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Question on Chamber Pressures using Quickload...Input welcomed.


VarmLR

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For all you QL users out there...is it possible, given the bullet data and MV for any given bullet/calibre that you can work backwards and estimate chamber pressures? I would be interested in seeing where I am with my loads now that I've developed some pretty consistent loads from the POV of barrel throat longevity!

 

I spent the day at the range with the Chrony and managed to get some decent data plus some very consistent groups. Data as follows, all groups shot at 100 yds (5mph left to right wind average, gusting between 2 to 8mph):

 

.223 calibre: 26 inch barrel

 

55g SGK BT bullets, N133 @24.2g; Seated length 2.25".....Average MV (10 shot string) = 3150fps...SD 17, Ave group = 0.34" (4 groups)

 

60g Vmax, N133@ 23.2g; Seated length 2.244" (10 thou off lands).....Average MV (10 shot string) = 3140fps, Ave group = 0.274" (6 groups)

 

69g STMK, N140 @ 23.6g; Seated length 2.26" (100 thou off lands).....Average MV (10 shot string) = 2786 fps, Ave group = 0.212" (5 groups)

 

 

0.308 Calibre: 24 inch barrel

 

150g SGK, N140 @ 43.1g, seated length 2.75" Average MV (10 shot string) = 2650 fps, ave group (3 groups) = 0.347"

 

 

Any info that could be supplied would be most welcome.

 

The only noteworthy things from the data collected are:

 

1. Boy, are those 69g TMKs consistent and tight grouping!

2. The MV for the 60g V-max is appreciably above book figures whilst all other bullets are below book figures for MV...go figure?

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Internal ballistics for pressure does not require SD/ES/ or group size but were they my groups, I'd have tossed them in too! ;) ~Andrew

 

(I don't use Quick Load but wouldn't the question be near moot if Vhit would include pressures in their loading data like most manufacturers do??)

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Internal ballistics for pressure does not require SD/ES/ or group size but were they my groups, I'd have tossed them in too! ;) ~Andrew

 

(I don't use Quick Load but wouldn't the question be near moot if Vhit would include pressures in their loading data like most manufacturers do??)

 

Hi Andrew

 

thanks for that. Yes, one of the reasons for asking was that Viht don't include pressure data, just the velocities. Group data was more for establishing part of the equation for consistency although I was surprised that with some very consistent groups, ES was surprisingly high and SD wasn't anything to write home about (perhaps in the 15 to 30 region) suggesting that one or two rounds per string were quite a long way off the majority (largest ES was about 70, and for the tightest groups, it was around 10 or 15). Could be I need to weigh my charges more carefully or it could be variations in neck tension...I don't know, but the groups seem ok. It would just round things off development-wise if I could establish approx pressures to ensure I'm not in barrel-burning territory!

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To answer your question... yes it can be done. As to how close these figures are to reality... who knows?

 

For an accurate attempt at giving you estimations of the pressures, you'd need to provide case volume and barrel lengths.

 

A crude attempt at the 308 gave the same velocity with the standard case volume and a 23" barrel. Pressure for this load was about 46,000 PSI.

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This is really interesting and I don't know any of the answers, but will be watching closely.

 

I have recently started using Quickload, and part of the reason I decided to buy it was because I suspect it could be a useful tool for replicating loads for which a double rifle has been regulated.

 

Modern doubles will have been regulated for a given factory load (mine is regulated for Hornady DGX). This stuff s crazy expensive and I need to replicate it in something so I can afford to shoot it enough to get used to it.

 

I can give the actual velocity of the regulated load, and could break one down and obtain the powder weight, but doubt Hornady will tell me which powder it is, so I need to get an idea of both chamber pressure and barrel time based on the known powder weight, bullet type and actual velocity. To add to VarmLR's original question, how would one go about this?

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To answer your question... yes it can be done. As to how close these figures are to reality... who knows?

 

For an accurate attempt at giving you estimations of the pressures, you'd need to provide case volume and barrel lengths.

 

A crude attempt at the 308 gave the same velocity with the standard case volume and a 23" barrel. Pressure for this load was about 46,000 PSI.

