Seating depth…what does it do?

[Jager]

Interesting and useful info. I’m just trying to learn here. Curiosity I guess.
It would seem to me that the pressure curve would also be different for jump versus jam. Even if total pressure is the same. For jump does pressure build more slowly because of less initial resistance then increase sharply when the bullet contacts the lands? As opposed to jam where it would seem initial pressure would rise very quickly because there is increased resistance from the beginning? Of course I have no idea if that would matter but it’s still interesting

You'd think there might be an initial pressure spike as an unjammed cartridge initially fires, a brief fall-off of that pressure as soon as the bullet releases, and then a second spike a fraction of a millisecond later when the bullet engages the rifling. But I've never seen a chamber pressure curve from a piezoelectric transducer that looked like that.

What we do see are a very rapid, nearly straight line rise in chamber pressure regardless of whether a bullet is jammed or jumped. A jammed bullet will invariably present a higher maximum pressure than its un-jammed counterpart - the pressure curve is indeed very different between the two. But the profile is going to look similar.

We tend to think of bullets, especially jacketed bullets, as being "hard." But they're not, really. They engrave into the rifling, or obturate, quite easily.

It all happens very fast, max pressure being reached within 2 to 4 tenths of a millisecond. By the time the bullet has moved an inch, the show is largely over. The bullet still has lots of pressure behind it as is still being accelerated... but pressure is rapidly declining as the bullet advances and the bore volume behind it increases.

It's really a remarkable sequence of events, when you think about it. And when you consider the precision with which we're able to place that bullet, way out yonder... almost magical.

 

[Turpentine]

You'd think there might be an initial pressure spike as an unjammed cartridge initially fires, a brief fall-off of that pressure as soon as the bullet releases, and then a second spike a fraction of a millisecond later when the bullet engages the rifling. But I've never seen a chamber pressure curve from a piezoelectric transducer that looked like that.

What we do see are a very rapid, nearly straight line rise in chamber pressure regardless of whether a bullet is jammed or jumped. A jammed bullet will invariably present a higher maximum pressure than its un-jammed counterpart - the pressure curve is indeed very different between the two. But the profile is going to look similar.

We tend to think of bullets, especially jacketed bullets, as being "hard." But they're not, really. They engrave into the rifling, or obturate, quite easily.

It all happens very fast, max pressure being reached within 2 to 4 tenths of a millisecond. By the time the bullet has moved an inch, the show is largely over. The bullet still has lots of pressure behind it as is still being accelerated... but pressure is rapidly declining as the bullet advances and the bore volume behind it increases.

It's really a remarkable sequence of events, when you think about it. And when you consider the precision with which we're able to place that bullet, way out yonder... almost magical.

I thank you for the explanation. It’s a little goal of mine to learn at least one new thing every day. I’m ignorant enough that I doubt I’ll ever run out of topics to learn about. I would assume that rapid drop in pressure as the bullet travels down the barrel is why we see improved performance, to a point, with slower powders in longer barrels. They continue to burn and push longer in a longer barrel which can add velocity. Or on the flip side why shorter barrels and faster burn rate powder tend to play well together.
This is off topic somewhat. I build and shoot ARs for myself and others in various calibers. They can be hard on themself and on brass. So one topic that I try to wrap my head around all the time is the relationship between burn rate, port size, port pressure, projectile weight, dwell time,and the subsequent drop off in pressure for the different length gas systems and barrels. Plus how all of that effects bolt unlock and extraction timing. The idea is to optimize rifle life, smoothness, and brass life. I have a pretty good handle on it, to the point that I pretty much can tweak, tune, and optimize the system. I also know what just doesn’t work well together. But the ”Why” escapes me sometimes. Perhaps it might be a good question for another day and forum.

 

[wkdickinson]

Regarding pressure, and how it changes as one makes increasing changes in seating depth... here is that information for the seating depth test I uploaded in post #90:


View attachment 1389576

If you start from a bullet jumping, and march towards the lands, you'll see very slight decreases in pressure and velocity at each increment, as case volume slowly increases. That will then reverse as you approach hard jam, as the bullet suddenly has much more resistance to initial movement.

Those pressure curve and velocity changes are absolutely part of the picture. But they are very, very slight for each increment of change.

Here, "Predicted Velocity" and "Predicted Pressure" are from QuickLoad. CSTO is cartridge-shoulder-to-ogive.

Maybe I am missing something here, but I don't see "predicted" pressure, "predicted" velocity or "actual" velocity increasing as your COAL increases, in the data you presented? I see decreases at each increment. What don't I understand?

 

[BartsBullets]

Maybe I am missing something here, but I don't see "predicted" pressure, "predicted" velocity or "actual" velocity increasing as your COAL increases, in the data you presented? I see decreases at each increment. What don't I understand?

As the bullet is seated deeper in the case it gets slower. That’s what his chart shows.

 

[Coyotefurharvester]

2.2610 coal 2947 fps
2.2875 coal 2935 fps
Predicted pressure also drops as PERCENTAGE of case fill drops. As cartridge length increases.

 

[wkdickinson]

As the bullet is seated deeper in the case it gets slower. That’s what his chart shows.

But that is not what the data in his table shows? The data shows that as the COAL increases, the predicted pressure/velocity decreases. The observed velocity also does the same thing.

