Velocity Vs OAL (or bullet jump)

NYDAN

Handloader
Sep 17, 2013
1,985
1,630
My old reloading manuals state that when you seat your bullet close to the lands you may experience higher pressures than if you seat your bullet with the OAL they list with their reloading data. I think I have some data here that clearly illustrates that point.

On January 12th I posted some results I obtained shooting:

270 Win
New Norma Brass
WLR Primer
150 Gr. CTBST
RL 22
Bullet Jump of .025"

I got very good accuracy but I seemed to get very large velocity increases with .5 Gr. powder charge increases. I wasn't sure if I had some bad velocity data or if I was getting high velocity increases because of a short bullet jump. I indicated I was going to rerun some tests with two different bullet jumps and compare the results. I finally had an opportunity to shoot the trials on Monday.

I loaded cartridges with 56.0, 56.5, 57.0, and 57.5 grains of RL 22. One batch was loaded to an OAL of 3.400" which would give a bullet jump of about .025". The other batch was loaded to an OAL of 3.380" which would give a bullet jump of about .045".

The velocity results are listed in an image below. The results are also plotted on a graph along with data from the Nosler reloading guide.

The data clearly shows that, in the range of charges tested, the loads with an OAL of 3.400" (.025" jump) (red line) had an increase in velocity of about 100 FPS per Grain of powder whereas the loads with an OAL of 3.380" (.045" jump) (blue line) had an incresae in velocity of about 50 FPS per Grain of powder charge. The loads with a bullet jump of .045" more closely approximates the Nosler data (black line).

I think this illustrates exactly what the reloading manuals caution us about when seating the bullets close to the lands and the need to work up loads carefully.

By the way, the four shot (four different charge weights) group with an OAL of 3.380" measured only 0.6" C/C.

The next tests will be with a powder charge of 57.0 grains and an OAL of 3.380".
 

Attachments

  • Velocity_Vs_OAL.jpg
    Velocity_Vs_OAL.jpg
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pressurecurvr_zpsaeb4a2df.gif


pressuregraph.jpg


seatingdepthvpressure.jpg
 
Steve4102,

Thanks for the great illustrations. It is always great to see supporting empirical data.

Dan
 
Same load but at different seating depth, note the progressively lower velocity with at deeper seating seating depths

1211-1.jpg


Also note the progressive lowering of Extreme Spread and Standard Deviation (ES & SD) to .060" off the lands and the progressive increase at jumps over .060". Low ES & SD will make a more consistent load with less influence from altitude or temperature variables.
 
THIS is the stuff I've always wanted to see:

pressurecurvr_zpsaeb4a2df.gif


I have read countless times that when you seat "close to or touching the lands" you will see pressure increases. I have always wondered what "close to" means. I'd like to see what the results are touching the lands and backing off by about 0.002" at a time.

QuickLOAD indicates that if you seat touching the lands you should add 7200 psi to the start pressure. But it also indicates that pressure increases as you seat the bullet deeper (and this is true, with caveats). What I've realized is that you have to consider two totally different effects: one is pressure changes due to changes in internal volume, and the other is pressure changes due to proximity to the lands.

I ran some numbers on 7mm-08 in QL. These first results are with a COAL of 2.900" and with 7200 psi added to the start pressure (default value is 3626 psi start pressure) for being seated against the lands:
Code:
Cartridge          : 7 mm-08 Rem.
Bullet             : .284, 150, Nosler PART SP 16326
Useable Case Capaci: 49.315 grain H2O = 3.202 cm³
Cartridge O.A.L. L6: 2.900 inch = 73.66 mm
Barrel Length      : 22.5 inch = 571.5 mm
Powder             : Hodgdon H4350

Predicted data by increasing and decreasing the given charge,
incremented in steps of 2.0% 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

-20.0   86    38.16   2303    1767   40572   7693     90.7    1.305
-18.0   88    39.12   2355    1847   42777   7914     91.6    1.276
-16.0   90    40.07   2407    1929   45125   8130     92.5    1.247
-14.0   92    41.02   2459    2013   47629   8341     93.4    1.219
-12.0   94    41.98   2511    2099   50297   8545     94.3    1.192
-10.0   97    42.93   2563    2188   53132   8743     95.0    1.166
-08.0   99    43.89   2615    2278   56137   8933     95.8    1.140  ! Near Maximum !
-06.0  101    44.84   2667    2370   59327   9115     96.4    1.114  ! Near Maximum !
-04.0  103    45.79   2720    2464   62715   9287     97.1    1.089  !DANGEROUS LOAD-DO NOT USE!
-02.0  105    46.75   2772    2560   66320   9450     97.6    1.065  !DANGEROUS LOAD-DO NOT USE!
+00.0  107    47.70   2825    2658   70179   9601     98.2    1.041  !DANGEROUS LOAD-DO NOT USE!

