Variability in OAL?

[DaveA37]

This may be a dumb question but what would be considered out of spec when it comes to overall cartridge length?

For example, my Rem. 700, 7 mm Express (280) has a throat depth that allows for 3.350 OAL using Nosler 140 gr BT's and, there is still room to spare for increased length, before reaching the lands.
These feed fine without any problems.

Hodgdon shows 3.230 as C.O.L.

I'm curious, how much is too long and how much is too short?

 

[Guest]

That's the trick of handloading. Too long, for me, is "it won't feed through the magazine." Too short? Well, that's when too much bullet is protruding into the case below the neck, taking up space that could be used for more powder to get more velocity! Anywhere in between is fair game.

 

[RiverRider]

If the finished cartridge fits the magazine and feeds, it cannot be too long unless your bullet is jamming into the lands when you close the bolt. The problems with jamming the lands are that an increase in chamber pressure will have to be accounted for when selecting your charge weight, and you run the risk of pulling the bullet out of the neck when extracting a loaded round.

With all else remaining unchanged: seating deeper will increase pressure, and conversely seating longer decreases pressure---until the bullet is in contact with the lands. Pressure will then suddenly become about 7000 psi higher with must jacketed bullets.

 

[DrMike]

The previous two answers pretty well cover the spectrum in providing you the information you seek. There is a lot of freedom in hand loading. This is the reason it is a good practise to begin from a known recipe and branch out as you gain knowledge of what is happening when you pull the trigger.

 

[woods]

Seating deeper will decrease pressure and seating closer to the lands will increase pressure

http://www.hornady.com/ballistics-resource/internal

"To illustrate the effects of variations in bullet travel before the bullet enters the rifling, we'll compare a standard load with adjustments made only in the bullet's seating depth.

In a "normal" load with the bullet seated to allow about one 32nd of an inch gap (A) between the bullet and the initial contact with the rifling, pressure builds very smoothly and steadily even as the bullet takes the rifling. Pressure remains safe throughout the powder burning period (B), and the velocity obtained - 3500 fps - is "normal" for this load in this rifle.

Seating the bullet deeper to allow more travel before it takes the rifling, as in these next two illustrations, permits the bullet to get a good running start (C). Powder gases quickly have more room in which to expand without resistance, and their pressure thus never reaches the "normal" level. Nor does the velocity; with the same powder charge it only comes to 3400 fps (D)."

 

[RiverRider]

Here is some data I have generated using QuickLOAD.

.25-06
120-grain Partition
60.0 grains H1000
OAL = 3.300"
peak pressure = 56000 psi

.25-06
120-grain Partition
60.0 grains H1000
OAL = 3.100"
peak pressure = 62600 psi

In this example, seating the bullet .200" deeper increases peak pressure by 6600 psi.

Here's another example:

6mm-284
95-grain Partition
52.0 grains H4831 SC
OAL = 2.575"
peak pressure = 50000psi

6mm-284
95-grain Partition
52.0 grains H4831 SC
OAL = 2.375"
peak pressure = 54000psi

An OAL decrease in this instance causes a 4000 psi increase in peak pressure.

Here's a good one:

.223 Remington
55-grain V-Max
24.0 grains H335
OAL = 2.260"
peak pressure = 46750 psi

Seat the same bullet to an OAL of 2.160" and pressure will increase to 51000 psi. Seat that bullet another .100" deeper and pressure will increase to 56000 psi. We gained 10000psi in the smaller .223 case by seating .200" deeper while the larger cases showed pressure increases about half that.

I have to wonder about this disparity. I can imagine where extremes in bullet weight might magnify or diminish the importance of OAL in some cases.

I'll confess that I have not played with a whole lot of combinations of powders and bullet weights with QuickLOAD, but every combination I did play with yielded similar results.

Was that chart from a Hornady manual?

