My 6.5CM journey

shuz

Active Member
If it's all about fit, how come Sevenfan can get fantastic groups at slower speeds but when he speeds them up, the groups turn into shotgun patters?
My understanding is that the fit was the same at both speeds!
 

Brad

Benevolent Overlord and site owner
Staff member
The bullet changes shape some under pressure. Different alloys behave different under pressure. Some bullets rely on no change under pressure and others are designed to handle the change.

Fit is far more than just sizing to the bore dimensions. Dynamic fit, the fit under pressure, is often the culprit. Bullet needs to enter the bore straight. Bullet needs to not deform in a manner that causes problems in the throat- like riveting.

A bore rider can do really well at speeds in the 1600 fps range but try to get them to be accurate at 2400 is an exercise in futility. That bore ride nose doesn’t hold up under the pressure very well and the bullet won’t stay concentric.

I think he biggest factor in high velocity cast bullet success is bullet design.
 
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Bret4207

At the casting bench in the sky. RIP Bret.
If it's all about fit, how come Sevenfan can get fantastic groups at slower speeds but when he speeds them up, the groups turn into shotgun patters?
My understanding is that the fit was the same at both speeds!
No, the fit changes with pressure, where the pressure peaks in the barrel, how fast the pressure peaks and a whole lot of other variations. For that matter the shape of the bullet changes with pressure. At his lower pressures his bullet is fitting the barrel and there's a balance point met that gives him good groups. Raise the pressure and the bullet reacts differently and he's pushed it beyond what that particular bullet (and everything that goes with it) in that particular barrel wants. So he gets worse grouping. Dynamic (an object in motion) fit.

ETA- I didn't see Brads post prior to making this one. He covers it pretty well.
 
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RBHarter

West Central AR
One of my "studies" used 3 powders one bullet and alloy.
I ran Red Dot up until it didn't group then I started just be low that point with Unique . I think 4350 was all I had then for rifle powder . What I found was that about where Red Dot peaked for speed was about where Unique started to come together.

. I did have some spectacular failures with fast twist 30s . Leading and group failures . I needed 1900 fps MV and I just couldn't get there and have the bullet expand.

In a 7mm groups in 1-8.5 went flat from 2100-2400 fps but that was patched bullets and I'm confident that a grease and check won't make it .
I haven't exhausted that yet .

It again comes to little groups , going fast , and desired terminal effect . You can have all 3 on paper but you're better served in the woods to pick 2 .
 

fiver

Well-Known Member
when you add in hunting your talking another whole level of performance and a totally different set of expectations.
if your gonna go faster you need less meplat.
that's for on target results not aerodynamics.


anyway if your gonna think about bullet design you have to think about what happens to it when the pressure is applied to the base.
then your next thought is gonna be well i'll just use some lino-type.
yeah,,,, no.
that's the exact opposite thought you should have.
think.. your... way... through... what happens to the bullet.... remember pressure is applied from one direction only.
yeah,yeah yeah it goes in all directions, but the bullet only gets it applied from one.
 

Bret4207

At the casting bench in the sky. RIP Bret.
when you add in hunting your talking another whole level of performance and a totally different set of expectations.
if your gonna go faster you need less meplat.
that's for on target results not aerodynamics.


anyway if your gonna think about bullet design you have to think about what happens to it when the pressure is applied to the base.
then your next thought is gonna be well i'll just use some lino-type.
yeah,,,, no.
that's the exact opposite thought you should have.
think.. your... way... through... what happens to the bullet.... remember pressure is applied from one direction only.
yeah,yeah yeah it goes in all directions, but the bullet only gets it applied from one.
The problem with thinking your way through it is that many of us, me included, either don't factor in things that happen or don't think things that happen even exist. That's where reading the musings of guys like you and Ian (and many others!) come in, 'cuz you thought this stuff out and bring new ideas to the table that the Larrys and Joe Brennans of the world never considered.
 
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Ian

Notorious member
If it's all about fit, how come Sevenfan can get fantastic groups at slower speeds but when he speeds them up, the groups turn into shotgun patters?
My understanding is that the fit was the same at both speeds!

Because every system has limits. After the trigger is pulled, you no longer have what you started with. Add more powder, it changes even more and usually not for the better because the whole set-up isn't correct to deal with the changes that will happen.

You can do a lot of things wrong and still have decent results with wheelweight alloy, common production moulds, Lyman manual reloading techniques, and book loads up to about 1700 fps, but when you start to push things, you find out it stops working so well. Some people blame it on excessive RPM, some on pressure/alloy strength limits, and others on the unattainable perfection of the hand-cast bullet. Each of these is true to a degree, but only part of the bigger picture where a factor not often discussed or understood plays the biggest role in cast bullet accuracy.
 

Bret4207

At the casting bench in the sky. RIP Bret.
Because every system has limits. After the trigger is pulled, you no longer have what you started with. Add more powder, it changes even more and usually not for the better because the whole set-up isn't correct to deal with the changes that will happen.