 

[gunsandgunsmithing]

In bore time can be more or less while both peak pressure and/ or muzzle velocity remain the same due to different acceleration rates and pressure curves. You can't just say that a specific pressure or velocity equates to the bullet spending the same amount of time in the bore. Obviously the time difference is tiny but this amount of time can be huge in terms of tune. Massive really.

 

[BartsBullets]

But that is not what the data in his table shows? The data shows that as the COAL increases, the predicted pressure/velocity decreases. The observed velocity also does the same thing.

You’re right my mistake!

 

[Jager]

Maybe I am missing something here, but I don't see "predicted" pressure, "predicted" velocity or "actual" velocity increasing as your COAL increases, in the data you presented? I see decreases at each increment. What don't I understand?

You're seeing it correctly... pressure and velocity decrease as COAL gets longer. As the bullet is seated further and further outside the case, moving towards the lands, there is more and more case volume at each increment.

What will reverse that is if/when you hit hard jam.

 

[wkdickinson]

You're seeing it correctly... pressure and velocity decrease as COAL gets longer. As the bullet is seated further and further outside the case, moving towards the lands, there is more and more case volume at each increment.

What will reverse that is if/when you hit hard jam.

OK, I get that. I thought your data went through and included "hard jam", now I understand it - thanks!

 

[David Milisock]

300WSM, 220 Matchkings, Benchmark barrel, factory trigger, Vortex 6-36.
Shot 3x3 shot .008 stepped seating test. Then the best one back to back here.
19 shots into a new bullet.
Depth tuning makes your first bullet hole your target. Never had so much fun never being happy.
View attachment 1387638
View attachment 1387636View attachment 1387637

You should try color management in graphic output processes.

 

[David Milisock]

Seating depth would change
1) Effective case capacity
2) Time between initial ignition and time when the bullet engages the rifling
3) Initial bullet speed when said bullet engages the rifling
4) Initial case pressure rise and peak pressure
5) Bullet barrel time +/- time in 2)

BINGO!
I'll add this, (assuming perfectly uniform reloaded ammunition) the physical properties of the rifle change on a microscopic level with each round fired.

Bad shooting barrels sometimes can be polished up to become a good shooting barrel, all good shooting barrels will shoot well until they don't.

Some combinations of components will shoot well without touching the lands, other combinations will shoot well only when they tough the lands. Others like varied distances.

It's your rifle, they're your loads, experiment and find out. Just remember error induced by you and uncontrollable ambient conditions make an absolute conclusion nearly impossible. Multiply that by the distance of your test firings.

I have three rifles that shoot on average 1 1/16" groups at 300 yards. I only load to fit the magazines. If I seat to a different positions near the lands the powder charge would need to change but that does not mean that my average groups would be smaller. I say average because if I shoot 20-5 shot groups over any period of time the average will always be 1 1/16".

If a world champion shooter wins an event one day with a world record group, they can with the same rifle and loads the next week not even place and shoot their worst group ever with that rifle.

Consistency in the loads, the testing and the shooting technique is all you can do. Ambient conditions will be what they are and have their effect.

 

[BoydAllen]

What I read here is a fair number of shooters who have not done simple experiments that anyone could do, and many that have.

Personally, my first move is to find a powder charge by doing a pressure test. I should mention that I have had the proper tools and have mastered the techniques to be able to find where a bullet touches the rifling, and the length of jam (old school definition, length that a bullet is pushed back to when seated long). For this test I use a seating depth that is around .006 into the rifling. and start with something near the lowest powder charge in the manual, firing one shot per charge, increasing in increments that are suitable to the case volume. For something the size of a PPC that is .3 gr. I have quite a bit of experience reading pressure signs and I keep increasing the charge to the point where I see a little more pressure than I want to use on a regular basis.

I fire all shots on the same target, using wind flags and keep track of where each successive bullet hole is located so that I can see if there are any clusters to investigate with three shot groups.

I have found that if you make small changes in charge weight and seating depth that there have been no problems with pressure spikes, with powders that are optimal for a particular cartridge bullet combination.

In summary, I suggest that you test,. and I have found that loading at the range facilitates this process.

Given the number of shots that are usual for most big game hunts, I think that it would be reasonable to remove the firing pin assembly from your bolt and run all of that ammo through the rifle to make sure that there are no problems. If everyone did this before a hunt, I do not thing that there would be any stuck bullet issues in the field.

 

[Doom]

It’s interesting that predicated pressure and predicted velocity decrease and increase with seating changes that suggest a point of impact shift on the target and a chronograph variations beyond the normal ES but why do we not see it on the target ?

Not sure how Boyd stumbled into this thread! But if I may propose something here to explain what you have observed. When you consider short range accuracy (100-200yd) the requirement for accuracy is not a strong function of velocity. In fact the BC of the bullet or a 100 fps variation in muzzle velocity as a very small affect (you can verify with a ballistic calculator). What determines the final point of impact is the direction the bullet is traveling when it leaves the barrel and various effects of random dispersion. Probably the largest factor in quality components and barrels becomes consistent in the barrel pointing, hence vibrational effects. However those velocity variations that are negligible at 200 yds become increasingly significant as the range increases. At some point they become the determining factor in the dispersion.