 

[Ridgerunner665]

QuickLoad doesn't account for long throated (Remington, Weatherby) rifles...rifle powder burns slow enough that the bullet gets shoved into the rifling long before peak pressure is reached...that means the combustion chamber is bigger on a rifle with a long throat....that was Roy Weatherby's recipe for all of his Weatherby magnums....use the throat to increase powder capacity.

I don't really think rifle powders burn fast enough to cause any serious problems from bullets seated too deep...at least not large rifles (30-06, etc)...223 and such MIGHT be different, but I kinda doubt it.


Pistols, on the other hand, with their very fast burning powders....can become VERY dangerous VERY fast when bullets are seated too deep.

 

[woods]

Hey RiverRider

If you look at the 2nd graph you will notice that the pressure goes up when you get past .250" off the lands. Perhaps you are starting out with the initial seating length a distance from the lands and then adding .200" to it, then it would increase according to the chart.

But most of us reloaders work within kissing the lands to .250" off. Some long throated rifles like Weatherby's or RUM's have close to .250" long throat, but no one really wants to seat deeper than that and that usually fits in the magazines.

Pressure has a correlation to velocity (more pressure = more velocity) all other things being equal, so here is a typical seating depth test of mine. Starting at .015" off and seating deeper (away from the lands) at an additional .015" at a time. Note the progressively lower velocity (pressure?) when moving away from the lands

Now this is all in the range of the 2nd graph between against the lands and .250" off where pressure decreases progressively.

The link to Hornady is to find the quote I posted and look at their associated pics.

The graphs I have picked up here and there and have had for years.

Seating deeper = lower pressure is a general rule and there are anomalies and inconsistencies in isolated instances but is correct (generally).

Here is some more reading from a Barnes test

http://www.barnesbullets.com/resources/ ... -bullet-n/

*Note: beginning on the left with shot No. 1, distance is closest to the lands, and ends with shot No. 5 on the right, which was seated at the greatest distance from the lands. This applies to all charts.

Perhaps you should plug into Quickload an OAL that is fairly close to the lands and then vary it by .030" or so and see what it says

 

[runtohunt]

I agree with Riverrider. On my 7STW, I decreased the OAL from 3.370 to 3.360 with the same load of H1000 and suddenly experienced extractor marks when I had not had marks before. The first two shots were touching. The third shot was a 1/4" out but the bolt was sticky. I then looked over the three pieces of brass and noted the extractor marks on all three pieces .

 

[RiverRider]

Ridgerunner, that's not quite so. QuickLOAD's results are not influenced by proximity to the lands until you are basically *in there,* as in "engaged." As I stated before, pressure will decrease as OAL increases---this only stands to reason---until you begin to engage the lands. At that point, pressure will increase by about 7000 psi. I do believe that older data and information is simply wrong as so many other conclusions that were "once true."

Woods, QuickLOAD does not operate by entering any information regarding proximity to the lands. It simply predicts what the pressure would be with a given case capacity, bullet design, burn rate & charge, and COAL. The caveat is that once you engage the lands, expect an increase in pressur of 7200 psi with conventional jacketed bullets. The program does not operate on any assumptions as to throat dimensions whatsoever.

 

[RiverRider]

I agree with Riverrider. On my 7STW, I decreased the OAL from 3.370 to 3.360 with the same load of H1000 and suddenly experienced extractor marks when I had not had marks before. The first two shots were touching. The third shot was a 1/4" out but the bolt was sticky. I then looked over the three pieces of brass and noted the extractor marks on all three pieces .

Honestly, I think that if only .010" difference in OAL makes that much difference you're probably well over---or at least to close to the edge of safe pressures already. I would back off some, unless the signs could be explained as owing to some other factor.

 

[woods]

John Barsness (a well known gunwriter and expert)

http://www.24hourcampfire.com/ubbthread ... ut_Full_Le

Yep, seating bullets deeper in the case in rifles decreases peak pressure. You can even "see" the effect in a rifle through your chronograph. Start out with the bullet seated out as far as possible, then using the same powder charge, seat it progressivly deeper. Average velocities will drop--because peak pressure is dropping.

Compressing modern rifle powders has no effect on pressure.

