velocity vs pressure relationship

[bilydean]

Help me understand this. Been reloading for years. Trying out a new powder in my 308, 18" barrel. Shooting thru a chronograph I get a speed of 2860 fps with a load of 45.0 gr of TAC with a 155gr ELDM bullet. Per Hodgdon website, using TAC and 155gr Amax bullet, 45.1 is the max. I started my ladders at 43.8g of TAC. No pressure signs till I got to 45.0 with slight swipe marks, so I would really call that the max for me.

My 18in barrel with this powder loads are getting 24in barrel speeds. With 44.4g, I was getting 2800fps with really good grouping and will likely go with that. No pressure signs in the brass.......

Does this seem right? Not that Quickload is the same as the mfg data, but QL predicted a much slower speed.

I was looking at a website that based on fps, barrel length, bullet dia & weight, etc, would ball park a peak pressure. That formula shows me below peak pressure.

What is the deal? Follow the book and be happy with speed/accuracy OR be concerned I am overpressurizing my barrel?

thx
Dean

[ar15barrels]

How good are you with quickload?
Are you using standard parameters or exact measurements for everything including your case volume and test temperatures?

When you account for everything, you should be able to make quickload match your test results and only then can you model different loads to see what will happen in your proofed model.

[ar15barrels]

To look at it another way, velocity and pressure absolutely track together so when you have higher velocity, you have higher pressure.
Even if you have not optimized your quickload model, the standard test conditions will give you a nicely correlated map of pressure to velocity even when the charge weights are wrong because your test conditions are wrong.
Look at your quickload powder charge steps output and find your chronograph velocity and the pressure on that line will be close even if the charge show for that velocity is different that our actual charge.
Ignore the charges for a moment and jus look at the correlated pressure/velocity.
Once you tune your model, all 3 data points will be hitting your actual results.
Make sure to account for velocity differences between your chronograph to muzzle distance too.

[tabascoz28]

What kind of chrono? Radar, cameras, mag? For years I had my chrono too close and was getting flash on my camera.

[bilydean]

Interesting. I put in all my data into QL (which I don't know real well). Bullet, weight, adjusted length of bullet, COAL, barrel length, case capacity (which is 56.0), temp of 56 deg, and 45 gn of TAC. No where near close to what my gun did. QL says 2820 fps, peak pressure at 51k. Reality - my rifle shot that pill at 2850 fps thru my chrono at 10 feet out. Based on what folks have said here, I was actually at peak pressure of 75k, which is WAY over pressure. Brass didn't show it that way.

-What other fields can I intelligently manipulate in QL to mirror my reality. (fairly new to QL)
-2nd, based on comments, my top fps should be around 2680 fps based on what QL shows for pressure. I am fine with that as well. Just want to understand

[ShaunBrady]

If that case capacity is measured from a case fired in your gun, the next variable to manipulate to line up velocity with charge weight is start pressure. Depending on the barrel, the chamber, and the amount of jump it can vary from 3,0000 to maybe 12,000 psi with the same bullet. The start pressure increases in new barrels giving the increase in velocity you see. With overbore cartridges, it'll continue to rise until the barrel is done. With something like a 308, a drop in start pressure indicates the barrel is near it's end.

A chrono and QL can provide a pretty harsh reality check on your impressions of pressure signs. If there are any marks on the brass face at all, you're over 70,000 psi. If it's Lapua or boutique brass and you minimize shoulder bump, it might take another 5,000 psi before marks start.

Kittens don't die as soon as you're over SAAMI pressure. If you measure the extractor groove expansion on the first firing of the brass, you can get some idea of how many firings you'll get before the primer pockets are gone.

[bilydean]

Thx Shaun. I will definitely be backing my charge way down.

I appreciate the response.