 

Thanks for that. I'll measure some cartridges to obtain volume using a lee dipper or syringe. Barrel length is stated in post #1.

 

I'm guessing that the 46K PSI was for the 308 loads stated? that sounds very reasonable if so as I started out with over 44gr but was starting to get pressure signs so backed off a full grain. Coincidentally, my developed load is identical to that stated in Sierra's 5th Ed load data as their "accuracy" load but is at the lower end of Vihts recommended load.

 

I can give the actual velocity of the regulated load, and could break one down and obtain the powder weight, but doubt Hornady will tell me which powder it is, so I need to get an idea of both chamber pressure and barrel time based on the known powder weight, bullet type and actual velocity. To add to VarmLR's original question, how would one go about this?

Good luck with that Bushdog....not sure how you get barrel time as the MV won't give you that. The bullet is accelerating all the time it's in the barrel and de-accelerates from the point of exit...presumably. You may be able to do something around the pressure curve and see if a crude estimation average barrel time can be made from that and the measured MV knowing that V(zero) = 0fps and Mv = XXXX fps?

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If you really want to know what the pressures are, you can submit a small batch to the Proof House and they will test it and produce a nice report for you. But I'm not sure how much they charge for this service these days.

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It was more out of interest/curiosity. I don't think that sending batches for proof house testing would be economically justifiable or needed. I was just curious as to whether QL could provide an estimate which might be in the ball park.

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Thanks for that. I'll measure some cartridges to obtain volume using a lee dipper or syringe. Barrel length is stated in post #1.

 

I'm guessing that the 46K PSI was for the 308 loads stated? that sounds very reasonable if so as I started out with over 44gr but was starting to get pressure signs so backed off a full grain. Coincidentally, my developed load is identical to that stated in Sierra's 5th Ed load data as their "accuracy" load but is at the lower end of Vihts recommended load.

 

I can give the actual velocity of the regulated load, and could break one down and obtain the powder weight, but doubt Hornady will tell me which powder it is, so I need to get an idea of both chamber pressure and barrel time based on the known powder weight, bullet type and actual velocity. To add to VarmLR's original question, how would one go about this?

Good luck with that Bushdog....not sure how you get barrel time as the MV won't give you that. The bullet is accelerating all the time it's in the barrel and de-accelerates from the point of exit...presumably. You may be able to do something around the pressure curve and see if a crude estimation average barrel time can be made from that and the measured MV knowing that V(zero) = 0fps and Mv = XXXX fps?

No problem. I'll await the average cartridge volume and then use that data to recompute the figures for both rifles.

 

Yes, the program computed a figure of 46K PSI (well 45,865 PSI) for the 308 case, N140, 23" barrel and the 150 gr SGK (2125) seated to 2.750" COAL. The figures for this combination on the Vihtavouri website (link) is a max of 47.1 gr, the bullet seated to 2.756". I concur with the Sierra manual, the manual giving the max load as 44.7 gr of N140 with this bullet. Quite a difference for the max charge and velocities, but then again, Sierra are using 26" 1:10 rifle and Vihtavuori are using a 24" 1:12". The former would likely give slightly high pressures for the same change weight due to the higher twist rate.

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My barrel is the same length as the Viht test barrel (610mm or 24 inches) and 1/11 twist so may give very slightly higher pressures than Viht suggest. I'll try and sort the case volume out later. Presumably this is case full to top, trimmed to 2.008 inches, or is it to the base of the neck?

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My barrel is the same length as the Viht test barrel (610mm or 24 inches) and 1/11 twist so may give very slightly higher pressures than Viht suggest. I'll try and sort the case volume out later. Presumably this is case full to top, trimmed to 2.008 inches, or is it to the base of the neck?

It's the volume (in grains of water) to the neck of a fired unsized case with a used primer in it. The 'How To' from the QuickLOAD manual.

 

For most RIFLE CALIBERS and other guns using peak pressures above 30,000 psi (2,000 bars): use a case fired in the specific gun you are working with. Do not resize the case before

making the capacity measurement.

For most PISTOL CALIBERS and other guns using peak pressures below 30,000 psi (2,000 bars): use a resized case.

