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Old January 20, 2009, 11:09 PM   #1
Idlechater
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OACL Question

I am just getting into serious reloading and have purchased a Hornady OACL gauge.

Tonight I was checking things out for my M77 in 30-06 caliber. I have some 125 Grain Nosler Ballistic Tips that I have loaded for several years for my son and shot without problem. They are loaded to 3.370"

The Speer Manual list the OACL for the 30-06 as 3.340, so I am over this by 30 thou. If I use the rule of thumb that says you should seat your bullet one caliber deep, my OACL would be 3.265. If I use my Hornady OACL gauge, it suggest that I need an OACL of approximately 3.4'ish, which puts the bullet way to far out to get a reliable crimp.

So, my questions are:

1) Any problem with exceeding the OACL published by Speer, assuming that the cartridge will fit in the magazine, feed reliably, and not be crammed into the rifling?

2) Any problem with having the bullet so far from the rifling? Any way that you look at it, I am between 60 and 135 thou off of the rifling. While I have not had any problems to date, this just sounds like I'm getting one heck of a running start and might put a bullet down the tube sideways.

3) Am I all wet on my rule of thumb of seating the bullet one caliber deep?

I appreciate you all sharing your wisdom with me.
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Old January 21, 2009, 05:01 AM   #2
Shoney
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Idlechater: you have touched on one of the sweet mysteries of reloading. It is not a simple answer, nor can it be answered in a few lines. I hope you can wade thru this and can understand whats I'm trying to say. Before I answer the questions, 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. Normally it is quite short and the rifling starts about one 32nd of an inch from the bullet; but in some rifles the freebore might allow over half an inch of bullet travel before it contacts the rifling firmly.

It has been written that the OAL was set by SAAMI for each cartridge, not for placement of the bullet away from the rifling, but moreover to fit in all magazines.

Many consider the “ideal distance” between the bullet and the initial contact with the rifling, to be about one 32nd of an inch gap (0.030”). This allows the pressure to build very smoothly and steadily even as the bullet hits the rifling. Pressure remains safe throughout the powder burn, and the velocity obtained is "normal".

Seating the bullet deeper into the case, like most factory ammo, allows more travel before it takes 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 their pressure thus never reaches the "normal" level. Nor does the velocity; 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. 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 you joked, 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.

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 at dangerous pressure.

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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Last edited by Shoney; January 21, 2009 at 05:09 AM.
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Old January 21, 2009, 07:48 AM   #3
SL1
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Idlechater wrote:,
Quote:
Any way that you look at it, I am between 60 and 135 thou off of the rifling.
Here's the way to look at it most accurately. It works particularly well with plastic-tipped bullets.

Select a bullet that appears to be unblemished, particularly in the area where the ogive transitions to the straight section of the bullet. That is the part that will be controlling the measurement you want to make.

Open the bolt and push the bullet into the chamber lead with a pencil so that it gently contacts the rifling. While holding there, push a flat-ended rod down the barrel from the muzzle end until it gently contacts the tip of the bullet. Then, while holding the bullet ogive aginst the lands from both ends, with your third hand or wife's help, make the position of the muzzle on the rod. Then, remove the pencil, rod and bullet, close the action, reinsert the rod so that it contacts the back of the action (bolt face in your M-77) and mark the position of the muzzle on the rod again. The distance between the marks on the muzzle is the length of a cartridge with THAT bullet that will make th bullet just touch the lands.

A few tips: (1) Make sure that the end of the rod is flat and square with the axis of the rod, otherwise the measurements will be off. Do not use a cleaning rod with a hole in the end, because the bullet tip will go into the hole but the bolt face will not, making the measurement too short. (2) Make sure that the rod contacts the face of the action for the second measurement, and does not get held-away by an ejector, a protruding firing pin, or some other object. (3) Marking the position of the muzzle on the rod needs to be precise. A magic marker isn't going to allow a sufficiently precise measurment. I wrap tape around the rod so that it is parially inside /partially outside the muzzle at the two locations that need to be marked. I make the "marks" with a single edge razor blade held flat against the muzzle to make cuts ih the tape. This gives really sharp lines to use for the measurements. (4) Use a good caliper to make the measurement between the two marks. Rulers aren't precise enough.

Also, keep in mind that different bullets from the same box may differ enough in the ogive to give slightly different measurements. If you plan to seat the bullets right against the rifling, it would pay to check several bullets this way to make sure that you know if there is much variation in the length needed to touch the lands. (I realize you don't want to do that, but others will read this too.)

This measurement technique does not require expensive gauges, and is more accurate for YOUR rifle because it does not count on the gauge having the same configuration as your rilfe's throat where it transitions into the rifling.

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Old January 21, 2009, 10:07 AM   #4
IllinoisCoyoteHunter
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Check out varmint al's page on how to make a poor mans gauge to determine where to seat your bullet. It works great!!! I have found that instead of cutting slits in the neck of the brass like varmint al does, I set my lee collet neck sizer to only slighly resize the neck so the bullet moves in and out with a couple pounds of pressure....enough to hold the setting. You will probably need a cleaning rod to gently bump the bullet off the lands. Good luck!!!
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Old January 21, 2009, 10:18 PM   #5
Idlechater
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Shoney:

Outstanding reply!!! Thanks for sharing your knowledge...

Sl1 and Illinois:

I already have a OACL gauge... Basically it is telling me that the leade on my M77 is so great that I cannot seat a 125 Grain Nosler BT bullet as I do not have sufficient bullet length. Shoney's reply suggest that while it is not ideal to have excessive leade, it is not necessarily a dangerous situation.

Regards,
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Old January 22, 2009, 09:31 AM   #6
SL1
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Idlechater,

It is not a safety problem to have a lot of bullet jump to reach the lands.

It may not be an accuracy problem, either.

With respect to having a caliber-length of bullet shank seated into your case neck, that is not necessarily the most accurate either. And, some of the newest case designs don't even HAVE a neck that is a caliber long. So, you can seat out a little more and see if it will improve accuracy.

If you find a length that is significantly more accurate than your current length, and it will feed from your magazine, then you can do a test to see if hand feeding single rounds is more accurate than feeding from the magazine (unless you have a controlled-feed action, like M-77 MkII - - the original M-77 is push feed). If feeding from the magazine does not push the bullet out of alignment enough to see the effect on your target, then I would be happy with that length.

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Old January 22, 2009, 05:07 PM   #7
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Idlechater,
You will need to check your OAL for rifle cartridges in two different ways:

1) (Physical) OAL from tip of nose to end of base: to make sure the cartridge will fit and feed properly from your rifle magazine.

2) (Effective) OAL from contact point on bullet ogive to end of base: the curve/ogive of bullet is what actually touches the barrel and the distance from that point on bullet to end of base is the EOAL which I always used to seat my bullets. Several different measuring tools will allow you to find and measure that distance. Note that the cartridge must then be measured as to (Physical) OAL to be sure the rounds will fit into and feed properly from magazine.
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