## NOW I'm confused about OAL ##

[rsplante]

I thought I had a good handle on OAL. I thought I was doing all the right things. NOPE! What I have been doing until now is pulling out my handy-dandy Hornady OAL gauge (straight one so far) Using one of the bullets I'm about to load, I make three measurements, lightly tapping the end of the plastic rod with one finger. I got three measurements within 0.002" measuring from ogive to the base of the modified brass I bought from Hornady. Even writing it down, it seems so obvious to me. Why did I miss it before? I was relying on the Hornady case having the exact same shoulder to head length as what I am shooting.

That begs the question, do I want to compare the Hornady case to one I have fired or one that I have full length sized. Obviously again, the one I have fired should be the same size as my chamber length, right? But when I went to measure brass fired in my 5.56 Larue OBR, lo and behold one of them was 0,003" longer than most of them. But how can the average fire formed length be shorter than any single case? Are they getting work hardened after my single firing? or might one firing not be enough to fire form them? (At this point you are probably wondering why he is worried about OAL in a gas gun? That's because I'm loading single round for 200yd prone competition, not magazine length.)

Hold the presses, this bothered me so much while writing that I remeasured the cases and subjected the errant case to further scrutiny. Turns out, the extractor had set up a tiny divot. I filed that flush and voila, it now measures the same as the others. One problem solved (I actually recorded this process here, instead of burying my initial error by just deleting this part, in case it can help others.)

I'm still a bit confused though in that my fireformed cases are 0.004" longer than the Hornady case I am using for measuring my chamber. The Hornady method relies on a shoulder to ogive measurement in the chamber, and then an ogive to head measurement out of the rifle to produce an OAL; however, it seems to be dependent on the THEIR shoulder to head measurement. My head is starting to hurt. Then it occurrs to me, does it really matter what my jump is as long as I can reproduce what works in my rifle? i.e. is it really just a relative measurement (as long as everything else stays the same: same amount of shoulder bump, same bullet, same brass when loading?)

 

[rsplante]

As I continue to think about this, two more thoughts occurred to me. One situation where the ACTUAL measurement is important is the condition where I think I am near or at the lands, but might actually be jamming any at all and could cause an overpressure situation. Then, deciding to do the actual calculations to be safe, I realized that maybe I was wrong to pick the fireformed case which represents the chamber size, since in setting up my press, I will be measuring full length sized brass. This GD&T stuff is why I'm not an ME.

 

[Mike_in_FL]

I'm no expert but I will give you my opinion as I own the tool as well. I'm not sure where Hornady sets their modified case at, SAAMI, min., max., or some mid-point. What I think you're missing is that using the tool, you're pushing the shoulder of the case into your chamber. Therefore I choose to think of it in these terms, the datum is in the same place (regardless of case length) so the measurement to the lands is accurate based upon the bullet you're using.

If you read anything about what happens after you pull the trigger, the first step is generally that the case if pushed forward by the firing pin until the shoulder contacts the end of the chamber. How much "play" is in the chamber depends upon how much you're pushing back that shoulder when you FL resize or resizing in general. I was pretty surprised when I measured my cases using an RCBS FL die in .308.

Do you have BOTH tools from Hornady? I know you say you have the straight OAL gauge, but that doesn't measure the headspace of the chamber based upon fired cases and how far you need to push back your shoulder.

I also think trying to compare your fired brass to Hornady's case is a mistake. If you want to compare it, size your brass first if you're this concerned. I think it's a non-issue.

If you want what you think is going to give you the best accuracy just bump the shoulder of your fired brass back say .003 or .004 from your actual headspace measurement.

As far as the jam question, this is my understanding. If you chose to develop a load with the bullet jammed, you only go up (obviously) to where you still have a safe load, no pressure signs. As the throat of the rifle wears, you create a jump, which reduces pressure. In what you're describing and comparing to Hornady's case, the only way I can see you having a problem is if you let the neck get too long and didn't trim it.

 

[rsplante]

I'm no expert but I will give you my opinion as I own the tool as well. I'm not sure where Hornady sets their modified case at, SAAMI, min., max., or some mid-point. What I think you're missing is that using the tool, you're pushing the shoulder of the case into your chamber. Therefore I choose to think of it in these terms, the datum is in the same place (regardless of case length) so the measurement to the lands is accurate based upon the bullet you're using.

If you read anything about what happens after you pull the trigger, the first step is generally that the case if pushed forward by the firing pin until the shoulder contacts the end of the chamber. How much "play" is in the chamber depends upon how much you're pushing back that shoulder when you FL resize or resizing in general. I was pretty surprised when I measured my cases using an RCBS FL die in .308.

Do you have BOTH tools from Hornady? I know you say you have the straight OAL gauge, but that doesn't measure the headspace of the chamber based upon fired cases and how far you need to push back your shoulder.

