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.



P

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.

https://www.24hourcampfire.com/ubbt...nce-due-to-seating-depth-barnes-120-ttsx

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.

Thanks for the definition and the info.

Our hobby certainly has its share of unsubstantiated lore, but I don't think that this fits in that category.

Vic in Va posted a graph from two very reputable sources, Floyd Brownell and Harold Vaughn. So I think that there is data to support the idea. I have both of their publications, and do remember seeing the graph.

It would be very interesting to compare the USMC data, and see if we can figure out why we have the discrepancy.

Agreed. I don't have the data, am going only from memory. It was a time when "Everyone knew the repetitive chambering of cartridges was causing rifles to blow up".

Mr Boone,
I'm guessing our friend GySgt G was involved in that test?

Denton and Vic in Va,
I'd like to know more about that adaptation of Brownell's paper. Lame' iirc was a thin walled, closed cylindrical vessel. I'd like to add another credible source to look up. His full name escapes me, but his last name was Watson, and his handle was OK Shooter. He self published his experiences playing with an Oehler PLB and the 30-06, changing variables and recording their effect on pressure and velocity. What I recall is that he observed a "J" shaped curve when plotting seating depth vs pressure with a significant increase in pressure occurred when you approached the lands.

OKShooter was Stan Watson, and it appears that I'm one of the few that remember him as a google search only turned up my previous posts here.

I don't recall the source, but there is one formula for shotguns (thin walled) and a more complex one for rifles (thick walled). That may be the source of the correction. I have both formulas somewhere around here.

Brownell was using Tektronix equipment, and was trained by some friends of mine when I worked there. I don't know for sure, but I believe he was using an odd crossbreed system: A constrained rod into the chamber, like the piezoelectric and copper crusher system, with a small strain gauge on the rod.

Looking at the graph, it extends from the bullet seated into the lands to the bullet newrly 1/2" back from the lands . That's a lot.

LOL! I think in engineering terms, thick wall is considered a vessel with wall thickness greater than 1/10 of the vessel diameter. Cartridge cases (rifle, shotgun and pistol) are generally considered thin walled.

Right.

I should have been more clear. The formulas apply to the barrels of shotguns and rifles, rather than to the cartridge cases.

Straight wall case increase in pressure as the bullet is seated deeper

You know it! Hope you are doing well.

Base to ogive, not tip.

Tight barrel. Been there. Normally, tight barrels are more accurate but slower. Loose barrels are faster but less accurate.

Nope.

Different OAL.

As a matter of practice, I have always started load development close to the lands, usually.005 off the lands and seated deeper if this doesn't produce the results I want. This way I have the highest pressure at my initial load and am reducing pressure as I back off the lands.

But with a Weatherby, it's a different process, because of the free bore. Doing load development a few years ago for a friend's 30-378 produced higher velocities as I seated deeper into the case. This follows mule deers note that pressures increase as you seat deeper if your initial load was over .250" off the lands.

Yep, & improved accuracy.

Hmmm….While the term “ogive” is often used to describe the particular point on the bullet where the curve reaches full bullet diameter, in fact the “ogive” properly refers to the entire curve of the bullet from the tip to the full-diameter straight section — the shank. Understanding that the ogive is a curve, how do you measure a seating depth to it?

Asking for a friend, not being a smartazz. Happy Trails

I use a Sinclair Ogive Gauge

Hornady comparator for me, but the same idea.


https://www.hornady.com/reloading/precision-measuring/precision-tools-and-gauges/lock-n-load-bullet-comparator#!/




P

Oops…posted pics to wrong thread.

Well, at least you were able to post them in a manner that they can be read. I have no idea what I was doing wrong. Thanks!

If they were boob pics, don’t be sorry.

The world needs more boob pics.

This is what you should get. The initial drop in pressure and velocity as you move off the lands is due to the ease of engraving the bullet with a running start. The increase past that is due to loss of case capacity/smaller starting volume.

The magnitude of the two effects changes with bullet diameter, because the amount of case capacity lost per distance set back increases as the square of the diameter.