Seating Depth

[nicky]

I know to expect variations in OAL when seating a bullet off of the ojive.

Since we cannot measure the the seating depth, but figure that if the ojive is more consistent then the nose of the bullet, what variations you were getting in OAL.

[Wobbly]

Can you clarify, please ? Rifle or pistol ?

And can you doctor that last sentence.

[nicky]

I'm sorry. Pistol

[Wobbly]

I'm still not exactly following. Maybe it the virus

• When we do the CZ Forum 'push test' we don't care if the obstruction that pushes the bullet back is the shoulder or the ogive. As long as we set back 0.015" further than that, we should be OK with all the clearances.

• We do know the seating depth. We can't see it or measure it directly, but we can calculate it. And that's as good as being there. (Case Length + Bullet Length) - OAL is a formula that gives you the seating depth.

• Even if our Seating Die is pushing the bullet into the case by the ogive, we're still using the OAL measurement. The CZ Forum 'push test' started with an OAL measurement, and we need to follow through with using that system of measurement.

• On bullets like the Precision Delta 124gr JHP, where there's no where else to push except the ogive, there is some small variation in the finished cartridge OAL that's more than (say for instance) a Flat Point bullet might end up measuring. Some is due to different amounts of lead exposed at the mouth of the hollow point's copper cup, some is due to seating by the ogive... but the numbers are so small it doesn't really matter. If I had to guess, I'd say my FP cartridges vary by 0.005" and my PD cartridges vary by 0.007".

When I say it doesn't matter, some of that is because I'm shooting mixed brass. And the brass makes a bigger difference in performance that .002 or .003" in OAL. But things may be may be different for you.

Does any of that make sense

[nicky]

That makes a lot of sense.

I do know how to figure the seating depth and I now see that the question I asked I should have figured out myself. With the bullet nose not being consistent, there is no way to account for how much the variation the OAL would be.

Thanks

[DeltaGray]

It seems that Wobbly makes sense when he speaks (writes informative comments).  New or experienced ammo loaders hopefully are careful of pressure levels based on seating depths and maximum powder charges based on printed sources of pressure guesses and Internet posts. Case ruptures can be dangerous based on what I have seen.  My intent is for people to be safe and careful. Your reloads may not be safe if used by a friend or stranger in their own equipment.

[Wobbly]

That makes a lot of sense.

I do know how to figure the seating depth and I now see that the question I asked I should have figured out myself. With the bullet nose not being consistent, there is no way to account for how much the variation the OAL would be.

It's OK to ask for help. That's hopefully what this forum is all about. This, even if the answer seems painfully obvious once explained. The truth is, once explained, everything is painfully obvious !! It's not the technical part that keeps us from seeing the truth, more than often than not it's our own assumptions that form the biggest barriers.

I was reading last night about the experiments the French did in the late 1700's with "hot air" balloons. They were convinced that smoke was a necessary component of the balloon's contents in order to get airborne. We can chuckle at that now, but the greatest minds in science of that day were convinced.

Now in all truth, I didn't ever fully understand the question... but I genuinely wanted to help. So I used sort of a shotgun approach to answer, hoping that one of those statements (I can't call them answers) at least touched on a tiny corner of the issue. This was not to belittle anyone or make the question smaller... or anything even close to that. In my own ignorance, I was simply hoping to dance around the subject matter long enough to get to a better explanation. 

Those that understand me will know that I don't "think though" problems as much as I "write though" them. So as I finally came to understand the question, I thought the insight behind it was excellent. And quite frankly, one I had never actually considered.

So thank you !

 

[nicky]

And I thank you for being the guy that helps this to be the best place for those of us handloaders.

[newageroman]

I think what he is asking is what is the variance of COAL(bullet nose) when we seat using the Ogive.
So say we load using the ogive(not flat points), plunk test and such. Come up with COAL for the round and all that.
What is the variance of COAL (from nose of bullet) across 10 bullets or so...

For me, from memory, it is maybe .0005 or so. I do know that when I pull my pre-set 650 loading heads down to make a run of the same bullet (the dies are GTG), I do double check COAL and if its off more than that I'll recheck/readjust stuff.

What would be really neat is to have a measurement tool that fits on calipers and would measure the COAL off the O-give vs nose, kind of like the Rifle Case comparators do to measure the shoulder of the case. If we are seating with Ogive, why aren't we measuring with ogive?

Lets say there is one bullet that has a short nose and higher ogive. You measure it, Coal is right on. It goes into the "good" pile. Woops, might be an issue. If you plunk your ammo, you would know though.

So when we plunk, we can see if it passes or not, but I can't tell the variance of ones that don't pass by eyesight alone. I mean you can tell if they spin or not, but if not, how many .0000 are they too long? You could measure to bullet nose, then set it further and further each time till it spins and then recheck the bullet nose, that would give you variance, but I would hate to do that just for testing/data purposes. The only other think I can think of is plunking ammo in the barrel and using calipers to measure from base of case to some known/repeatable place on the barrel (like the edge of the first lug). I just don't know if that is possible, or easily repeatable though. It would also be nice to know the variance in Ogive vs bullet nose across some of the major bullets we use too.

