 12:37 PM   #18
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Thanks for the info.

There are a LOT of people out there that are interested in reloading.
Most of us realize that the intention is not only to economically mass produce a round but to work up loads for a particular application.
The subject can be intimidating for a beginner as we try to digest the basics.
All of the responses to my questions have been from reloaders that recognize the need for clarity on the subject matter.

My application is long range shooting - specifically whitetails at 300+ yards.
Every year at least 3 or 4 of these shots present themselves.
- Usually broadside shots.
I pass on them due to the inconsistencies I've found with factory ammo.
Quality ammo is just as important as accurizing your weapon and honing your shooting skills.
I am confident that one day I will produce that kind of quality.

All of your input is appreciated as I apply it toward the selection of tools and equipment.
I'm sure my questions will become increasingly more specific in the future.
This forum serves a good cause.
Thanks again.
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Old May 25, 2009, 01:57 PM   #19
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All of your input is appreciated as I apply it toward the selection of tools and equipment.
I'm sure my questions will become increasingly more specific in the future.
This forum serves a good cause.
Thanks again.
You're welcome. But to really get started we need to know just what caliber, make, and model rifle you're shooting. The more specific you can be, the better we can answer your questions.

Loading good, accurate, long range hunting ammo narrows the search quite a bit. High ballistic coefficient bullets, that also have good terminal performance are not that hard to find. Then matching those bullets to good, well prepared brass, the correct primer, and consistent powder, results in accurate ammo.

For hunting rounds, I load to mag length, then do load work ups staying at that Over All Length,(OAL). I want my hunting ammo to work in ALL situations. If I were to ONLY sit in a permanent stand on the edge of a big field, I would consider purpose built,(loaded), ammo that would NOT fit in the mag. Meaning longer that normal, to be loaded single shot.

There's tools that you will need to know exactly where you throat ends, and the rifling starts in your rifle. Stoney point made such a tool, they've been bought out by Hornady.

http://www.midwayusa.com/viewProduct...tNumber=570611

You'll also need the converted caliber specific case for your rifle.

Another tool that's a good idea is a precision micrometer made by RCBS. It tells you just what the actual headspace is for your chamber, so you can set your full length sizing die to just bump the shoulder of the case the right amount. Hornady also sells a set of bushings that do the same thing.
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Old May 25, 2009, 03:00 PM   #20
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snuffy,

I shoot a Remington BDL Custom Deluxe in 270 Win.(a gift from the wife.)
It wears a 4.5x14x40 LR Leupold scope and has a Jewell varminter trigger.
I will also be loading 223 and some 270 & 300 Weatherby loads for my Dad.

I walk for miles in the coulee country in extreme NW South Dakota during the day and sit on stand in the evening.I also want my ammo to work in all situations.

The Hornady Overall Length Gauge tool you mentioned is the reason I asked the original question at the beginning of this thread.
Until I noticed the tool in their catalog I had assumed that a cartridge was loaded at no more than a standard measured length per published spec info.
I'll add that tool to my list.
Is the RCBS micrometer in particular a superior one or is it simply your personal preference?

What's your take on the RCBS & Hornady case prep stations?
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Old May 25, 2009, 04:18 PM   #21
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I have this one;

http://www.midwayusa.com/viewProduct...tNumber=565099

RCBS calls it a case trimmer, it is not. It IS a case prep tool that works quite well. The 5 stations all rotate at once, slowly, and have 8-32 female threads on the tool points. That accommodates the primer pocket brushes, and other case prep tools. Including the primer flash hole uniformers. I use mine for when I'm going to do match case prep, or for my hunting rounds.

The RCBS precision case mic, is what I use, for .223 and .308.

http://www.midwayusa.com/viewProduct...tNumber=198327

It uses a factory loaded round, or a new case loaded by you, then fired in your rifle to tell you exactly what your headspace is. Then, you can set your full length die to just bump the shoulder back 2 or 3 thousandths. This vastly minimizes case stretch, and eliminates case head separations. It also makes the shell a tighter fit in the chamber, which aligns the bullet better in the chamber.

It also has a comparator as part of it, BUT if your bullet has a different ogive,( the portion of the bullet from the full diameter on the front sides of the bullet, to the tip/point), than the one on the comparator, it is only close to where your rifling actually begins.
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Old May 26, 2009, 05:52 PM   #22
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Shoney "Great" post, I new what to do but know I understand it much better.
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Old May 27, 2009, 11:04 PM   #23
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Great forum.

Most of the technical feedback is beyond me right now but I'm all ears.
I honestly believed reloading only involved duplicating factory ammo...
It's looking to be very interesting science.
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Old May 29, 2009, 12:16 AM   #24
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I have found that the freebore of most factory barrels is long enough to accommodate the heaviest bullets that are produced for that caliber. So…if you want to seat….say a 50 gr. Bullet in a savage .223 and you try to seat that bullet at .010 off the lands, you will not have enough of the bullet in the case to keep it straight or as in my case the bullet would not even stay in the case. Whenever I start load development on a new rifle, I start as close to the lands that I can and load 5 rounds. I then start seating the bullet deeper in .010 increments…5 rounds each until I have at least the diameter of the bullet seated in the case. In every case and with every rifle that I have, I have seen my groups get smaller with each set-back up to a point then they will start to grow. At that point I know where my seating depth is for that bullet. Then and only then will I start my load development. Seating a bullet on or close to the lands works great if you can still get enough of the bullet in the case to give even pressures and even neck tension. I think most BR shooters have their chambers cut with reamers made for specific bullets that they want to shoot. I.e. short freebores. My stock savage 110FP shot a .1653 inch 5 shot group at 100 yards and it is seated .067 off the lands. I guess what I’m trying to say is that getting a bullet as close to the lands in a factory rifle is not always the best in my opinion.
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Old May 29, 2009, 02:29 AM   #25
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Very interesting stuff.

By varying the bullet jump what sort of accuracy difference can be expected ?
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