Deeper bullet seating has the opposite effect in handguns, because of the very fast-burning powders--especially revolvers, because there's more bullet travel before the bullet hits the cylinder gap.
_________________________
John

Last paragraph of this Guns magazine article (it won't let me copy and paste)

http://fmgpublications.ipaperus.com/FMG ... 1/?page=10

The handy thing about seating bullets deeper is that if you start with bullets way out, any increase in seating depth reduces pressure. So if your rifle doesn't shoot all that well with bullets seated as close to the lands as possible try seating them a little deeper.

Some more Barnes

http://www.barnesbullets.com/resources/ ... -bullet-n/

Question: I’m trying to knock some sense into a hard-headed “armchair” ballistician regarding seating depth vs. pressure.

I maintain that seating a rifle bullet farther off the lands (giving the cartridge a shorter OAL) reduces pressure somewhat due to the bullet having a greater “run” before engaging the lands, even though the available powder space is reduced by the shorter OAL. True? (Thanks for a great product!)

-Joe Johnson

Answer: Yes, you are correct. Seating farther off the lands does reduce pressures. In addition to that, we have found that experimenting with seating in a range of .030″ to .100″ off the lands –sometimes even greater with TSX and MRX bullets– will normally improve accuracy.

Look at it this way: Put your fist against the wall and try to punch a hole in it. It’s going to take some serious pressure behind your fist to put a hole in the wall. Now back your fist up and give it a good swing. You’ll easily make a fist-size hole that will need repairing. This small amount of “jump” helps reduce the initial starting pressures. The practice of seating bullets off the riflings is especially critical when loading maximum charges.

Thanks—Ty

I can keep going

 

[RiverRider]

Well, everybody cannot be right. Either all these people have been relying on "the conventional wisdom" for decades and it has turned out to be wrong, or the folks who put QuickLOAD together are wrong.

I have a ton of my own data and I've looked through it. Some suggests one answer and some of it suggests the other. The problem with it is that my seating depth experiments typically don't cover a range of more than .050 inch, and also that I typically fire three or five shots of each depth. There are no answers in my own data for sure.

I think I will write to the folks at QuickLOAD. Maybe there is an explanation that would benefit all.

 

[woods]

I have read many posts where posters have said that QuickLoad indicated a lower pressure with small increases in seating depth within the normal range of load development (kiss-the-lands to .200" off). Can't find but one right now but Mikecr is a very knowledgeable reloader and his post is #9 here

http://www.longrangehunting.com/forums/ ... ndex2.html

Running it through QuickLoad, I predict that with as much as 25thou deeper seating(quite a bit), you could expect a 1Kpsi/45fps drop.
No reason to do that, but the point is pressure WOULD drop, even with a big adjustment deeper than you're at.

I don't run QuickLoad but it is my understanding that there are a lot of critical parameters and I have seen data from it that is counter-intuitive in the past.

 

[RiverRider]

I've spent the last hour reading the QuickLOAD User Manual. It is pretty informative and gets pretty technical, and comes right up to the point of addressing the question we're discussing here but does not quite get there. Aargghhhh.

A comment on "intuitive" understanding---it is generally over-rated. The nature of so many things we see and deal with every day just isn't really quite as we perceive!

I will email the folks at NECO. Maybe I'll get a response, maybe I won't...

As I've had some more time to digest, I have a few thoughts that might be relevant:

1. There is no question that the bullet resting against the lands will raise pressures and this is presumably because bullet movement is retarded or delayed by means of physical confinement.

2. The same effect, proportional to bullet weight and due to inertial confinement must certainly exist; in other words a heavier bullet will tend to resist movement.

3. All the stuff I just read about burn rates and such makes the whole picture even more complicated than I had always thought. For example, I had been told many years ago that smokeless powders have progressive burning characteristics. Not always true! Slow-burning rifle powders are generally progressive in burning characteristics, but the fast-burning (shotgun and pistol) powders may actually exhibit regressive burning characteristics.

