QuickLoad vs Chrono'd Velocities; Effect of Shot Start (Initialization) Pressure

[South Pender]

I'm returning to a question I raised over a year ago on this forum--but with a slight change. I've been experimenting with QuickLoad in connection with my .270 Win. and have found big differences in estimated pressures and muzzle velocities when the Shot Start (Initialization) Pressure is set at the default value of 3625 as opposed to when it is set to account for bullets seated close to the lands. The QL instructions say that you should add 7200 to the 3625 (i.e., use the value 10,825) for bullets seated to actually touch the lands. Then subtract 29 for each .001” seated off the lands. So as an example, with my .270 Win with bullets seated with a .030” jump (.030” off the lands), I would subtract 870 (29 x 30) from the 10,825 for a value of 9955 as the Shot Start (Initialization) Pressure. I should note that there are some guys who suggest that you should use the default value of 3625 for any jump. When I run QL for my .270 Win., I get very different pressure values and velocities when I use the 3625 value as opposed to when I use the adjusted value of 9955.

So here's my question: Do you guys who have been using QL and chronographing your loads find that you get the best agreement between what QL says and what your chronograph says

(a) when using the default value (3625) for the Shot Start Pressure or

(b) when you adjust that for closeness to the lands, in which case the Shot Start Pressure you enter will be much greater than 3625 as shown above?

 

[rebeltex]

I believe you have that backwards.
Shot start pressure should be lower on the load with a jump and higher with the bullet into the lands.

 

[Murray Brook]

3626 psi plus 7200 psi when touching lands . Minus 145 psi for every .005 off the lands. Seems to give good results.

 

[Jager]

Here's the hover help bubble for Shot Start Initiation...




Like rebel and Murray note... you're going to want to leave Shot Start Initiation at its default of 3625 unless you're clearly into the lands, or are using a non conventional bullet.

 

[South Pender]

I believe you have that backwards.
Shot start pressure should be lower on the load with a jump and higher with the bullet into the lands.

I probably wasn't clear enough in my post. I do have it lower with a jump. With a .030" jump, the Shot Start Pressure is 9956, whereas with the bullet touching the lands it would be 10,826 (3625 + 7200). However, this 9956 is far higher than the QL default value of 3625 (which presumably would correspond to a bullet seated about .25" off the lands). The chart in Jager's post shows the adjustments.

What appears to be the generally agreed-upon adjustment for various jumps is to subtract 29 for each .001" of jump. So with a jump of .03" as in my example, we would subtract 30 x 29 or 870 from the touching value of 10826 or 9956 as noted above.

Put this way, my question is which of the Shot Start Pressure values--3625 or 9956--will agree best with chronographed velocities.

 

[South Pender]

3626 psi plus 7200 psi when touching lands . Minus 145 psi for every .001 off the lands. Seems to give good results.

If I'm not mistaken the 145 is for every .005 off the lands, not every .001. (This corresponds to 29 for every .001 off the lands.) At least that's how I've seen it in the past.

 

[Murray Brook]

Yes . Thank you for pointing that out. I believe you are right. I will edit my post so as to not steer others wrong.

 

[South Pender]

I've rewritten my opening post in the hopes of making the issue clearer.

 

[rebeltex]

I believe the 9956 will be closer to agreeing but there are other factors at play in initial pressure such as differing amounts of bearing surface between different styles of bullets, bullet fit to bore size etc… but I think you are on the right track.

 

[RegionRat]

If you are playing with ladder testing, and you have an array of charges and velocities, I adjust the values to match the velocities and then examine the pressure. Initial pressure is one parameter, but you also have the powder properties to use as well.

On occasions when I have strain gage data, I often don't see a huge increase in pressure from jam compared to what others have reported. Is this due to the bullets, the reamers, etc? I guess I don't sweat it.

 

[C_Ackley]

Does QL have anything to say about engraving pressure of Soft and Hard Cast Lead?
I would think even high BHN lead would still be lower than jacketed.

 

[RegionRat]

Does QL have anything to say about engraving pressure of Soft and Hard Cast Lead?
I would think even high BHN lead would still be lower than jacketed.