Next, I ran the program with the same OAL except with the start pressure value at the default value, as if we had reamed a bunch of freebore into the chamber:

Code:
Cartridge          : 7 mm-08 Rem.
Bullet             : .284, 150, Nosler PART SP 16326
Useable Case Capaci: 49.315 grain H2O = 3.202 cm³
Cartridge O.A.L. L6: 2.900 inch = 73.66 mm
Barrel Length      : 22.5 inch = 571.5 mm
Powder             : Hodgdon H4350

Predicted data by increasing and decreasing the given charge,
incremented in steps of 2.0% 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

-20.0   86    38.16   2219    1640   31517   7521     87.2    1.483
-18.0   88    39.12   2273    1721   33619   7768     88.5    1.443
-16.0   90    40.07   2328    1805   35863   8010     89.7    1.404
-14.0   92    41.02   2383    1891   38259   8246     90.8    1.363
-12.0   94    41.98   2438    1980   40819   8475     91.9    1.323
-10.0   97    42.93   2493    2070   43556   8696     93.0    1.284
-08.0   99    43.89   2548    2163   46485   8909     93.9    1.246
-06.0  101    44.84   2604    2258   49620   9112     94.8    1.210
-04.0  103    45.79   2659    2355   52979   9305     95.7    1.174
-02.0  105    46.75   2714    2454   56554   9486     96.4    1.141  ! Near Maximum !
+00.0  107    47.70   2770    2555   60377   9656     97.1    1.108  ! Near Maximum !
+02.0  109    48.66   2825    2658   64480   9814     97.8    1.076  !DANGEROUS LOAD-DO NOT USE!
+04.0  112    49.61   2880    2763   68890   9957     98.3    1.046  !DANGEROUS LOAD-DO NOT USE!
+06.0  114    50.56   2936    2871   73634  10086     98.8    1.016  !DANGEROUS LOAD-DO NOT USE!
+08.0  116    51.52   2991    2980   78747  10201     99.2    0.987  !DANGEROUS LOAD-DO NOT USE!


And last, here are the results with the OAL reduced from 2.900" to 2.400 and the default value (3626 psi) for start pressure:

Code:
Cartridge          : 7 mm-08 Rem.
Bullet             : .284, 150, Nosler PART SP 16326
Useable Case Capaci: 41.313 grain H2O = 2.682 cm³
Cartridge O.A.L. L6: 2.400 inch = 60.96 mm
Barrel Length      : 22.5 inch = 571.5 mm
Powder             : Hodgdon H4350

Predicted data by increasing and decreasing the given charge,
incremented in steps of 2.0% 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

-20.0  102    38.16   2406    1928   43376   7373     90.7    1.299
-18.0  105    39.12   2468    2029   46826   7584     92.0    1.255
-16.0  108    40.07   2530    2133   50584   7786     93.1    1.212
-14.0  110    41.02   2593    2240   54678   7976     94.2    1.171  ! Near Maximum !
-12.0  113    41.98   2656    2350   59149   8155     95.2    1.132  ! Near Maximum !
-10.0  115    42.93   2719    2463   64041   8320     96.1    1.094  !DANGEROUS LOAD-DO NOT USE!
-08.0  118    43.89   2782    2579   69413   8470     96.9    1.058  !DANGEROUS LOAD-DO NOT USE!
-06.0  120    44.84   2846    2698   75319   8605     97.6    1.023  !DANGEROUS LOAD-DO NOT USE!
 
It looks to me like you changed the H2O capacity drastically from 49.315 to 41.313 so your pressure is naturally going to go up.

My problem with this is that it is not the internal capacity of the case that matters but the internal capacity of the chamber, which will not change. The case will get blown out to the chamber walls with very little pressure and essentially you will have the same combustion chamber size. The difference will be the added jump and subsequent easier engraving of the bullet with a larger jump which will lower pressure.

Why did you change the case capacity to such a large degree?
 
I think that is the available powder space under the bullet, as seated in each instance. I can change the overflow water capacity, but I did not touch it.

What this is illustrating is the same effect as seating a pistol bullet deeper. In small cartridges like 9mm, you can raise pressures dramatically with relatively small changes in OAL. It's not nearly so pronounced in cartridges with more volume, like this rifle case. Notice that I changed the OAL by 1/2 an inch, which is pretty drastic but makes the result more evident.
 
