Home Forums Hunting & Shooting Ask The Gunwriters Help me understand the relationship between velocity and seating depth

I recently developed a load for my brother’s suppressed T3 Superlite 7mm-08. His previous load, at 2.156”, chronoed 2790. His new load has a different OAL but everything else was constant. The new OAL is 2.306”, and the velocity is 2840. At 2.306” the bullet is .005” from the lands.

Help me understand the relationship between depth and velocity. I always thought that, absent the bullet into the lands, that a smaller combustion chamber created higher velocities. Such is not the case in this instance, unless I’m doing something wrong, which is always a distinct possibility.



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I can’t add but am curious as well. I’d be thinking the same thing you are.

Just for clarification....

Was previous load development suppressed also?

And what was the round count on the rifle for previous load development vs load development this time?


I always understood it that seating deeper into the case gave you higher initial pressure or spike because of smaller case volume, but lower overall pressure.

Curious myself.

I don't care how fast the bullet is going, I just want it to go where I send it.

Thanks for the link Whttail. Interesting.

With the shorter COAL length the bullet is farther from the lands and moves freely before engaging the lands therfore lower pressure.
The longer COAL is much closer to the lands and engages the lands sooner creating higher pressure, thus higher velocity

Yep. The longer "jump" to the lands allows the bullet to accelerate more before being engraved by the lands, resulting in lower peak pressure--which usually occurs shortly after the bullet enters the lands.

Only when a rifle bullet is seated deeper than about 1/4" from the lands (or even more) does pressure--and hence velocity--rise.

I believe that part of the origin of the theory that deeper seating increases pressures is due to assuming handgun and rifle handloads result in the same sort of pressure curve. They don't, for two reasons: Handgun cartridges have relatively little powder capacity, and many handgun powders are not only faster-burning, but not "progressive" powders, like those used in most modern rifle loads.

Progressive means the powder starts burning out slowly, then burns faster as the bullet travels down the bore. Many handgun powders are degressive, meaning they start out burning faster--in order to get pressure up quickly, resulting higher muzzle velocities in shorter barrels. Some are essentially neutrail, meaning the burn at about the same rate during their trip down the bore.

But it's been proven over and over again in pressure labs that seating rifle bullets slightly deeper tend to reduce pressures as the jump to the lands increases--which is why Roy Weatherby started using "freebore" in his cartridges, to flatten the pressure curve.

This also also why muzzle velocities tend to drop a little the farther a bullet's seated from the lands--and rise when they're seated closer to the lands.

To an extent you are creating an expansion chamber effect. Increase in velocity is not out of the question.....again, to an extent.

This makes sense.

I must say that it irks me that he has better velocity than I do. I am, after all, the source of ammunition.

If you jam the bullet into the lands, you get higher pressure.

If you seat the bullet far back into the case, you get higher pressure.

In between, there is a zone where you get lower pressure, and hence lower muzzle velocity.

All that said, if the temperature of the barrel and the ammo was not carefully controlled, you might just be seeing the effect of temperature variation.

The Lyman manual has an article on this. Longer jump equals lower overall pressure assuming all else is equal.

This. As MD said, somewhere around 0.250" off the lands is where the relationship between jump-to-lands and powder capacity results in minimum peak pressure.

Fifty feet per second difference is within the realm of shot to shot variation for a typical (not great) load. A fouled bore can increase velocity 50 fps. Need to test the load more to see what it averages out to be. Any change in components can do this as well, like a different lot of primers, bullets, powder, cases, etc.

This is a good explanation
https://www.hornady.com/team-hornady/ballistic-calculators/ballistic-resources/internal-ballistics

This ^^^^^^^^^^^^^^^^^^^^^^^^

I have shot the exact same loads on different days and got different velocities just from temperature variation, in the OP's original post this was not addressed

The variance is more likely due to other factors, possibly a bit of carbon fouling , also a suppressor can add 50 fps

Am I missing something here? Case trim length for a 7mm-08 is 2.025” and COAL for Barnes loads runs 2.735 to 2.80”

Seems like the OP length is a wee bit short, but what do I know….

The USMC ran a test, that I participated in, of seating the projectile progressively deeper in the case and actually pressure testing the round according to SAAMI procedures.

This was done because of a concern that multiple chamberings of a cartridge (think unloading then re-loading) would push the bullet back and create an over pressure situation, ultimately resulting in a catastrophic destruction of the rifle.

We never saw (measured) a pressure increase.

Perhaps it is another case of "Unfortunately, the facts are not supportive of the fairy tale".

This was a test of one type of cartridge (M855). I don't know if other cartridges/propellant types would have similar results.