Neck tension - looking at it the wrong way?

[VA_XTC_Shooter]

Most of what I have read on neck tension refers to the diameter of the sized brass to create and interference fit with the bullet. The idea is, the tighter it is, the more force required to push the bullet. The AMP seating press is advertised to tell you how much pressure it takes to seat your bullet, inferring that similar seating pressure should result in similar dislodging pressure.

Are we looking at neck tension the wrong way? What I mean by that is, at least the way I understand the explanation, is that it's function of friction between the bullet and the case.

However, during the firing process (per Whelen Vol. 2 pg. 4), it doesn't work quite like that. As the pressure in the case builds, the bullet is not pushed forward with a tight neck. On the contrary, as pressure in the case builds, it expands the neck freeing the bullet to then move forward.

So my questions are, if bullet movement is dependent on pressure and neck expansion, is measuring interference fit and seating pressure the correct approach? I would think the plasticity/ductility (not sure which is correct term) of the brass neck would play a role. Or, is interference fit a close enough measurement that a handloader can measure, so that is why it is used?

Of course, "neck tension" as it applies to interference fit does play a role with bullet movement before firing like in magazine jostling, loading a round in chamber, etc.

 

[jelenko]

as pressure in the case builds, it expands the neck freeing the bullet to then move forward.

How does the gas get between the bullet and the inside of the case so that the gas can expand the neck?

 

[NZ_Fclass]

You are likely correct - that the brass moves and that we can only measure that by interference fit, We anneal the case. It has been proven that consistent ductility is important to accuracy. - University study - dunno - but my scores improved once i started annealing. IMHO neck tension and load length are more critical facors that powder load, Let the target tell you otherwise.

 

[VA_XTC_Shooter]

How does the gas get between the bullet and the inside of the case so that the gas can expand the neck?

pressure, expands neck to chamber walls, then bullet moves. Some gases move around bullet between case neck forward until bullet moves forward and seals against the forcing cone.

 

[Straightshooter1]

pressure, expands neck to chamber walls, then bullet moves. Some gases move around bullet between case neck forward until bullet moves forward and seals against the forcing cone.

The bullet will start to move first if the pressure to expand the brass is greater than the pressure it takes for the bullet's interference to be overcome first. In the beginning of the ignitions cycle, pressure starts at zero and when the pressure reaches, let's say, 30psi, will the brass actually begin to expand before the bullet has begun to move if it takes 30 psi to overcome the interference?

 

[Tommie]

Each time neck tension comes up - I’m thinking of Guffey.

Scarred for life…

 

[VA_XTC_Shooter]

The bullet will start to move first if the pressure to expand the brass is greater than the pressure it takes for the bullet's interference to be overcome first. In the beginning of the ignitions cycle, pressure starts at zero and when the pressure reaches, let's say, 30psi, will the brass actually begin to expand before the bullet has begun to move if it takes 30 psi to overcome the interference?

I don't know for certain, I am just quoting the process from Whelen.


I do wonder if there is a difference between bullets seated where the base is above the neck/shoulder junction vs below. I could see pressure acting differently in those two scenarios.

 

[dryhumor]

Wouldn’t pressure be equal on every exposed surface it can touch? And, the metal’s ability to react and move with pressure’s influence (and heat) would be dependent on its softness and uniformity? Wouldn’t necks of the same thickness, concentricity, and hardness allow a repeatable release point of bullet tension? Brass, softer than copper, provides the ability to release under pressure, and copper, subjected to the same pressure begins forward movement at the time of release. Providing internal case capacity and charge weight are near equal creating a uniform pressure curve upon ignition.

And, wouldn’t the cleanliness and smoothness of bore, and heat expansion of the barrel have an effect on the longitudinal friction of the copper moving through it in regard to the pressure curve being varied? Which in part could be why barrels can speed up after X amount of rounds, even if having been lapped for uniformity? Would X amount of powder charge variance provide more consistent speed (in relation to pressure) as a barrel changed state due to heat or cleanliness (fouling) during longer strings of fire, or 2-3 shots for lighter profile barrels?

Just asking/observing as I see things I do in regards to necks/tension having varied effects on group size.

ETA: Would a variance in neck tension compensate for bore constriction or movement due to fouling/heating? Wouldn’t neck tension variance allow a physical manipulation of the pressure curve to keep it more uniform?

 

[46and2]

Wouldn’t pressure be equal on every exposed surface it can touch? And, the metal’s ability to react and move with pressure’s influence (and heat) would be dependent on its softness and uniformity? Wouldn’t necks of the same thickness, concentricity, and hardness allow a repeatable release point of bullet tension? Brass, softer than copper, provides the ability to release under pressure, and copper, subjected to the same pressure begins forward movement at the time of release. Providing internal case capacity and charge weight are near equal creating a uniform pressure curve upon ignition.

And, wouldn’t the cleanliness and smoothness of bore, and heat expansion of the barrel have an effect on the longitudinal friction of the copper moving through it in regard to the pressure curve being varied? Which in part could be why barrels can speed up after X amount of rounds, even if having been lapped for uniformity? Would X amount of powder charge variance provide more consistent speed (in relation to pressure) as a barrel changed state due to heat or cleanliness (fouling) during longer strings of fire, or 2-3 shots for lighter profile barrels?

Just asking/observing as I see things I do in regards to necks/tension having varied effects on group size.

ETA: Would a variance in neck tension compensate for bore constriction or movement due to fouling/heating? Wouldn’t neck tension variance allow a physical manipulation of the pressure curve to keep it more uniform?

I don't believe brass is softer than copper. We use copper jackets so it can be engraved by the rifling.

