Effects of OAL on velocity?

E

Ervin

Moderator
Can anyone provide a link of someone who tested how accuracy or velocity changes due to different OALs and bullet seating depths in rifle ammunition?
Or if in fact it does change mainly because of inconsistently seated bullets.

Im interested to find out what a 0.010" difference would make in velocity atleast.


At the bottom of this page is some data that says a .060" difference in bullet seating only equals 20fps difference.
http://calguns.net/calgunforum/showthread.php?t=255240
 
Here's a link to a thread in which the first post is a letter from Berger discussing it for their VLD shapes. Most benchrest shooters I discussed it with say a 0.010" difference in seating depth is about the smallest at which they've ever been able to detect a difference.

There are several approaches to this. One is to try different depths with a load light enough to be safe touching the lands (about 10% below what is safe with the bullet at normal SAAMI COL). Then tune the load. Unfortunately there is an interactive dynamic, so another approach is to find an accuracy load, then tune depth to see if it gets still better, and if it does, then retuning the powder charge to see if it gets tighter, still.

In the 1995 Precision Shooting Reloading Guide, Dan Hackett describes how, in changing bullets to a 50 grain Nosler BT in his 220 Swift, he accidentally turned the micrometer the wrong way on his seating die and wound up with 20 rounds 0.050" off the lands instead of 0.020" off, as he'd expected would be best because, well, lots of people said so. To his surprise, this gun, which had never previously shot better that 3/8" 5 shot groups, turned in two 1/4" groups and two bughole groups in the 1's with that "wrong" seating depth.

As to velocity, that goes up very roughly as the cube root of peak pressure. So, a chronograph can give you at least some idea what the pressure difference is due to seating depth in your particular gun. Below is a graph taken off some very old 1965 data for a round nose bullet in .30-06. There is also a pressure plot on RSI's site showing a 6 PPC bullet's pressure dropping 20% in going from touching the lands to 0.030" off the lands. So this depends on bullet shape and position.

In the plot below, the study authors suggest the drop in pressure coming away from the lands (left side of plot) is due to increasing gas volume bypassing the bullet as it goes forward into the throat. When it finally is deep enough into the case, pressure rises again because the amount of space the powder is confined in gets small enough for the added confinement to overwhelm the gas bypass influence.

Obviously the exact depth at which you see these effects will depend on the freebore length in your chamber and on the capacity of your cartridge case and the diameter of the bullet, as that determines what percent of the powder space an additional 0.010" deeper seating depth takes up. In other words, the amounts vary dramatically with the particular cartridge.

I believe the Speer manual says they'd seen seating a 9 mm 0.1" deeper nearly double pressure. In a .300 RUM it might make 3 or 4% difference in pressure and maybe 1% difference in velocity.

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"Effects of OAL on velocity? "

Sorry, it just ain't calibrated all that well, far to many variables for it to ever be. You'll have to do your own experiments IN YOUR RIG to learn anything useful.
 
Im interested to find out what a 0.010" difference would make in velocity atleast.
Click to expand...

This is nothing personal, but who cares?? Unless you are loading for long range HUNTING. Speed means nothing it is just a unit of measure (one you can use to see if your are close to a published load, and even then not really since they are using a universal barrel and not a rifle or at least not your rifle and same lenght.)

There are two schools of thought, the first is to load that cartage to the max and hope the bullet reaches the animal before it knows it is dead. The scecond is accurate shot placement and small groups.

If you are shooting for groups (target) you will want to finesse the bullet to just the perfect sweet spot of your rifle. (correct speed, OAL, rifle twist and bullet weight and length and amount of powder.)


My rifles require a jump to the lands, this is generally 0.050 from the max OAL for the chamber. I use a Hornady LNL OAL gauge and the bullet I will be loading for to find out what is that MAX OAL (FOR MY RIFLE, IT WILL BE DIFERENT ON YOURS.) Then I back off that Max OAL by 0.050 inch for the OAL and usually start 0.50 to 0.80 grains under the Max and work up my load by 0.20 (2 tenths of a grain) grain. Most accurate loads tend to be 0.50 to 0.40 grains under max (for my rifle).

This works for me and I do not worry about the speed out the barrel, but how the load groups on the target.

Just my 5 1/2 cents.
Jim
 
Looking at the graph in Unclenicks post #2 it's obvious that in bottlenecked rifle cartridges, the deeper you seat bullets (farther from the lands) the LESS pressure and velocity you will get. Yes, the low point for pressure was a full 1/4" off the lands, and even when pressure started rising again because the powder space was considerably lessened, the pressure didn't get back to the maximum at even 1/2" off the lands. Nobody I know or have even ever heard of loads bullets this far from the lands.

Straight walled pistol cartridges are the exact opposite. Every increment the bullet is seated deeper in the case ADDS to pressure and velocity.

Therefore, the safe and effective way to load for rifle cartridges is to seat at maximum length (for throat and magazine box) until safe maximum pressure is reached, and then seat bullets deeper until the accuracy sweet spot is found. You may lose 25 fps or so, but if the load is more accurate, you're still better off.
 
Another way to look at it is if you set COL for a pressure minimum, you can add more powder. That raises remaining pressure at the muzzle, even if the peak is the same, so average pressure is raised, increasing velocity. You see the same thing with adding powder to get back to a given peak pressure when using bullets with dry lube coatings, like moly. More velocity at equal peak pressure.

The problem is, how do you find the minimum pressure COL? The graph I posted was for a round nose bullet (only numerical data I had) in a 1903 Springfield rifle, but a spire point can find the pressure minimum a shorter distance off the lands. RSI's information on their Pressure Trace instrument has a pressure plot of a 6 PPC round's pressure dropping 20% just from coming back 0.030" off the lands. Unfortunately, you need instrumentation to know where it will happen with your particular bullet and chamber and whether that's a practical objective for you or not?
 
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