4. The more I learn, the less I know. At times I wonder why I even bother...

 

[Antelope_Sniper]

I agree with Riverrider. On my 7STW, I decreased the OAL from 3.370 to 3.360 with the same load of H1000 and suddenly experienced extractor marks when I had not had marks before. The first two shots were touching. The third shot was a 1/4" out but the bolt was sticky. I then looked over the three pieces of brass and noted the extractor marks on all three pieces .

Is that a 7 Mag, or a 7 STW you are loading for.
3.360 is about right for a 7 MAG
COL for a 7 STW should be closer to 3.60

 

[RiverRider]

I got a response from NECO. Didn't expect it so soon.

Seating the bullet deeper in the case reduces volume available for a given amount of powder thus increasing pressure. Seating the bullet farther out increases the volume available for a given amount of powder thus reducung pressure. This is true until the bullet is seated against the lands. At this point the pressure spikes significantly. A normal load that approaches max pressure thus becomes a dangerous load.
Ed

I suspect that a lot of the old "conventional wisdom" originates from old experiments conducted using old instrumentation and techniques. That may be the source of some of the discrepancy.

It is well known and accepted that seating a bullet deeper than usual in a cartridge like the 9 x 19 can raise pressure significantly due to the reduced volume behind the bullet. One aspect of that scenario that is important is that a very small change in seating depth has a much greater effect on the volume behind the bullet, resulting in elevated pressure, than the same seating depth change on a larger cartridge. So, the pressure increase may be much less significant, but it should exist.

There may be some range of proximity to the lands (and I have no guess to offer) that causes the pressure increase in the way everyone agrees it does. In the data I generated and posted earlier using QuickLOAD, the OAL decreases that produced higher pressures are admittedly pretty drastic. It may well be that typically no one decreases OAL enough to produce a noticeable increase in pressure (as evidenced by a velocity increase).

I think it's pretty important to recognize that we are talking about two separate events. My search for higher resolution pressure traces came up short, so I assume the instrumentation does not exist that can illustrate exactly what goes on during the time between the primer strike and the bullet entering the leade.

I personally will never accept the idea that decreasing the volume behind the bullet does not raise pressures, but it's not so hard to accept the idea that most of us probably operate in the OAL ranges where the most significant effect of OAL on pressure is seen because of proximity of the bullet to the lands.

 

[Oldtrader3]

I think that it matters what type of ogive design on the bullet as well. Intuitively, secant type ogives will be different then radius based ogives and will give totally different pressure curves at different seating depths. Secant ogives are typically higher chamber pressure. Programs such as Quickload, are based on a particular bullet shape and therefore are only appropriate and accurate for that bullet profile.

Look at the Hornady (secant ogive) curve charts shown in post #5 versus the one for the .30-06 Springfield above it. This illustrates the effect of using different ogives on pressure curves because of different bullet shapes.

 

[RiverRider]

No doubt the shape of the bullet matters. A tangent ogive will engage the lands more gradually compared to the abrupt engagement that would result from the profile of a secant ogive design. I would imagine that it would be extremely difficult to quantify the difference though since we'd be trying to examine what happens during a very small particular segment of bullet displacement.

I wonder how much difference there would be between two bullets that were identical in every way except for one being a tangent ogive and the other being secant ogive profile. Intuition tells me the difference would be pretty darned insignificant...but intuition is often wrong!

 

[woods]

Well all I am saying is that if you take a 180 gr 30-06 AccuBond and seat 5 in each of these OAL's:

.020" off the lands
.040" off
.060" off
.080" off

etc

that you WILL get a progressively average velocity LOWER WITH INCREASED SEATING DEPTH (just like Barsness, the Guns magazine writer, Hornady, Ty Barnes, the people who made those graphs and I say). Does lower velocity indicate lower pressure. Well I have no way of verifying but IMO yes.

Does that totally negate what your QuickLoad or one guy at NECO says (what are his credentials?), ain't saying that. Just that you need ACTUAL data not opinions or calculations.

Do the seating depth your self. Ain't that much work