[ar15barrels]

Interesting. I put in all my data into QL (which I don't know real well). Bullet, weight, adjusted length of bullet, COAL, barrel length, case capacity (which is 56.0), temp of 56 deg, and 45 gn of TAC. No where near close to what my gun did. QL says 2820 fps, peak pressure at 51k. Reality - my rifle shot that pill at 2850 fps thru my chrono at 10 feet out. Based on what folks have said here, I was actually at peak pressure of 75k, which is WAY over pressure. Brass didn't show it that way.

-What other fields can I intelligently manipulate in QL to mirror my reality. (fairly new to QL)
-2nd, based on comments, my top fps should be around 2680 fps based on what QL shows for pressure. I am fine with that as well. Just want to understand

I don't have the lengths of an ELD-M 155gr to model it so I don't know how much longer that bullet is than the old 155 amax or even the 155BTHP.
Also, what's your actual loaded OAL?
And how close to the lands are you?

One method you will see talked about is that you take your starting pressure for the bullet type and then add 7200 like you are touching the lands and then you subtract 29psi per 0.001" of bullet jump between your loaded length and when the bullet first starts contacting the lands.
So for a traditional jacketed bullet like an ELD-M, that's 3625+7200=10,825 and now let's say you are 0.020" off the lands so subtract 20X29=580 from 10,825 to get a starting pressure of 10,245.

Using the more detailed specs you gave about temperature and modeling the 155gr hornady BTHP, I can't get anywhere near 2800fps with 44.4gr of tac in an 18in barrel in QL so something is really wrong in QL or your pressures are really high or your barrel length is really wrong.

Cartridge          : .308 Win. (SAAMI)
Bullet             : .308, 155, Hornady BTHP Match 3039
Useable Case Capaci: 48.949 grain H2O = 3.178 cm³
Cartridge O.A.L. L6: 2.800 inch = 71.12 mm
Barrel Length      : 18.0 inch = 457.2 mm
Powder             : Ramshot TAC *C ?, Temperature: 56 °F

Predicted data by increasing and decreasing the given charge,
incremented in steps of 1.033% of nominal charge.
CAUTION: Figures exceed maximum and minimum recommended loads !

Step    Fill. Charge   Vel.  Energy   Pmax   Pmuz  Prop.Burnt B_Time
 %       %    Grains   fps   ft.lbs    psi    psi      %        ms

-10.3   89    43.40   2450    2066   51689   9543     91.1    0.973
-09.3   90    43.90   2478    2113   53189   9669     91.6    0.961  ! Near Maximum !
-08.3   91    44.40   2505    2160   54741   9793     92.0    0.950  ! Near Maximum !
-07.2   92    44.90   2533    2208   56348   9915     92.5    0.939  ! Near Maximum !
-06.2   93    45.40   2560    2256   58013  10035     93.0    0.928  ! Near Maximum !
-05.2   94    45.90   2588    2305   59737  10154     93.4    0.917  ! Near Maximum !
-04.1   95    46.40   2616    2355   61524  10270     93.8    0.906  !DANGEROUS LOAD-DO NOT USE!
-03.1   96    46.90   2644    2405   63375  10384     94.2    0.895  !DANGEROUS LOAD-DO NOT USE!
-02.1   97    47.40   2671    2456   65295  10496     94.6    0.885  !DANGEROUS LOAD-DO NOT USE!
-01.0   98    47.90   2699    2508   67284  10606     95.0    0.874  !DANGEROUS LOAD-DO NOT USE!
+00.0   99    48.40   2727    2560   69348  10713     95.4    0.864  !DANGEROUS LOAD-DO NOT USE!
+01.0  101    48.90   2755    2613   71489  10817     95.8    0.853  !DANGEROUS LOAD-DO NOT USE!
+02.1  102    49.40   2783    2666   73711  10918     96.1    0.843  !DANGEROUS LOAD-DO NOT USE!
+03.1  103    49.90   2811    2720   76018  11017     96.5    0.833  !DANGEROUS LOAD-DO NOT USE!
+04.1  104    50.40   2839    2775   78414  11112     96.8    0.822  !DANGEROUS LOAD-DO NOT USE!
+05.2  105    50.90   2868    2830   80904  11204     97.1    0.812  !DANGEROUS LOAD-DO NOT USE!