You can do a lot of things wrong and still have decent results with wheelweight alloy, common production moulds, Lyman manual reloading techniques, and book loads up to about 1700 fps, but when you start to push things, you find out it stops working so well. Some people blame it on excessive RPM, some on pressure/alloy strength limits, and others on the unattainable perfection of the hand-cast bullet. Each of these is true to a degree, but only part of the bigger picture where a factor not often discussed or understood plays the biggest role in cast bullet accuracy.
Come on Ian, don't be shy! I sat here thinking, (scary, I know!), and came up with 3 or 4 possibilities of what you meant by that. Take pity on an old man with a head overflowing with a million other things and give us a hint!!!
 

Ian

Notorious member
I was trying to save some typin' like Fiver back on page one.

IIRC Dimner started a thread on this very subject and it kinda petered out, we should keep it going.

The next thing is that the gummy worm is only gummy at the back end and progressively less so toward the front, how much depending of course on alloy, dynamic fit (particularly the rate of rise of engraving resistance), and how the powder charge reacts to that resistance, i.e. shape of the pressure curve....which comes around again to how easily the bullet deforms which affects pressure curve and pressure affects burn rate, and around and around again. In college they called it Calculus III or Differential Equations. Short version, cast bullet accuracy boils down to the relationships between the alloy, the bullet fit, and the pressure. Get it all right and the bullet leaves the muzzle in good form and flies true, regardless of "rpm" or "peak pressure". Get it wrong and it won't hit a barn from the inside. Most people shooting cast bullets for accuracy are barking up entirely the wrong tree because they haven't thought about what's really happening inside the gun hard enough. Now I've written a lot and given you the answer but really communicated nothing, these things aren't really all that simple to explain.
 
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Bret4207

At the casting bench in the sky. RIP Bret.
Well, maybe you didn't do as bad as you thought 'cuz the line about "particularly the rate of rise of engraving resistance" is another one of those light bulb moments for me. Never considered that aspect before. So, thanks! And yeah, that thread you refer to was something I looked for but couldn't find when I was trying to figure out what you meant. And yes, we should try to keep it going.

Gummy only at the back end...now we're back to thinking in nanoseconds. Sigh...
 
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Ian

Notorious member
What do you think happens to that gummy back end when the solid front end slams into a brick wall at 400-600fps and the pressure spikes from 5,000 to 30,000 almost instantly? Why isn't it so bad if you just back off 100 fps? Is it the RPM? Bad casting? Bullet alloy too weak? How about.....none of the above.
 
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Sevenfan

Member
So, back to my situation and what I have, or more accurately stated, could find, to work with.

- the 8t barrel is not ideal for what I'm trying to do but is what I have
- the bullet I'm using is the best compromise of moulds I could actually obtain (I see the NOE 140 mould and nose sizing dies are now available)
- my Savage 12FV has no throat

Top to bottom, a loaded round, bullet reverse seated in shortened neck (I use chamber gauges), the case in which I tried to reverse seat the bullet (gas check bottomed on neck and could not close bolt).

throat.jpg

A lot has been written since I last checked in so I'm going to spend some time reading and attempting to digest the "Fit is King" topic. I really don't want to invest much more time on my 6.5CM as I will be switching to 7BR in the near future.
 

Ian

Notorious member
I like the looks of the bullet except it looks like the nose is hanging in space. Does the top cartridge in the photo chamber as-is? How does an un-sized bullet fit nose-first into the muzzle? Use a bushing neck sizer die or Lee collet sizing die to give yourself about 1.5 thousandths loaded neck tension and start out with something like 2400 powder. I bet it shoots.
 

fiver

Well-Known Member
does the neck shorten that much when the case is fire formed?

anyway.
a lot of the discussion is about why things DON'T work.
that is half the battle when your trying to go fast.
just like fishing.
most guys go well they just ain't biting.
the ones that usually show up to the dock with a stringer full, go: well i eliminated that portion of the Lake, let's go try something else.
 

Bret4207

At the casting bench in the sky. RIP Bret.
What do you think happens to that gummy back end when the solid front end slams into a brick wall at 400-600fps and the pressure spikes from 5,000 to 30,000 almost instantly? Why isn't it so bad if you just back off 100 fps? Is it the RPM? Bad casting? Bullet alloy too weak? How about.....none of the above.
That's all part of dynamic fit to my addled mind. Finding a balance point between what the specific firearms internal dimensions and style are and what the bullets specifics are and what the powder charges specifics are in that gun. I put the question you ask on the "100fps" down to exceeding the point of balance. I don't have the definitions of all the little pieces parts down yet. There's an awful lot happening awfully fast.

Fiver- We do tend to concentrate on what fails rather than what succeeds, good point. It's the missing links that we don't grasp that keep us from discussing what works I think. And I'm not even trying to go fast! 2200 is about my upper limit. It's pretty easy to fail even there or lower.