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How do boolits like this for example https://www.reddit.com/r/reloading/c...gr_hp_in_4570/ start to spin?. This boolit is full diameter so the front of the boolit will engage the rifling and want to start turning yet the rear of the boolit is still in the case, so what happens?, does the boolit twist like a piece of licorice or does it start to spin while still inside the case?
The pressure of the expanding gases forces the brass outward while shoving the bullet down the bore. The bullet starts to spin, but it is not tight in the cartridge case.
I would think it would start spinning while still inside the case. Otherwise, you would have a projectile that would exhibit some kind of 'gain twist' characteristics. This is entirely an uneducated guess, but, an informed one based upon my education garnered from wiser members on this site.
I clicked on the link but did not see anything there ? But thinking on a standard bullet / cartridge situation the expanding gas relieves the neck tension and in the first 1/2" the bullet is moving forward there is very little spin before the bullet is out of the case. This happens in a millisecond.
Jedman
PAT303 -- I may be "missing something" here. The photograph which appeared (pasted, albeit I cropped image here) looks -- to me -- as a plain-Jane rimmed case with a bullet in it.Attachment 253383 When the cartridge is seated, and the priming compound is activated to ignite powder, the bullet starts its journey down the barrel where the rifled lands and grooves are engaged by the bullet -- starting its spinning. The rim at base of case keeps said case from moving.
Again -- perhaps 'cause it's rather early in morn here ;) (?) -- I do not "get" the jyst of your query -- sorry -- albeit, I hope this helps.
What has boggled me is the actual r.p.m. bullets generally spin at. This is of importance for many reasons -- the good vis stability of bullet's flight; the bad that the spinning -- if too fast with, say, lead alloys, may cause the bullet to fly apart from centrifugal force.
A good article on spin, you may enjoy reading, is at http://bulletin.accurateshooter.com/...and-stability/
. A quick formula for determining bullet spin rate is to multiply the Muzzle Velocity times 720. Then, divide this product by the barrel's Twist Rate -- to give you the bullet's RPM.
BEST!
geo
I gather the OP question is "does the nose of the bullet hitting the lands begin the nose rotation while the base of the bullet remains in the case and has not begun to rotate"? I suppose one could mark the side of a bullet, fire it and recover it and hope the lands do not obliterate the mark to see what can be shown. This would be the time to fire test rounds into a snow bank and await spring to recover the bullets.
More to the point though is the rate of twist of the 45/70, which appears to be the cartridge shown, will vary from 1 in 18 to 1 in 30 inches depending upon maker. That suggests that the base of the bullet would enter the lands after the nose has only traveled an inch or so down the bore. The pressure on the base would tend to deform the base and compress the base against the nose for a very tight fit of the bullet in the bore. With the slick sided bullets, there is no relief groove as the lube grooves provide and the displaced lead would likely cause significant deforming of the base and also perhaps trailing edge fins on the base of the bullet. That would seem to have more affect on the flight than any perceived twisting of a bullet if I understand the question. But do shoot some bullets into a snow bank to see what really happens. Otherwise it is all guessing and speculation. I would think there are a lot more things to be concerned with than just being curious.
I can't say about this particular instance.
I have recovered boolits that I fired from handguns where the the rifling marks on the side of the boolit were much wider at the front of the boolit.
This indicates that the momentum of the boolit, as it engaged the rifling, caused a shear that made the rifling marks in the sides of the boolit wider at the front of the boolit. As the boolit continued into the barrel, the marks became narrower as the shear was reduced until the marks were the same width as the lands of the barrel.
A lot depends on the strength of the lead and it's ability to resist the shear. Also the coating, if any, would have some input here as well.
If the apparent shear continues to the base of the boolit, you will normally get leading in the barrel due to gas blowing past the boolit.
The timing of all this happening suggests that the boolit may begin spinning while the base is still inside the case.
This almost certainly happens in the case of jacketed bullets that are long for caliber.
The case may well release the bullet due to expanding pressure from the gasses produced by the powder burn.
Much depends on the individual cartridge and firearm.
If the rifling is sufficient enough to grab the bullet and the bullet strong enough to resist, the bullet will start to spin in the case; jacketed bullets. But if the rifling happens to be shallow and or rounded, and the bullet maliable enough then it will jump the rifling until enough of the bullet engages to overcome the forward thrust of the powder and begin spinning; cast bullets at high pressure.
It probably happens this way more than we realize because most of us don't inspect our fired bullets with any regularity nor with the aptitude to be able to interpret what's being looked at. I suspect too that with softer alloys, the alloys most commonly used for all of it the most extreme rifle applications, there will be some level of backfilling of the groove caused by skipping the rifling.
I would venture to say that when the case expands out against the walls of the chamber to seal the gasses off from the breech that the case has effectively turned loose of the cast enough for the cast to spin up asap.