I also think trying to compare your fired brass to Hornady's case is a mistake. If you want to compare it, size your brass first if you're this concerned. I think it's a non-issue.

If you want what you think is going to give you the best accuracy just bump the shoulder of your fired brass back say .003 or .004 from your actual headspace measurement.

As far as the jam question, this is my understanding. If you chose to develop a load with the bullet jammed, you only go up (obviously) to where you still have a safe load, no pressure signs. As the throat of the rifle wears, you create a jump, which reduces pressure. In what you're describing and comparing to Hornady's case, the only way I can see you having a problem is if you let the neck get too long and didn't trim it.

Yeah, I guess I have my die set wrong, I just checked and found that I am bumping .005"-.006". Once I fix that, I guess I should be pretty much lined up with the Hornady case and this issue will basically go away. As far as jamming goes, I understand if I had done it correctly. A few weeks ago, I asked on this site whether I should adjust charge or OAL first. Unfortunately the concensus was charge. Well, the drawback to that is that my best charge came in hot, at 25gr of Varget. Now I am afraid to take a chance of setting the OAL longer as I could inadvertantly jam the bullet possibly resulting in a KaBoom. So that answers my previous question. If you think you are close to jamming the bullet, you should set OAL first, THEN charge.

 

[rsplante]

I'm no expert but I will give you my opinion as I own the tool as well. I'm not sure where Hornady sets their modified case at, SAAMI, min., max., or some mid-point. What I think you're missing is that using the tool, you're pushing the shoulder of the case into your chamber. Therefore I choose to think of it in these terms, the datum is in the same place (regardless of case length) so the measurement to the lands is accurate based upon the bullet you're using.

.

That was what I thought before, but there is a potential error built into this method. Like you, I thought I was getting an accurate measurement since it is measuring the difference between where the shoulder hits the chamber and the ogive hits the chamber. That part is correct, but what you may be forgetting, as did I, is that the Hornady does not perform any "measurement", it merely represents parts of the chamber which you can in turn measure. But, you have to remember that while the relationship between the bullet and the shoulder is established in the chamber, when you take it out to measure that relationship, where are you applying your caliper? You are measuring between the bullet ogive and the head of Hornady's modified case (which probably is not the same as your case). Now comes the tricky part. You just measured the distance between the Ogive of your bullet and the head of Hornady's case. When you measure your completed round and set the ogive to head distance to the same distance as you measured, if you have a different shoulder length, that means the distance between your ogive and your shoulder will be different than that relationship in your chamber. In a worst case scenario, when the round is pushed forward as you say, if that distance is different than you thought, by the time the shoulder makes contact with the chamber, it is possible to jam the bullet into the lands. If you are running a max load, you could overpressure the round possibly resulting in a KaBoom. This stuff can get complicated fast. One of the surprisingly difficult concepts in Mechanical Engineering (at least based on the differences in opinion on the subject between Mechanical Engineers) is the subject of Geometric Dimensioning and Tolerencing or GD&T. I'm a Civil Engineer, as as I have been told many times, Mechanical Engineers make weapons, and Civil Engineers make targets.

 

[Sniper Uncle]

Hornady makes an OAL Measurement Tool. You take the OAL Tool, which you have, and measure five bullets, to get the average MAXIMUM COAL. Then, you use your sized cases, and determine an Ogive to Case Head measurement. Your Hornady OAL Tool is only measuring the chamber relative to the bullet and case chosen. Then, you use cases and a bullet comparator to get your ogive measurement for your reloading. Does that make sense, how I put it?

 

[rsplante]

Tim, I will try a different way to explain it. The Hornady OAL tool does not measure anything. It is sort of like making a mold of a footprint. It produces a negative of the chamber so that you can get to make your measurements. The problem with the system, or should I say an added variable, is that while the idea of letting the shoulder of the Hornady modified case hit the end of the straight part of the chamber and the ogive hit the start of the lands will accurately reproduce that physical relationship, THAT is not what you actually measure with your calipers. If there was a way to actually measure that relationship directly, we'd be golden.

The issue is that when you make the measurement, you apply the calipers (with the Hornady LNL bullet comparator attached) to the Ogive of the bullet, and the Head of the Hornady modified case, NOT to the Shoulder of the case. Therefore, you are no longer directly measuring the relationship between the Ogive and the Shoulder of the case. When you measure from your own case head to the ogive you are ignoring the fact that something important has changed, the difference between the head to shoulder length of the two different cases. I agree that the Ogive to Head distance will be constant, but that is not what you are really concerned with in my understanding.

