design and alloy

popper

Well-Known Member
To renew the alloy/bullet design subject. Go back to the 'marshmellow' down the barrel example. Assume the bullet does NOT fit throat/freebore/bore perfectly. No case/neck for this discussion. Push on the base and it must expand into all empty space (incompressible alloy) , nose isn't moving yet. Wall friction keeps the 'skin' stationary for a while. Center (core) FLOWS toward the nose, expanding the nose possibly into the grooves. Alloy FLOWS from throat into bore (reduced diam.). Nose begins to move. Nose & body start to spin but not at the same rotational speed until body is entirely into the rifling. The soft alloy has lots of stress (pressure) and strain (movement) but as it's 'gooey', so no fracture. As to design, the longer the bullet (length to bore ratio), the more differential in inertia, base moves before nose. Initial engraving increases the differential. All this happens quickly but get a good visual/mental picture of what is happening.
Now, harden the alloy some. What changes? Nothing! The alloy just FLOWS less easily/rapidly! What can occur? It is assumed flow occurs geometrically perfect (proper throat/freebore/bore). We also assume that the alloy CAN flow and NOT create stresses that will distort the bullet. The highest bullet acceleration occurs when it is just in the bore ( directly related to base pressure) therefore most of the groove spin force occurs there. Note: velocity is the summation of acceleration and time. So now we want minimal flow (distortion). So far we haven't spoken of lube or friction. Friction coefficient is related to the materials at the junction. Lube reduces the 'effective' coefficient. Actual friction is determined by the force applied at the junction. Difference in friction around the bullet can cause distortion. Lube grooves. Strength of a rod is determined by material strength and diameter - so - grooves weaken the bullet. Empty grooves weaken the compression resistance of the grooves. So shallow grooves (with angular or rounded shape) makes a stronger bullet.
Nose shape? Why not just flat (wad cutter)? BC - aerodynamics! But remember rod strength from above. Bore rider? Lots of metal in the bore that doesn't have to flow from the throat but if too strong it won't flow into the grooves. So we want an alloy that will flow through the irregularities of the entrance and then NOT flow. Un-attainium! Best we can do is experiment with alloy and real powder pressure curves (burn rate).
End of physics lesson.
As we are talking HV mostly, let's talk GCs. Interestingly pressure is the same on all surfaces of a closed container. If the base is not square with the bore, there is a radial force on the base which can cause the body to be forced to one side of the bore. Additionally, the GC must go through the 'flow' process with no distortion or upsetting the alloy flow. IMHO, annealed GC eliminated forming stress, softens the Cu closer to alloy and allows better flow.
 

Ian

Notorious member
Good explanation, and yes, a person really has to figure out how to visualize this stuff in their head in order to "see" how fit, alloy, and pressure curve all work together dynamically. I was taught that velocity is the derivative of position (on a straight vector) with respect to time, not summation of acceleration and time, acceleration being the second derivative of position with respect to time...not that it matters to the discussion.

I would also move that eliminating as much of the supposed "bore riding" part of the bullet as possible from the design would be of great benefit to HV cast shooting. Narrow lands don't support the nose for beans at even medium velocity and will allow the whole bullet to tilt and dig into one side or the other of the bore, which is yet another reason that poor dynamic fit by design causes some wild theories about twist rates. The less "bore diameter" you have, the less you have to worry about bumping the bullet to fill the grooves in a concentric fashion, or alloy flow characteristics through the core under pressure. Ogive radius is also a big consideration when referring to torpedo-shaped noses, think about metal pushing outward on the ogive from the inside, how shape will affect flow, and think about nose radius fitting into the ball seat shape. The disadvantage of the torpedo-shaped bullet is the difficulty in steering it from behind with sloppy chamber neck tolerances. Here we have to have a harder alloy so the bullet can self-align without getting bent in the throat, or we simply design a bullet with minimal bore-riding section which is closely fitted to the throat angle and has a body diameter sized just a fraction of a thousandth smaller than throat entrance diameter, or freebore diameter if any.
 

fiver

Well-Known Member
that's also why we tend to slide the powder speed to the right, and to push the bullet in as far as possible from the start.
we are doing just about everything we can to move the not moving object into the little round hole without changing it's shape.
 

popper

Well-Known Member
Force (on the base) = mass * acceleration (of bullet). So acceleration follows the pressure curve in the barrel.
a = dV/dT so dV = a/dT integrate both tho get v = Integral (a/dT). Just looking from the other side.
 