What happens if you leave the case capacity the same and just change the OAL length?
 
woods":38pt88pb said:
What happens if you leave the case capacity the same and just change the OAL length?


I told you. I did NOT change the case capacity. The figure you are looking at is the powder space that remains after seating the bullet. The program automatically reduces the "useable case capacity" when you reduce OAL.

I CAN change the overflow capacity, but all three instances are run with the default value for that parameter.
 
Look, I don't run Quirkload and I don't know what you have changed or not. But the graphs, pressure trace and target above PROVE that pressure will decrease with deeper seating. If Quirkload does not account for this, then that is the problem.
 
It is NOT the deeper seating that decreases the pressure. It is reducing the proximity of the bullet to the lands. There are TWO different things at work here.
 
RiverRider":1dnc43uo said:
It is NOT the deeper seating that decreases the pressure. It is reducing the proximity of the bullet to the lands. There are TWO different things at work here.

Six of one half dozen of the other

How is seating deeper NOT reducing the proximity of the bullet to the lands

Semantics
 
I am not going to continue to entertain you with an argument. If you're sure you already know everything then you should be satisfied with that. Or better yet contact the originator of the software and let him know what a charlatan he is.
 
Very interesting topic. I really appreciate those who took the time to analyze and document their findings.

The physics of what happens in those 1 or 2 milliseconds is fascinating.
 
Not here to be entertained and decidedly not so. You're hanging your hat on Quirkload and I really don't know whether it is the program itself or the operator. Trying to figure that out. Don't know if the originator is a Charlatan or not but suspect that he is not.

If you need someone you would consider an authority then Hornady has a nice writeup

http://www.hornady.com/ballistics-resource/internal

down at the bottom

To illustrate the effects of variations in bullet travel before the bullet enters the rifling, we'll compare a standard load with adjustments made only in the bullet's seating depth.

In a "normal" load with the bullet seated to allow about one 32nd of an inch gap (A) between the bullet and the initial contact with the rifling, pressure builds very smoothly and steadily even as the bullet takes the rifling. Pressure remains safe throughout the powder burning period (B), and the velocity obtained - 3500 fps - is "normal" for this load in this rifle.

Seating the bullet deeper to allow more travel before it takes the rifling, as in these next two illustrations, permits the bullet to get a good running start (C). Powder gases quickly have more room in which to expand without resistance, and their pressure thus never reaches the "normal" level. Nor does the velocity; with the same powder charge it only comes to 3400 fps (D).

bullet seated to touch the riflingWhen the bullet is seated to touch the rifling, as in the accompanying illustrations, it does not move when the pressure is low (E); and not having a good run at the rifling as did the other bullets, it takes greatly increased pressure to force it into the rifling. As the rapidly expanding gases now find less room than they should have at this time in their burning, the pressure rise under these conditions is both rapid and excessive (F). Velocity is high at 3650 fps - but at the expense of rather dangerous pressure. Many rifles deliver their best groups when bullets are seated just touching the rifling. Seating bullets thus can be done quite safely if the reloader will reduce his charge by a few grains. The lighter load will still produce the "normal" velocity without excessive pressure.

You'll have to access the site to view the illustrations

Barnes had some work too and came to the same conclusion but I seem to have lost that link

Barsness on 24hour has often commented on the reduction in pressure and velocity with deeper seating

I'll take all this information over a printout of Quirkload with faulty parameters.

I just don't see how you can't see it
 
I am going to try ONE MORE TIME. If you read carefully what I have provided you will see that I have said that the effects of proximity to the lands has far more detectable influence on velocity and pressure because of the way we typically operate. We do NOT vary seating depth by 1/2 inch which id what I did in one of the three examples I gave above; I exaggerated that difference to illustrate the effect of seating deeper. Whether or not you can grasp it, it is real.

I am trying to explain to you that there are two different things operating here. If you were to duplicate loads with seating depth varied and shoot them in a rifle that was freebored to the muzzle, you would see higher pressures where the bullet was seated deeper. This is because reduced volume under the bullet causes higher pressure.

The effect of deeper seating operates independently of the proximity to the lands. It is a property of the cartridge itself. Because we shoot these cartridges in rifles that DO have a leade and very limited freebore we see the effect of proximity to the lands primarily. So, YES---overall we see that seating closer to the lands yields a NET gain in pressure and velocity, but that does not mean that the case volume available for powder is of no consequence. Keep seating deeper, and you will eventually see evidence of rising pressure.

I don't think it's all that difficult to grasp, but if you still can't comprehend it then I give up.
 
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