Hypothetically Speaking...

[painter]

The load manual oal is the 'shortest' safe load with that data.  That can leave what seems to be overwhelming options available.  That's why forums sometimes help with experienced suggestions.--or not  .

IMHO the statement in blue is not true, and here's why....

The ballistician chooses an OAL and bullet weight, then starts incremental loads. The OAL could be based on a popular gun, what the last ballistician did when the data was collected 10 years prior, or a number out of thin air. We don't know, and it really doesn't matter. What's important to him is the relationship between the powder, OAL and bullet. So the OAL listed in your loading manual is merely part of a report of what they did in the lab.

If you can remember back to high school science class when you put the powder in the beaker, heated the liquid to 104F and the liquid turned purple. You merely reported the volume of liquid, the weigh of the powder and the temperature of the visual change. You did not draw any conclusions as to what would happen with less liquid or more powder. You were not able to hypothesis what it would take to get a green color. You merely reported what steps you took and the results.

It's the same for the ballistician. He is merely reporting the results. He makes no claims about what would happen with a different OAL or if a different brand of bullet were used. It is entirely up to the reloader to make the recipe work for his bullet in his gun. The ballistician is in effect saying, "We did the following test and survived. If you follow this closely, then you will survive too."

So the OAL used in the load manual is not a promise of anything. It is not a guess at the "shortest safe load". It is merely a report of what they used in the lab and the results they achieved. It is entirely up to the reloader to interpret the results and apply the knowledge to their particular gun.

Novice reloaders should take the reported OAL and can use anything longer in complete safety. However, advanced reloaders can and do shorten OALs all the time.

 
[/quote]
As the title says this is all speculation, but I wanted to get some experienced reloaders ([cough] Wobbly, et al) opinions on tweaking OAL.

I'll begin by throwing some hypothetical numbers at you.

124 gr GDHP 5.2 gr Unique starting 5.8 gr max with an OAL of 1.06.

I perform a push test in my barrel like a good doobie and discover that this bullet needs to be loaded at 1.02 max in my gun.

I understand that by needing to go shorter the safety factor has disappeared.

How do you guys that have been doing this for a while, and still have all their digits and extremities, begin to develop a load for this barrel with those components?

Do you have a formula or a secret book or something?

[Wobbly]

Within short OAL changes you can proportionalize the load, but you MUST (absolutely MUST) begin at the new "starting load".

 5.2         X
-----  =  -----
1.060   1.020

So the new "starting load" becomes 5.0gr.  Just for safety, and since Unique does not display bad habits at low loads, you might even begin at 4.8 or 4.9gr.

This cartoon comes close to explaining....



? This is one case where a chrono becomes invaluable. If you can shoot the standard "starting load" in another gun, then you can easily correlate your new starting load "guesstimate" with velocity readings.

? Then, as always, you'd work up in incremental steps. For this load, maybe 0.1gr steps would be safer to start.

? Since the old Max Load was +0.6gr, the new max load becomes 4.9 + 0.6 or 5.5gr.

 

[painter]

Now I know why you get the big bucks Wobbly.

Your cartoon was fine BTW.

Thanks for a really clear concise explanation.

One more question...

Within short OAL changes you can proportionalize the load, but you MUST (absolutely MUST) begin at the new "starting load".

<snip>
 

Define, or characterize if you will, what you consider a short or small OAL change.

[armoredman]

It just shows I really need a chronograph...

[painter]

It just shows I really need a chronograph...

...and I told my wife I was going to 'save' money by reloading.

[armoredman]

Ah, I didn't save money, really, until I started casting. Now THAT'S some savings!

[painter]

Ah, I didn't save money, really, until I started casting. Now THAT'S some savings!

I used the reloading calculator that Cheezewhiz posted the link to.

I can pay for my reloading equipment in the first 2700 rounds.

It's actually less than that because the set up included bullets/powder/primers/casings along with the press.

I don't think I'd get a chrono past her after I picked up a tumbler too.

It's fine for the amount I shoot.