popper

Well-Known Member
Powder - pressure effects.
We tune loads with different powders to get different effects. Why? Let's look at the pressure (my version) curves.
Force (on base) = weight * acceleration. Alloy strength takes some pressure to get it 'flowed' into the bore as previously discussed. At some pressure, the bullet is 'flowed' into the bore. It has some value of acceleration. Some of the energy from the pressure is used to 'flow' the alloy, the rest is used to accelerate the bullet (some is for recoil, barrel expansion, etc.). In the above diagram (slightly slow powder) A is the bore entry time/pressure. The bullet is accelerating in the bore (good). As the powder burns, pressure decreases as does acceleration (volume increases). That's the peak of your load, more powder doesn't create more velocity. The steeper the part A of the curve the more the bullet gets slammed into the bore. B is the pressure required to just accelerate the bullet.
Let's look at what we desire for a curve.
Slow/continuous burning powder that gets the bullet into the bore and continues to create pressure to accelerate the bullet in the bore.
Yea, ideal but not what we get usually. We usually get a faster burn with EXCESSIVE pressure (needs a tougher alloy), oversized bullet/jammed to the lands to let this pressure build up. The excess pressure is needed to keep the bullet accelerating down the bore (gas volume). P * V=constant.
The best solution is what solid rocket motors use, a chunk of propellant that only burns on the surface at a constant rate and provides the constant pressure. We don't get that. Therefore, manufacturers use retarding coatings in an attempt to get the same effect. A problem arises, uneven burning. For several reasons, powder burns better under pressure. So enter 'filler', used to increase initial pressure - as well as oversized bullets/ jamming the lands. Does filler just hold the powder against the primer? Kinda. If you've ever seen primer testing, they have a blast front that can move the powder around causing inconsistent burn.
Just some added info to wrap your mind around.
 

Kevin Stenberg

Well-Known Member
Sorry for doing a replay from so far back. I have been trying to get my head around some new concepts.
In post 25 Ian showed the lands and gruve markings left from forcing a bullet into the start of a barrel. And I have read and understand a bullet fitting tight to the lands and gruves.
All of my guns are bolt action or single shots. If I load light contact to the lands of any of my rifles I can't completely close the bolt. So when you have bullet contact with the lands. Are you talking a heavy contact just barely touching or .001 jump.
 

fiver

Well-Known Member
there is 2 different directions of contact.
you have forward contact, and you have dimensional contact.
in a normal average speed discussion your talking about a dimensional contact.
in other words you have a nose that is .001 or .0015 larger than the bore diameter and it takes a firm amount of engraving when chambered.
this is important on bullets with long bore ride nose designs.
what your doing is taking the nose from just gliding along on the rifling and making it act like part of the engagement system.
nose diameter is very important for a couple of reasons.

now as I understand your question.
your asking about forcing a portion of the full diameter forward into the ball seat area or even further and jamming into the rifling.

on my first run with the XCB bullet I used full contact of the bullet, only .001 neck tension and allowed the bullet to seat itself when I closed the bolt.
this pushed the bullet back in the case about 15 thousands and I worked my loads.
this was with full length sized cases that needed to be blown out to fit the chamber.
now.
in the same rifle when I worked with the 165-A which has a lot more bore contact on the nose.
I fixed the bullet with .003 neck tension, worked the alloy slightly for diameter and stopped the front drive band about .005 short of hitting the end of the ball seat area so there was some slight movement forward.
I also used the diameter to engrave the rifling and for the front drive band to fill the ball seat area.
both of these things help align the bullet with the barrel.

trying to maintain a light touch of lead to steel is the most difficult of all because your talking a .000 tolerance from bolt face to a fixed point on the bullet.

it's much easier to measure and maintain a jump on 'fixed ammunition' than it is to use the 000 or the seat on closing method.
it's also a little more forgiving.
it also lends itself to some tuning with neck tension and length to lands.
 

Will

Well-Known Member
Thought I would bump this up for newer members to read through.
I’ve read this whole thread 3 times and pick something else up every time.
 

fiver

Well-Known Member
since we are rejuvenating this thread.
I'm gonna link it to something that at first glance is gonna make the reader go wtheck is this? and what does it have to do with this discussion.
well.
what it has to do with this is,,,, it totally illustrates with a vvvvverrrry good picture of a bullet that has slumped up from the rear but shows how the whole bullet can suffer from the affect.
if you note the BHN of the alloy it will also illustrate how a high BHN isn't gonna save your butt when attempting high velocity or high accuracy.
it also illustrates why a low BHN squirty bullet is maybe gonna let you down just as quickly too without the proper support, and launch.
when you see that picture think about the consequences of leaving too much free space, or not figuring out the proper centerline alignment....
 
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