Crooked launch: Dynamic Fit gone awry.
- Thread starter Ian
- Start date
Ian
Notorious member
Thanks. Dynamic fit is one of the factors I have found of critical importance to cast bullet accuracy but also one of the most frustrating to discuss because it is so difficult to quantify.
A fired bullet with clear evidence of having engraved with its geometric center line oblique to the line of the bore is as close as I can get.
A fired bullet with clear evidence of having engraved with its geometric center line oblique to the line of the bore is as close as I can get.
RicinYakima
High Steppes of Eastern Washington
It took me a while to get my head around what Ian was talking about. When he showed pictures of recovered bullets, then I could understand it.
Ian
Notorious member
It was probably Remington brass, I did my best to wear out a box of 20 at a out the time I would have been testing the XCB bullet in my LR-308.
I've played the thicker brass game to extremes and while it can fix some problems, it isn't the whole answer to the alignment problem (believe it or not). Someone asked if there was enough room for the bullet to yaw as much as the engraves indicated and the answer is no....but that begs the question of how then? Well, once it starts crooked a little bit, inertia keeps driving it that direction until finally the bullet begins to deflect true to the bore again, so the point of the nose can be considerably off the bore centerline with only a slight initial (static) misalignment.
This is why static trueness of cartridge and chamber (tolerance stacking and a chamber cut straight) and minimizing cartridge runout (static bullet alignment) are often so important to accuracy.
Except. There's always an except, isn't there? WHEN the bullet shape is just right for the system, the alloy is if the appropriate ductility, and the powder pressure curve is tuned to match, you can sail that bullet at the throat from fifty thousandths away, several thousandths off-center, and a couple degrees crooked and it will still group under one MOA. I've measured runout to prove two degrees and figured the off-center based on tolerances of some loads and seen the results consistently on target.
Ever since I finally figured out the dynamic fit secrets (not secrets really, but just starting to understand just what the fit/alloy/powder relationships mean to each other has taken me the better part of a decade) I've gotten a lot less picky about loaded neck clearance and for most of my loads (1.5 MOA is just fine to me for ammunition which must be universal to many different rifles) I don't even bother to turn the thick side off of the case necks unless I'm really fine-tuning a target load.
I've played the thicker brass game to extremes and while it can fix some problems, it isn't the whole answer to the alignment problem (believe it or not). Someone asked if there was enough room for the bullet to yaw as much as the engraves indicated and the answer is no....but that begs the question of how then? Well, once it starts crooked a little bit, inertia keeps driving it that direction until finally the bullet begins to deflect true to the bore again, so the point of the nose can be considerably off the bore centerline with only a slight initial (static) misalignment.
This is why static trueness of cartridge and chamber (tolerance stacking and a chamber cut straight) and minimizing cartridge runout (static bullet alignment) are often so important to accuracy.
Except. There's always an except, isn't there? WHEN the bullet shape is just right for the system, the alloy is if the appropriate ductility, and the powder pressure curve is tuned to match, you can sail that bullet at the throat from fifty thousandths away, several thousandths off-center, and a couple degrees crooked and it will still group under one MOA. I've measured runout to prove two degrees and figured the off-center based on tolerances of some loads and seen the results consistently on target.
Ever since I finally figured out the dynamic fit secrets (not secrets really, but just starting to understand just what the fit/alloy/powder relationships mean to each other has taken me the better part of a decade) I've gotten a lot less picky about loaded neck clearance and for most of my loads (1.5 MOA is just fine to me for ammunition which must be universal to many different rifles) I don't even bother to turn the thick side off of the case necks unless I'm really fine-tuning a target load.
Ian
Notorious member
One more thing, when seating considerably off the lands and loading for high velocity, consistent neck tension, or effective net "bullet pull" at launch becomes super-critical. I've shown targets over the years where I noted that groups were sorted according to felt pressure on the press handle and the difference in POI can be pretty dramatic.
Snakeoil
Well-Known Member
"Is it only me though that see a flaw in the second picture that looks like there was a void in the bullet that got exposed?"
I noticed that, too. Something else I noticed is in the top photo, it appears that the base/gas check area is either bent or offset of the bullet axis. Thought that was strange. Could be optical illusion. But in the second photo, that same area does not look in line either. In both photos it is most apparent on the left side. Maybe all this is cause by impact in the trap.
But getting back to your point, I thought it was odd that nobody mentioned what looks like a semi-melted hole on the left side in the top photo. I thought it was so obvious that it was some kinda newbie trap. Kept my mouth shut (well, fingers still, actually).
I noticed that, too. Something else I noticed is in the top photo, it appears that the base/gas check area is either bent or offset of the bullet axis. Thought that was strange. Could be optical illusion. But in the second photo, that same area does not look in line either. In both photos it is most apparent on the left side. Maybe all this is cause by impact in the trap.
But getting back to your point, I thought it was odd that nobody mentioned what looks like a semi-melted hole on the left side in the top photo. I thought it was so obvious that it was some kinda newbie trap. Kept my mouth shut (well, fingers still, actually).
Snakeoil
Well-Known Member
How about CNC turned bullets made from extruded bar stock? If I had a machine shop with CNC equipment, I'd be tempted to source the correct lead wire alloy and set up a machine to make bullets. Cutting tools would last forever. I'll bet you could make them for a very reasonable cost, too. Especially if the machine was sitting dormant a lot.
Ian
Notorious member
Have you ever tried to turn lead alloy on a lathe? Little things like altering a groove or facing off a gas check shank is ok, but I can't see doing a whole bullet.
Swaging is the way to go. Some people have swaged fresh castings while they're still soft and claimed it helped. Dan from Mountain Molds did some swaging for precision rifle and generated a decent amount of data that showed it didn't make any statistical difference.
I find that a very light bump sizing in a die shaped like the bullet to ensure perfect roundness and concentricity (and a square gas check base), together with careful weight sorting, is good enough for my purposes. YMMV.
Swaging is the way to go. Some people have swaged fresh castings while they're still soft and claimed it helped. Dan from Mountain Molds did some swaging for precision rifle and generated a decent amount of data that showed it didn't make any statistical difference.
I find that a very light bump sizing in a die shaped like the bullet to ensure perfect roundness and concentricity (and a square gas check base), together with careful weight sorting, is good enough for my purposes. YMMV.
Snakeoil
Well-Known Member
Yes, I have turned lead in a lathe. It is fraught with challenges. If the allow were hard enough, it would probably work, but it would require some serious development work. I could see a CNC type machine developed that cast the bullet and then moved to another station where the bullet is then swaged in the die before it is ejected. Need some bored millionaire to put up the funds to develop it.