 

After obtaining the weight of a dry empty case (with used primer in place), fill the case with cold water. Eliminate any air bubbles and bring water even to end of case neck. Reweigh water-filled case. Subtract dry weight from water-filled weight. This gives case capacity in grains of water, which is the standard unit of measure.

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My barrel is the same length as the Viht test barrel (610mm or 24 inches) and 1/11 twist so may give very slightly higher pressures than Viht suggest. I'll try and sort the case volume out later. Presumably this is case full to top, trimmed to 2.008 inches, or is it to the base of the neck?

It's the volume (in grains of water) of a fired unsized case with a used primer in it. There will be a slight meniscus at the top, just keep its height the same for each measurement.

 

The 'How To' from the QuickLOAD manual.

 

For most RIFLE CALIBERS and other guns using peak pressures above 30,000 psi (2,000 bars): use a case fired in the specific gun you are working with. Do not resize the case before

making the capacity measurement.

For most PISTOL CALIBERS and other guns using peak pressures below 30,000 psi (2,000 bars): use a resized case.

 

After obtaining the weight of a dry empty case (with used primer in place), fill the case with cold water. Eliminate any air bubbles and bring water even to end of case neck. Reweigh water-filled case. Subtract dry weight from water-filled weight. This gives case capacity in grains of water, which is the standard unit of measure.

 

Sorry, double post.

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Ok. Made a rudimentary gauge which was used to gauge the correct level of water to the neck in a number of fired cases trimmed to the same length. Did a number of measurements and took the average.

 

Average mass of water occupying a twice fired .308 Lapua case (FL sized prior to firing) was 47.32 grains water at 16 degrees Celius (ie temperature from tap).

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Nope. definitely 47.3 grains to the neck; 53 grains to the top of the cartridge case (trimmed to 2.008 inches). This broadly agrees with the case full (ie to the very top) capacity cited here for example: http://www.chuckhawks.com/rifle_case_capacity.htm

 

This is less than the 55 grains of my fire formed PPU, probably indicating thicker brass used for the Lapua cartridge case which in turn could mean higher chamber pressures for any given load.

 

Are you measuring your cases to brim full or to the base of the neck?

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The capacity is to the very top of the case (i.e. the mouth, aka top of the neck) not the bottom of the neck (i.e. the shoulder/neck junction). So this figure is 53 grains.

 

CartridgeCaseParts_10.jpg

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53 grains it is then but why then Post #11 or did you mean "mouth" instead of "neck"?:

 

It's the volume (in grains of water) to the neck of a fired unsized case with a used primer in it. The 'How To' from the QuickLOAD manual.

 

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53 grains it is then but why then Post #11 or did you mean "mouth" instead of "neck"?:

 

Post #11 was a double post, hence the later revised #12. I wanted to put a reference to top of the neck, as I noticed I had missed it out, but somehow clicked the post button. I then carried on with the edited version, unknowing that the previous one was already posted, eventually posting that too. I would have deleted the accidental previous post, but I couldn't. I should have said mouth to start with, that would have alleviated any later confusion.

 

Anyway, I put the figures into the program and the nothing married up. The computed velocity is about 88 fps more that you got (2,738 fps), the pressure 51,500 PSI, load ratio 101%, so very slightly compressed.

 

This illustrates, yet again, that QuickLOAD is purely a model that produces an approximation of cartridge's internal ballistics. What it computes often does not align with that produced by real world testing. This suggests the model doesn't work well with this combination, or data for N140 is a bit 'screwy'. Coincidentally, the velocity does align when you put in 57.32 gr capacity.

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Thanks for the best wishes!

I have so far managed to replicate it by trial and error (i.e. using normal loading data and fiddling with known loads until they regulate). Was just thinking it might be a useful tool to reduce the trial and error factor.

I'm a mathematical idiot, so am unlikely to be able to work this out through calculation, so probably sticking to trial and error might be my best bet!!

 

Quickload is definitely only a model - my .308 loads (with all measurable parameters input correctly, e.g. case capacity, barrel length etc) are consistently giving considerably higher velocities than predicted, measured on a magnetospeed chrono.