 

[gunsandgunsmithing]

Good subject. Glad to see this brought up. There was a good thread on this subject several years ago on brc. The answer there was simply that the lighter object will move first..in a nutshell.

 

[dryhumor]

I don't believe brass is softer than copper. We use copper jackets so it can be engraved by the rifling.

True. I was just thinking that the copper jacket is supported by a lead core though, and durable until engraved by the rifling. The “brass” of the neck, is thinner and unsupported, and more able to move. It takes a lot to ding a bullet, not a lot to ding or deform a case neck.

 

[VA_XTC_Shooter]

I am thinking the AMP seater and other gadgets that measure seating force don't really predict anything for accuracy/consistency. In my mind, the case consistency is more important.

What I mean by that is the rate the case expands to seal the chamber is more important because that plays a role in consistent pressure in the system (less variable leakage). If pressure is same, bullet is pushed through barrel the same velocity.

I read a few posts from a couple years back and @mikecr summed it up pretty well (bullet seating pressure tests don't show a difference in velocity, but neck tension does). So if that's the case, looking at the consistency case mouth/neck ductility would be important. To do this I am thinking you would anneal each firing. Measure the fired case neck diameter. Size the cases and measure neck diameter. The more consistent cases would be those with consistent measurement because they would indicate the same amount of change in neck diameter, thus ductility, so the expansion of the case neck upon firing would be expected to be consistent. Of course, playing with the varying amounts of sized neck diameter would be an option to test, but you'd still want the end result to be consistent.

 

[Alex Wheeler]

I agree that the neck expands first. If the bullet slipped out the gas pressure would equalize on the outside of the neck and the neck would not expand. Plus I have seen when with overly long freebores and bullets barely in the neck, this exact thing happened. The bullet slid out of the neck and no expansion occurred. Through testing probably 10 years ago I came to the conclusion its the bushing that matters not the friction of bullet seating. There is something going on with the friction as well though. You can ruin accuracy with some neck lubes. Too slick is a problem. Remember the K&M force indicator has been around a long time. I think I have had mine about 15 years. So much of this testing has been done.

 

[mikecr]

The bullet will start to move first if the pressure to expand the brass is greater than the pressure it takes for the bullet's interference to be overcome first.

Yes, but that's not what happens. If it was, everything about internal ballistics would be different today.

Consider two matters with this:
1. The entire case presents a lot of area for PSI applied. This expansion assists the order; mouth, neck, body-shoulder, mid-body, shoulder, webs. Mouth & neck are first to expand with significance, as brass is thinnest there, and it's open ended.
2. Bullet bearing is swinging in the wind with ANY neck release from it. So when a neck expands ~0.000000001" (or less) there is no longer any bullet grip at all.

Bullets 'can be' pushed a bit in necks before release,, like if you used a liquid case lube between bearing/neck.
I don't know why anyone would want that.
And neck release can be delayed with real crimping (into a groove). This is not because of increased friction, so seating forces would be completely useless for tension correlations with crimped necks.
Neck release is also delayed with neck sizing beyond seated bearing (FL sizing) for similar reason to crimping.

Given what I describe here, understand that neck tension(what grips bullet bearing), is a force best described in PSI. It is an area of spring back force applied to seated bullet bearing.
Interference fit (in inches), and friction (in push-pounds), are separate and different from hoop tension/stress.
There is currently no device or method with which we can measure neck tension.

 

[AlNyhus]

Through testing probably 10 years ago I came to the conclusion its the bushing that matters not the friction of bullet seating. There is something going on with the friction as well though.

Alex, I don't think most understand what's really happening when they see 'on target' changes with different neck bushing sizes.

Randy Robinett and I have had quite a few discussions about this and have come to similar thoughts on 'why'. When I started using one of Mike Ezell's PDT tuners this season, it was the perfect opportunity to verify our hunch.

Shooters want to measure everything that can be measured...even when it doesn't translate into any 'on target' differences. But give them something that can't be measured but shows up 'on target' and most will invent an explanation that fits with their thinking rather than letting the results guide the way.

Good shootin' -Al

 

[JEFFPPC]

I have been using a seating force press for about 5 years. I could never really see that it improved accuracy. I quit using it this year. I could not see any loss of accuracy. At this stage I will stick with Whelens thoughts.All my bench barrels are throated for minimum bullet in the neck. Way less than 1 caliber.I believe what the target tells me.

 

[ebb]

Good question. I was in a conversation about this and a national champion BR shooter that had a reputation as knowing his stuff, said that all neck tension issues were mute when the bullet was loaded to touch the rifling as it was chambered. I have always been curious if this was the fact.

 

[mikecr]

said that all neck tension issues were mute when the bullet was loaded to touch the rifling as it was chambered.

Likely true.
The starting pressure of a load is affected more with on/off land relationship than normal tension adjustments.

 

[SeabeeKen]

pressure, expands neck to chamber walls, then bullet moves. Some gases move around bullet between case neck forward until bullet moves forward and seals against the forcing cone.

Seems as though it would depend on which takes the least pressure to happen first. If soft annealed brass takes less pressure to expand the neck for release or less pressure for the bullet to over come interference. Seems to suggest 2 pressure curves that intersect at some point. Does this also suggest that bullet could start its trip from the primer impulse before powder ignition?

 

[JFrank]

Good question. I was in a conversation about this and a national champion BR shooter that had a reputation as knowing his stuff, said that all neck tension issues were mute when the bullet was loaded to touch the rifling as it was chambered. I have always been curious if this was the fact.

I don’t believe this to be accurate as with guys who shoot jammed rounds that still test bullet hold/ neck tension and see definite variations on the target.