Send me the bullet OAL as well as the boattail diameter and length as well as your rifle's touch length with the bullet and your OAL of the loaded rounds and re-confirm the 18" barrel length by marking a cleaning rod down the bore that's touching the boltface of the bolt closed on an empty chamber.
Mark the cleaning rod with a sharpie/pencil where it exits the muzzle and then withdraw the rod and measure from the mark to the end of the rod that was touching the boltface.

I'll play with it some more but also check your loaded round neck daimeter and your fired case neck diameters and let me know what both are.
Any chance you are massively jamming your bullets into the lands or that you have a 0.298" or 0.299" groove diameter barrel that raises pressure and velocity?

Besides starting pressure for small adjustments to match QL to reality, you can adjust your powder's burn rate (Ba) for bigger adjustments as there definitely are differences from lot to lot of the burn rate.

[Sandspider500]

Have you shot factory ammo over your Chrono?
Hornady 155 edl match is 1.195, and the VMAX is 1.218(although the QL bullet file lists it at 1.183).

[bilydean]

Randall,
Hence my dilemma why QL seems to be way off.
Bullet is similar profile to the AMAX. It is 1.205 in length
Barrel is 18
Loaded neck is .334, fired neck is .342
COAL is 2.80
I don't know on the lands question. This is an LR308, so magazine has always been my limiting factor.
Excellent question on the .298 groove. Don't know. It is a Faxon Gunner barrel.

What I have done at this point is set up QL to match Hogdgon site for 24' barrel to get speed and pressure they publish. I was adjusting the ratio of specific heat to match published data. Then changed to the 18" barrel to see what that speed would be based on peak pressure.

Then put in my load and velocity data and adjusted the specific heat to where speeds matched my ladder data. Looked up what my speed and pressure should based on published book adjustments to get that I should only load a max of 42.7g of TAC which should put me in the 2675-2700fps range.

[bilydean]

Have you shot factory ammo over your Chrono?
Hornady 155 edl match is 1.195, and the VMAX is 1.218(although the QL bullet file lists it at 1.183).

I measured my ELDM at 1.205. Haven't shot factory ammo in years. I have been using 8208 powder all this time. Picked up this TAC, which I have never used and encountered this issue. I have shot factory ammo from my 7mm over the chrono and it does match what it should be doing, so don't believe the issue is there, but is was a good point.

[ar15barrels]

When you question a velocity being correct, shoot out to 600 or 1000yds or some other nice distance that will need a significant elevation correction and then use the velocity truing function of your ballistic calculator with actual weather conditions where you are shooting to backwards calculate the velocity from the known elevation adjustment that was necessary to be on target at the given distance.

Match up published loads from loading manuals in quickload and then change your barrel length to see what you will get from the shorter length.
If you can match the loading manuals with quickload, your QL model is pretty damn tuned-in as the loading manual data is usually derived in a decent laboratory with good instrumentation.

[roostersgt]

Try using a different chronograph. There really is no way to get that kind of velocity from a 308 with a 18" barrel. I use TAC and get the same velocities as Varget using same number of grain. 22" barrel, 155
-165grn SP, and 2650~2720fps, depending on which brand of brass. I think your chronology is off.

[fguffey]

billydean, I have never had overpressure that could not be measured if measurements were made before firing.

There is always someone that know what cases are soft and subject to primer pocket expansion.

F. Guffey

[rhodesengr]

velocity and pressure absolutely track together so when you have higher velocity, you have higher pressure.

Is that really true? peak pressure depends a lot on the powder burn rate. Slower powders can produce lower peak pressure yet still produce faster exit speed because they have a more extended pressure pulse. They are pushing harder when the bullet is further down the barrel than faster powders.

I know in my Trap loads, WSH has lower peak pressure but can give a higher speed than WST for a 1 1/8oz load.