[tdogg]

What you need to know is the base to ogive distribution and the weight distribution for your bullet.  You can then remove the ones that are anomalies and sort the rest into different buckets of weight and length for different reloading sessions/load development.  Then you would need to weigh each charge and make sure the runout is zero for each finished cartridge.  Finally you would need to use a ransom rest to shoot them to be able to determine the accuracy.

If this all seems like a lot to do for pistol shooting, you're right it's overkill.   I'd argue that understanding the bullet weight distribution is valuable when starting a new bullet.   It will help make sure your Power Factor calculation is sound (use the lightest bullet weight if you wan't to be conservative but the mean it's acceptable in most cases).

Seating off the ogive vs the flat point is really a bullet shape decision.  Some bullets have small flat points and are better to be seated off the ogive.   Others you may be better off seating off the flat point.

Knowing your seating depth is important, especially if you know you're seating deep into the case.  Same goes for OAL, especially if you know you're seating long or short.

All these bullet shape variables are part of the reason for adding the fudge factor (typically 0.015in) when determining your load OAL.  Your case type/mix and ability to pull the handle consistently also introduce variability to the cartridge length.      In the end you want to find the LOAD that can handle all this variability and still shoot the center of the target repeatedly.

Cheers,
Toby

Sent from my SM-G950U using Tapatalk

[Duke Nukem]

When setting up a cartridge with a new bullet, I've read about arriving at an OAL using the push test and subtracting a safety factor.  For an example, the data I used gave a minimum OAL for 124gr. FMJ of 1.120.  We have noted that different brands/shapes of bullets are different lengths, meaning that for a given OAL the seating depth is varying, with will have an effect on pressure.  I recall there being an ideal seating depth with 9mm cartridges of .250.  With the bullets I'm using and the minimum OAL the powder manufacturer cites, I can't achieve a seating depth that great.

Is there an order of importance to the dimensions we work with?  Do I need a chronograph to tell me what I need to know?

[painter]

When setting up a cartridge with a new bullet, I've read about arriving at an OAL using the push test and subtracting a safety factor.  For an example, the data I used gave a minimum OAL for 124gr. FMJ of 1.120.  We have noted that different brands/shapes of bullets are different lengths, meaning that for a given OAL the seating depth is varying, with will have an effect on pressure.  I recall there being an ideal seating depth with 9mm cartridges of .250.  With the bullets I'm using and the minimum OAL the powder manufacturer cites, I can't achieve a seating depth that great.

Is there an order of importance to the dimensions we work with?  Do I need a chronograph to tell me what I need to know?

There is an order of importance.

Starting load is the most important.

If you need to vary significantly from the manufacturers test OAL then a chrono is the only way to know how close you are to the max velocity at starting load. If you only need to vary by a small amount, I just reduce max load by a tenth and call it good.

Wobbly will be by to disagree.

[skin]

 If you want to measure the base to ogive, why not use a sized and expanded case? You are going to be. 002 higher on the ogive, but....

[Wobbly]

If you want to measure the base to ogive, why not use a sized and expanded case? You are going to be. 002 higher on the ogive, but....

Because when you use a Sized case, the case mouth ID is made 0.002 to 0.004" smaller than the bullet. Therefore, the bullet will not "telescope" into the case without using immense pressure. When you set this combination into your press to make the OAL collapse, you'll loose the "feel" for all the small detail of the situation.

However, when the bullet is a "slip fit" into the case, you'll be able to "feel" the instant the bullet strikes the first obstruction.

Hope this helps. 

[Wobbly]

Is there an order of importance to the dimensions we work with?  Do I need a chronograph to tell me what I need to know?

There is an order of importance.

Starting load is the most important.

If you need to vary significantly from the manufacturers test OAL then a chrono is the only way to know how close you are to the max velocity at starting load. If you only need to vary by a small amount, I just reduce max load by a tenth and call it good.

Wobbly will be by to disagree.

• We currently believe that there is some accuracy to be gained if the bullet is seated 0.200 to 0.250" into the case. That is a goal, which we try to achieve when we have the option of a 0.280" Max to Min OAL range that we can use. But that doesn't always happen. You know, FREE added accuracy is a pretty good thing.

• As Painter pointed out, not going below (shorter than) the published load's OAL is a matter of personal safety. And your personal safety is paramount. So that step always takes first place. This is extremely important with 9, 40, and other high pressure cartridges that operate in the 30,000 psi chamber pressure range, because simply seating 0.010" deeper can put you off the pressure chart.

Now, having said all that... it's not the end of the world. Perhaps we can help you find another published load which has a shorter OAL figure, which might help you satisfy both points.

There are simply several ways to "skin this cat".