The case has the shoulder bumped back enough to make it possible to close the bolt. The OAL tool has allowed you to reproduce the measured distance between the ogive and the head (for two different cases) and in the process thrown out the measurement between the ogive and the shoulder of the case. Now here is where it gets a little fuzzy for me. Someone has mentioned that the firing pin drives the round forward until the shoulder stops. Unless you are intentionally jamming the bullet as with VLDs, you are trying to avoid having the bullet make hard contact with the lands. Apparently this causes a pressure spike in the short period of time before the bullet can overcome the added resistance of the lands. If you have already loaded a hot round, and stopped just short of obvious pressure signs, this added pressure spike can be enough to cause a catastrophic failure of the round, AKA a KaBoom, which can, in addition to ruining a perfectly good rifle, cause the loss of fingers, eyes, and possibly but not likely, serious death.

 

[elfster1234]

You might be over thinking this just a tad but it is a valid point.

Put it this way.

I just measured my hornady .308 modified case with my hornady headspace gauge and it shows a headspace measurement of 1.624...... Which i think is sami min spec (correct me if im wrong).... Regardless... This would just happen to be the very same datum headspace measurement with the hornady headspace gauge on some factory .308 rounds that i have.

Now, with that being said fire formed brass being ejected out of my semi auto rifle with a 7.62x51 chamber is always 1.631, and i like to do a bump back of .004 for a finished bumped back headspace measurement of 1.627....... This would mean my brass of 1.627 would be .003 longer (mostly due to having a 7.62chamber in comparison to a .308chamber) than the hornady modified case as described above.

This might be of some concern if i seated my bullets to touch the lands, but i usually shoot for a .020 bullet jump anyway. Weather or not it is a true .020 or .017 bullet jump or not, as long as what "i think is correct" in this situation shoots well or not i can make adjustments up or down so it really is a moot issue. Just as long as when i find a OGIVE seating depth that shoots well in my rifle that i can repeat that process for future reloads.

About the only time id be concerned about this is if i was touching the lands, which i never do with a semi auto rifle, but id do it with a bolt action no problem if i wanted that option.... If i was touching the lands with a bolt action id make a dummy test round anyway to see how it felt when chambered regardless.

I hope this helps.

 

[elfster1234]

Btw..... Big difference between seating bullets by OGIVE and OAL....... Yes, when seating by OGIVE you can figure out your over all length once the finished round is complete.

 

[Sniper Uncle]

I guess where I come out is that the actual COAL means nothing to me. All I really care about is the Base to Ogive measurement, to be sure that the bullet is not touching or jammed in the lands. I use the Hornady tool to figure max case head to ogive measurement, and then set the dies to match what I want my finished product head to ogive measurement to be. That is what I do....

 

[elfster1234]

this is spot on

I guess where I come out is that the actual COAL means nothing to me. All I really care about is the Base to Ogive measurement, to be sure that the bullet is not touching or jammed in the lands. I use the Hornady tool to figure max case head to ogive measurement, and then set the dies to match what I want my finished product head to ogive measurement to be. That is what I do....

 

[rsplante]

You might be over thinking this just a tad but it is a valid point...

This might be of some concern if i seated my bullets to touch the lands, but i usually shoot for a .020 bullet jump anyway. Weather or not it is a true .020 or .017 bullet jump or not, as long as what "i think is correct" in this situation shoots well or not i can make adjustments up or down so it really is a moot issue. Just as long as when i find a OGIVE seating depth that shoots well in my rifle that i can repeat that process for future reloads.

About the only time id be concerned about this is if i was touching the lands, which i never do with a semi auto rifle, but id do it with a bolt action no problem if i wanted that option.... If i was touching the lands with a bolt action id make a dummy test round anyway to see how it felt when chambered regardless.

I hope this helps.

We are on the same page. I have mentioned before (maybe a different thread) that the bottom line is that it is usually a RELATIVE measurement. Most times you do not need to determine the ACTUAL measurement, you just want to be able to reproduce what works for your rifle. The exception to that is if you are jamming your bullets AND running a hot load. While I am shooting a gas gun, I have optimized my handloads for what I am currently competing in, 200yd prone. They require us to shoot single round, so there is no need for me to load magazine length. I just gave up on 80gr JLK VLD bullets because they won't shoot in my 1:8 barrel; however, I am laboring under the assumption that VLDs like to be jammed. Now that I am back to 77gr SMKs, I am allowing a little bit of jump. This past weekend, those that shot the best were with a .03" jump. (I actually got an SD of 7 as measured with an Oehler 35P!!)

That raises a new question. I just installed a David Tubb CWS AR-15 Carrier Weight System - AR 15 PRODUCTS by Superior Shooting Systems and am wondering if I need to start over in my load development. The CWS adds 4.05 oz of weight to the bolt carrier group. Not sure I completely understand the internal ballistics of delaying the bolt release, but I cannot logically argue with an 11 time Camp Perry Champ. Only thing is that the guy I talked to at David Tubb's shop had no idea if it would have an effect on my load development. Oh well, back to the drawing board I guess.