Yes, as in Jager's post #4, they show a little difference between jacketed and lead without splitting hairs on hardness types, but it doesn't amount to a large share of the total pressure. 2175 for jacketed versus 1160 for lead.

This is a modeling method and isn't an attempt to mimic the actual physics as much as it is a way to match results.

 

[mikecr]

I've never heard of a linear adjustment to it (like 29psi per 1thou clearance).
But I am sure it's not linear.
With pretty much any jump, bullet's gain enough momentum alone to cover engraving forces.
There is still forward travel rate interruption, which raises pressure for an instant, and the default 3625 seems reasonable to cover that.

Since I choose off the land seating, I leave starting pressure at default for normal jacketed bullets.
Have not had a challenge calibrating QL for my results, that starting pressure would fix.

 

[Laurie]

I've never heard of a linear adjustment to it (like 29psi per 1thou clearance).
But I am sure it's not linear.
With pretty much any jump, bullet's gain enough momentum alone to cover engraving forces.
There is still forward travel rate interruption, which raises pressure for an instant, and the default 3625 seems reasonable to cover that.

That's how I've always read it too. back in the 80s when Viht powders first appeared in the UK, the company had a large free reloading guide that folded multiple times into a small booklet and had all sorts of interesting information as well as loads data. One little 'box' was called something on the lines of 'Seating Depth Pressure Effects' and gave pressure barrel results for a Lapua 7.62X51mm ball round load with the bullet seated progressively deeper. For the first three or four steps, there were no MV or maximum pressure changes, then when it had become really substantial, both metrics experienced a considerable drop. I lost that document and have never seen it repeated experimentally elsewhere. It is of course 180-degrees out from what most handloaders believe and which QuickLOAD has actually reinforced because of the COAL - PMax link which as I understand it has nothing to do with PMax per se, but with chambers / freebore and the COALs that they determine. (For example, QL will show a 2.26" COAL 223 loaded to magazine length as fired in a Wylde or 5.56 chamber as far higher pressure than when the same load is fired in the same chamber but has the bullet seated further out to be say 15 thou' short of the lands and single-loaded into the rifle. The chronograph shows no noticeable difference IME in such cases.)

I can understand why companies shy well clear of this issue for two reasons - a general desire to avoid people who don't know what they're doing 'playing around' with some of these factors; secondly, whilst this is the effect in rifles and standard rifle type loads/ propellants, the opposite effect does apply to many small capacity pistol cartridges like 9mmP, ie seat the bullet more deeply below the standard position and pressures rise, potentially to dangerous levels. So, a single general rule can be not only incorrect but potentially dangerously so.

When I've seated bullets mildly into the lands to the point where the tiniest bit of resistance is barely felt on bolt closure with the bolt's firing assembly removed, my experience of applying QL's full short start pressure additions is that the resulting MVs are way below the model's predictions, hence pressures are presumably so too. I usually end up increasing charges to where I would have been if I had left the short-start pressure setting at the default level. As a compromise, my first runs with a new untried combination will be modelled with a modest short-start increase in such cases, but nowhere near as high as 10,000 psi plus.

 

[cdgaydos]

Not claiming to KNOW the answer… I have not changed the shot start pressure unless looking at lead or monolithic or jamming… I’m sure there is some minimal “jump” where less would result in the substantial increase in pressures suggested in QL. I assume that beyond that minimal jump, case capacity would dominate the equation.

I have had decent success ensuring my fired case capacity was correct… and adjusting Ba to achieve results matching my chrono….

Perhaps I should consider shot start as a variable to tune the model… there is no reason to believe that shot start pressure would be a constant for all bullet, chamber, seating depth, power (you get the idea) combinations…

 

[jepp2]

So here's my question: Do you guys who have been using QL and chronographing your loads find that you get the best agreement between what QL says and what your chronograph says

Neither (a) or (b). I adjust the Ba value for the lot of powder I am using.

Now let me qualify my answer. I don't load into the lands a lot, but I will load from a few thousandths off to as much as 0.130" off the lands when running the test per Berger instructions for VLDs. I chono every single shot I fire when at the range. And I shoot many thousands of rounds per year at the range. I see nothing that indicates the kind of change in peak pressure that you are suggesting.