[1SOW]

Wobbly has a good method,  but I'm sure he  doesn't extend this to anything beyond small changes.  If the shorter oal leaves no open space in the case, or maybe even compresses the powder, ALL BETS ARE OFF as to what the pressures will be with any given powder.   

[Wobbly]

Define, or characterize if you will, what you consider a short or small OAL change.

That would depend upon cartridge and the load.

? In low pressure cartridges like the 45ACP bigger changes are permissible. Conversely, with tiny cartridges like 9x19 it doesn't take much to make a huge change. IMHO .040" change in a 9x19 is a substantial amount. That's why I suggested a further reduction in the calculated load and insisted on the "starting load".

? Here I consider that "the load" consists of a type of powder and an amount of powder.....

? So you'd be crazy to try something like this with Tite Group, Bullseye or Red Dot because faster powders get erratic very quickly. And some fast powders misbehave when you under-load them as well as when they you over-load them. Like being with a person who is nice one moment and then jumps up and threatens you the next. Luckily, Unique is not in that group and is a moderate speed powder, and therefore well-behaved.

? You got to realize that the pressure curve is not straight, nor is the ratio between chamber pressure and bullet velocity always 1:1. There comes a point where the pressure increases wildly, but the bullet doesn't go any faster. (In other words, your chrono will lie to you.) So it's important for your safety that the first load (your "starting load") land at the lower end of the 1:1 region. That's the only region to safely load and shoot in.


So what a chrono tells you is where you landed on the curve. If you first tried 4.8gr and the chrono reported a mid-range bullet speed, you'd know that 1) further reductions could be made, and 2) the max load is not as high as first assumed.

 

[painter]

Thanks Wobbly.

I don't really plan on doing any of this...

inquiring mind is all.

[Wobbly]

I've replaced the graphic above with a much more appropriate one.



The chamber pressure, plotted over the entire load range of any given powder and bullet is a curve. The exact shape of the curve is different for every bullet, powder, gun, and load. But if we simply consider the "starting load" for a single bullet and powder in 1 gun, and we zoom way in, the curve starts to look almost straight. How far you have to zoom in (1000x or 10x) depends again on many factors, but all curves are made up of tiny straight lines at some level of magnification.

So if your changes are "small", the proportion allows you to mathematically move up/down that straight line and remain at the same pressure as the load given in the manual. The object is to change the contributing factors of OAL and Load but achieve the same chamber pressure. You do not want to land in a zone too much below the red pressure line, because that would represent an increase in pressure. If anything, you want to land above the red line and end up with a lower pressure. You can see this by lowering the powder load and noting where the green New Load line intersects the New OAL line.

This is how 3 "knowns" can be used to solve for the forth "unknown".

 

[armoredman]

Hypothetically speaking, Wobbly is a cheerleader for the Dallas Cowboys...


Nice graphics, easy to read/understand.

[Wobbly]

Here's one I'm happy with. Consider the last graphic as a zoomed in view of just the portion of the curve near the "starting load". This one shows the "larger picture" of the whole load range.

You can see that between Max and Starting there is an area of predictable behavior (the green zone). You are free to go up and down in that zone all day long. Within that zone, bullet velocity and chamber pressure are related to each other. You can use a chrono to measure the bullet speed and thereby cheaply discern an accurate chamber pressure.

The pressure graph inside the safe zone is a curve, but a very gentle one. So within that area, and especially at the Starting Load, you can get reasonable "guesstimates" using proportional relationships. In truth, the safe zone is probably more pear shaped, so that at the lower end it's larger.

Once you get above the max load, the relationship stops. Chamber pressure builds much faster while bullet speed remains nearly the same. This is why guns blow up; adding 0.2gr at the bottom of the load range does not have the same effect as adding 0.2gr at the top end. The higher you get in the load range, the less powder it takes to make a big difference.

I think we beat this to death. Test on Monday.

 

[1SOW]

          --------
 

[painter]

<snip>
I think we beat this to death. Test on Monday.

 

We? 

Thanks again.