bullet seating & velocity

06 May 2004, 01:35

MINNESOTA_DICK

Would someone explain to me how bullet seating depth changes affects the velocity of a bullet. thanks

06 May 2004, 02:13

stubblejumper

I have found through chronograph testing that the deeper that a bullet is seated,the less the resulting velocity.

06 May 2004, 03:00

Bob338

Do a "Search" here. There was a long discussion on the effect, mostly theory.

In the real world, I'm working with a new 338WSM now developing loads. As you seat the bullets deeper you reduce the size of the powder chamber in the case, much the same as raising the compression ratio in an automotive engine. The higher the compression with the smaller chamber, the higher the pressure and the higher the velocity. You get the same effect with heavier brass (less internal capacity.)

In loads fired just this last Monday I'll give you the velocity and seating depths stated in the bolt-to-ogive measurement. The load was H4350-74g behind Nosler 210 Partition. You be the judge.

The longest measurement, the ogive closest to the lands, was 2.520", 2916fps. At 2.510", 2921fps. At 2.500", 2925fps. At 2.490", 2931fps. The was over an Oehler 35P.

THAT has been my experience consistently notwithstanding other "theories" expounded here in previous threads.

06 May 2004, 07:55

CDH

The basic assumed theory is that the deeper bullet seating, all else being equal, raises pressures due to the reduction in initial case capacity. To load bullets deeper to the same pressures, you have to reduce powder charge, thereby reducing velocity.

In my experience, the difference is very slight, and easily on the order of differences in chamber/barrel/powder lot/primer.

Just one more reason to work up YOUR loads using the books as a guide, not as a bible.

06 May 2004, 08:00

Curly

My results were different than yours, Bob. I was experimenting with 168gr TSX bullets and 77.5 grains of RL22. The only thing that changed was seating depth. The deeper they were seated, the lower the velocity. I was shooting over a CED Millenium chronograph with 4 shot groups. Extreme spreads were all less than 20.

OAL 3.379", V = 3,177
OAL 3.391", V = 3,186
OAL 3.410", V = 3,206
OAL 3.486", V = 3,220

So, based on the results you had and the opposite results that I had, I would say that there are not any hard/fast rules regarding velocity and seating depth. BTW - I would have expected higher velocity at the deeper seating depths.

06 May 2004, 17:16

stubblejumper

Quote:

As you seat the bullets deeper you reduce the size of the powder chamber in the case, much the same as raising the compression ratio in an automotive engine. The higher the compression with the smaller chamber, the higher the pressure and the higher the velocity.

Not totally true.In the case of firearms the combustion chamber is not limited to the cartridge case.The bullet is acted upon by the combustion pressures long after it leaves the case.Therefore the barrel is the combustion chamber.
For the record my own test results agree with Curly's results.

06 May 2004, 22:51

volk

Bullet engraving force is most significantly for pressure as changing case capacity via seating deapth. Less O.A.L. produce more velocity for bullet in engraving moment - and less pressure. Kinetic energy of bullet replace pressure necessary for bullet engraving.

08 May 2004, 02:54

Hot Core

Quote:

Would someone explain to me how bullet seating depth changes affects the velocity of a bullet. thanks

Hey MD, I guess ALL THE ABOVE cleared it up for you?!?!?!

So,here is a bit more for an added confusion factor.

From my experience, you will see higher relative Pressure, then lower relative Pressure and then higher relative Pressure as you vary the Seating Depth using the exact same Load.

Lets say you start out doing your Load Development with the Bullet Seated so it is jamed 0.010" into the Lands. (More than this and the Bullet will normally slip farther into the case as you close the Bolt.) Work up your Load to a SAFE MAX watching for all the normal Pressure Indicators.

Then begin Seating the Bullet deeper into the case. As you get away from the Lands a few thousandths, the Pressure will "typically" go down a bit - for awhile.

Then you reach a point where the "Peak Pressure" begins to rise again due to the smaller "initial chamber" volume.