Using my actual Ba values I find excellent prediction of velocities. I don't have the capability to directly measure pressure, but feel I am reasonable well versed in reading pressure signs. A few years ago, I was wanting to develop some safe starting points for the 20 Practical using the 39/40 gr bullets. There isn't much out there. So I took the powder I had on hand, calculate what I felt would be a good test point. You can see how well my measured velocities agreed with the QL prediction using my powder Ba values from testing.

 

[mikecr]

I believe QL's starting pressure is useful as a base setting for the internal math.
~3600 OTL,, ~triple it for ITL. Makes sense.
I just don't think it's linear to adjust from there, or even that it's truly an accurate pressure number, given all the different situations possible.
It's just one of a bunch of matters we can tweak to calibrate software to chrono numbers.

One thing is for sure: From QL, we can learn a tremendous amount about internal ballistics.
With it, I can prove and dis-prove common notions of what's going on -from a laptop and recliner.
So my calibration tweaking is important, but it's not as important as the learning overall.

 

[South Pender]

Jepp2, you have mentioned adjusting Ba values. I had thought that Ba referred to burning rate, and I see that QL inserts the appropriate values for the powder you have entered. Do you then tweak these from what QL gives you? And, if so, on what basis do you do this?

 

[South Pender]

That's how I've always read it too. back in the 80s when Viht powders first appeared in the UK, the company had a large free reloading guide that folded multiple times into a small booklet and had all sorts of interesting information as well as loads data. One little 'box' was called something on the lines of 'Seating Depth Pressure Effects' and gave pressure barrel results for a Lapua 7.62X51mm ball round load with the bullet seated progressively deeper. For the first three or four steps, there were no MV or maximum pressure changes, then when it had become really substantial, both metrics experienced a considerable drop. I lost that document and have never seen it repeated experimentally elsewhere. It is of course 180-degrees out from what most handloaders believe and which QuickLOAD has actually reinforced because of the COAL - PMax link which as I understand it has nothing to do with PMax per se, but with chambers / freebore and the COALs that they determine. (For example, QL will show a 2.26" COAL 223 loaded to magazine length as fired in a Wylde or 5.56 chamber as far higher pressure than when the same load is fired in the same chamber but has the bullet seated further out to be say 15 thou' short of the lands and single-loaded into the rifle. The chronograph shows no noticeable difference IME in such cases.)

I can understand why companies shy well clear of this issue for two reasons - a general desire to avoid people who don't know what they're doing 'playing around' with some of these factors; secondly, whilst this is the effect in rifles and standard rifle type loads/ propellants, the opposite effect does apply to many small capacity pistol cartridges like 9mmP, ie seat the bullet more deeply below the standard position and pressures rise, potentially to dangerous levels. So, a single general rule can be not only incorrect but potentially dangerously so.

When I've seated bullets mildly into the lands to the point where the tiniest bit of resistance is barely felt on bolt closure with the bolt's firing assembly removed, my experience of applying QL's full short start pressure additions is that the resulting MVs are way below the model's predictions, hence pressures are presumably so too. I usually end up increasing charges to where I would have been if I had left the short-start pressure setting at the default level. As a compromise, my first runs with a new untried combination will be modelled with a modest short-start increase in such cases, but nowhere near as high as 10,000 psi plus.

Laurie, when I've put this question out on another (Canadian) forum, one knowledgeable forum member mentioned that he went with a value somewhere between 5000 and 7000 psi as the Shot Start Pressure. This is approximately midway between the QL default value of 3625 and the "into the lands" value of 10,825. He claimed that doing so seemed to give QL predicted MVs close to his chronographed values with his particular loads. This practice sounds something like what you've described.

 

[jepp2]

Do you then tweak these from what QL gives you? And, if so, on what basis do you do this?

I do, to me it is the most consistent way to adjust the powder lot you are using to the results you are seeing. I wasn't smart enough to discover this myself, but I can follow the lead of others. So here is the process I use:

http://www.the-long-family.com/Tuning QL to achieve best results.pdf

I also use the OBT barrel time QL provides to help me find nodes. I adjust the Ba value to make my velocities match so I get the ms of barrel time.