 

Edited to add: My case capacity in fired .308 Lapua case came out at 55.85grains of water.

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Post #11 was a double post, hence the later revised #12. I wanted to put a reference to top of the neck, as I noticed I had missed it out, but somehow clicked the post button. I then carried on with the edited version, unknowing that the previous one was already posted, eventually posting that too. I would have deleted the accidental previous post, but I couldn't. I should have said mouth to start with, that would have alleviated any later confusion.

 

Anyway, I put the figures into the program and the nothing married up. The computed velocity is about 88 fps more that you got (2,738 fps), the pressure 51,500 PSI, load ratio 101%, so very slightly compressed.

 

This illustrates, yet again, that QuickLOAD is purely a model that produces an approximation of cartridge's internal ballistics. What it computes often does not align with that produced by real world testing. This suggests the model doesn't work well with this combination, or data for N140 is a bit 'screwy'. Coincidentally, the velocity does align when you put in 57.32 gr capacity.

 

 

Thanks for taking the trouble to do that, appreciated. I measured 3 different cartridges and all were around the 53 grains (55 to 56 grains with PPU brass). However, puzzled by your findings, I took another batch and measured those. It transpires that the first 3 taken were from an over-trimmed batch and were several thou shorter (some down to 2.003") which may explain why they were a way off on capacity. Also, I suspect that they may have had a little crud left inside them. I was surprised just what did come out when a vigorous cleaning with a Q-bud was put to them! The cleaned and more samples averaged 55.6 grains. My chrony may not be the world's most accurate, but it should be giving reasonable results. It's a Digital Pro-Chrony and was set up 6 feet in front of the rifle with the diffuser screens in place as it was a bright, sunny day when I did the testing. Loads were weighed on a Lee safety scale.

 

I did some other water capacity tests on the 223 brass and got 29.8 grains average case capacity. That was for a 26 inch barrel with 1/8 twist.

 

I'm guessing that re-evaluating, the results might be a little closer now. Apologies for not having thoroughly checked all the brass before troubling you with the QL predictions.

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It's no trouble doing this VarmLR, it just goes to prove how very small changes in the input data (or in this case, reality) can change the output so greatly. And this is without fiddling with the propellant data. How close each batch propellant is to that in stored in the program files, and how close the bomb calorimeter data is the that within the chamber of a gun, is something I don't know. There are also so many variables not covered by the program, that it will always be just a basic model, a crude approximation. Though that is true, it's better than no data at all!

 

I put the 55.6 gr data on the 308 in, along with all the other supplied data, the output was as follows: MV = 2,682 fps; Pmax = 46,382 PSI; and load ratio = 95.7%. With the max load from the Sierra manual, 44.7 gr, the output was: MV = 2,770 fps; Pmax = 51,797 PSI; and load ratio = 99.3%.

 

The 223 Rem data is as follows.

 

55g SGK BT bullet (#1365), N133 = 24.2 gr, COAL 2.25": MV = 3,214 fps; Pmax = 50,558 PSI; and load ratio = 104.0%.

60 gr V-Max bullet (#22281), N133 = 23.2 gr, COAL 2.244": MV = 3,111 fps; Pmax = 53,427 PSI; and load ratio = 105.9%.

69 gr TMK bullet (#7169), N140 = 23.6 gr, COAL 2.26": MV = 2,939 fps; Pmax = 51,833 PSI; and load ratio = 103.4%.

 

Not much correlation.

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Thanks for that. There's not exact correlation but you know it's not that far off. The 60g .223 are the closest being just 29fps off measured average MV. I didn't think that I was compressing by 5%...it seemed less than that but I guess powders vary from batch to batch. The 69g TMKs were quite a bit off measured, by a few hundred fps. The 308 was much closer to actual so seems to be a nice accurate load without excessive pressure. I'll stock with the loads developed. Only shot them at a max of 22 degrees C, so will need to watch for rising pressure with the Vmax 60g loads especially if we get a summer where we can add another 5 or 10 degrees to that!

 

 

Thanks again. very interesting exercise.

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