This is easier to see(happens quicker) in straight wall cases like a 444Mar, 45-70, 357Mag, 44Mag, etc., but the same thing applies to bottleneck cases as well.

08 May 2004, 03:07

<eldeguello>

Quote:

In the real world, I'm working with a new 338WSM now developing loads. As you seat the bullets deeper you reduce the size of the powder chamber in the case, much the same as raising the compression ratio in an automotive engine. The higher the compression with the smaller chamber, the higher the pressure and the higher the velocity. You get the same effect with heavier brass (less internal capacity.) In loads fired just this last Monday I'll give you the velocity and seating depths stated in the bolt-to-ogive measurement. The load was H4350-74g behind Nosler 210 Partition. You be the judge. The longest measurement, the ogive closest to the lands, was 2.520", 2916fps. At 2.510", 2921fps. At 2.500", 2925fps. At 2.490", 2931fps. The was over an Oehler 35P. THAT has been my experience consistently notwithstanding other "theories" expounded here in previous threads.

How many rounds per seating depth did you fire to give you these averages?

From your figures, it appears obvious that there is no statistically significant difference in the velocities you obtainsed, since the shot-to-shot variation between IDENTICAL loads is usually greater than the mere 15 FPS you report! In short, your results do NOT prove that the miniscule reduction in case volume, and resultant increase in loading density one obtains by seating a bullet a little deeper, has any effect on chamber pressures or bullet velocity at all!!

(NOTE: These conclusions pertain to RIFLE ammunition, NOT pistol cartridges that use much quicker powders!!)

09 May 2004, 04:28

MINNESOTA_DICK

Well, I guess you guys really got me confused now.
So, I give up on it.

Just a note, I can't reach the lands on any of my guns using light varmint bullets, so I have just settled on seating all guns (223 rem, 17 rem, 220 swift, 243 win) at 3/16 into the case neck. The way I do it is I draw the case on Quick-cad, the measure the bullet length and insert the bullet 3/16 into the case drawing and measure the overall length of the drawing.

The reason I asked the question is because when checking out the velociy of my loads I can never get what the good books publish, am alway on the shot side when it comes to published velocity.

Thanks vor the replys, and information.

09 May 2004, 08:41

sonofagun

I suspect ("perhapsfully") the answer may depend very much on the powder you're using?

Sounds like something to discuss with a ballistician at one of the powder makers. If anyone does so (or has already), please let us know what you find out.

09 May 2004, 13:15

sonofagun

I emailed ricciardelli to get his opinion here.

09 May 2004, 15:23

ricciardelli

Oh oh!

There is no definitive answer to this question.

The variables have all been mentioned above. And each will change the velocity of the load one way or another.

The first response from most will be something along the lines of, "If you seat a bullet deeper in the case you are going to increase pressures, therefore you are going to increase velocity."

HOWEVER, this is not true, because it depends on where the bullet was seated before. If it was crammed into the rifling before, the pressure will probably decrease.

If you had a tight roll crimp before and now you don't, the pressure will probably decrease.

Like I said, it depends on many, many variables. The cartridge, the chamber of the firearm, the length of the barrel, the weight of the bullet, the style of the bullet, the powder used, etc.

The only way to find out is to use your firearm over a reliable chrony and see what happens...

10 May 2004, 15:46

franke

Dick,
This just recorded a month ago:
300 Savage cartridge.
OL 2.570-1673 FPS
OL 2.60 -1697 FPS
OL 2.630-1704 FPS
Frank

11 May 2004, 17:00

turtle71

Quote:

The way I do it is I draw the case on Quick-cad, the measure the bullet length and insert the bullet 3/16 into the case drawing and measure the overall length of the drawing.

Wouldn't it be easier to measure case length & bullet length then add 'em together & subtract 3/16"? Unless you're using boattails & want 3/16" of shank held by the neck. Even then it'd be easier to hold the bullet into a sized case & caliper OAL then subtract 3/16".

As an aside anyone know of downloadable case drawings in .dwg format or similar?