The nose of the bullet is tapered, and I doubt if it is jammed into the rear of the rifling upon being loaded as this would cause astronomical pressure when fired. So the bullet has to move forward some to engage the rifling, but as stated by others the gas pressure expands the case and releases the bullet. I would anticipate accelerated wear to the throat and first couple of inches of the rifling using such a large projectile, especially jacketed.
I regularly inspect fired rifle bullets and pistol/revolver bullets.
Little rifling slippage is evident in rifles and autoloading pistols that usually do not have excessive freebore. The bullet enters and is gripped by the rifling in a short distance while it is still moving slowly. All evidence indicates the entire bullet spins immediately or very nearly so to the extent any slippage is difficult to see. The land marks at the front are the same width as those at the rear.
Revolvers are the exception due to the long jump bullets must make before hitting the rifling origin. The bullet goes straight for a short distance before the rifling starts to spin it because it has built up considerable speed. This is why the grooves left in the bullet are wider at the front.
I have not seen this in rifles and autoloading pistols. Disclaimer is that I have none with long freebores.
Under these conditions I have not found that soft bullets skip or tend to have significantly widened front rifling marks so that seems to be pretty much a non factor for rifles and autoloading pistols.
The shear strength of the outer layer (the thickness equaling the Rifling groove to Bore dimension) of the boolit is no where near what would be needed to actually twist the entire boolit.Quote:
does the boolit twist like a piece of licorice
We call it Skidding, when a boolit fails to start turning after it engages the rifling.
Search "skidding" in the castboolits website to learn more about what's happening ...and the causes of it.
Yes, I'd say there's spin inside the case but it would be so insignificant that it would hard to detect, measure or compute even if it does exist.
Take a Marlin M1895 with a 20" barrel. Assume a bullet of 1" length. Throw in some throating in front of the chamber. Say the bullet is seated .5" into the case for argument. The bullet base will "bump up" upon hitting the rifling and the bullet will shorten somewhat. I'm betting that there would be very little twist imparted at this point in time and the bullet base would probably already be out of the case. There are just too many variables to throw in in the case of the rifle but mechanically, the sequence of events indicates that it should start to spin.
Pistols are another case. Just my opinion.\beagle
Maybe I am oversimplifying in my own head but here is how I am looking at this situation.
1" boolit, 1/2 of which is inside a smooth sided case neck.
Using simple numbers, lets say a 1 in 10" rate of twist. 1/2" of boolit will travel 1/2" before making contact with lands/grooves.
As the gas expands the case neck expands sealing the chamber and releasing the boolit.
10" broken into 1/2" segments is 20.
360 degrees of rotation divided by 20 gives 18 degrees of rotation within a smooth sided case prior to making contact with the lands/grooves.
In my simple mind, the smooth sided case neck will have little or no impact for that 1/2" as the lands and grooves are surely going to more force on the projectiles. It seems to me the first half of the boolit making even and concentric contact with the lands and grooves would have more effect upon accuracy than the back end rotating 18 degrees within a smooth sided case neck. Can somebody help me out here?
I agree with rintinglen, case expands to "release" the bullet. Perhaps the extra long bullet will experience some twist (as a softer metal the front portion may start to spin a bit sooner than the rear), but I believe this is a different situation than skidding, where the entire bullet looses "grip" on the rifling and skids. But all this is just speculation as I've never experienced bullet skidding, nor see any indication of bullet twist. Ninety nine percent of my cast bullet shooting is in handguns with most in the 10-12 BHN category so perhaps one proficient in rifle cast bullet design can help...
You have to picture the FORCE driving the bullet is from behind. Forget about the rpm's at that point, the spin of the bullet is relative to the length it travels. Picture it in super slow motion. You are jamming a lead slug into the twisted grooves of the rifling. The spin is imparted as it starts down the rifling, There is very little torque applied to the base of the bullet at that point relative to the force upon the base of the bullet driving it forward. If you had a VERY VERY long bullet, say maybe 6" it may come into play but I wouldn't think anything within what is used would have a problem.
Well that question has got you all thinking hasn't it, I appreciate the replies.
The case has to expand before the bullet goes anywhere. The pressure will expand the case enough to let it turn. A chevrolet engine only has a bout .002 thou clearance in the main and rod bearings. Check to see how much a case expands after being fired. It is generally more than that and is why cases need to be resized (most of the time).
The boolit simply moves forward with a twisting motion. Either way, it is moving in the neck. Its movement in the neck is exactly the same as its movement in the bore. It doesn't spin in the neck.
My personal expectation is that the case expands during the combustion and lets go of the boolit. Thus allowing it to spin freely.
However, lets say this is not the case, and the boolit is still firmly crimped. When the boolit starts spinning from engaging the rifling, this should cause the case to start spinning as well. Easy way to see if this is what happens: take a magic marker and make a mark on both the case and the breech. If the marks are still lined up after firing, you know the case didn't spin.