Author Topic: pressure, deformation, and rifling (Read 745 times)

ScatterGunner · « **on:** December 09, 2004, 03:36:58 PM »

this has been bothering me for a while;

when the cartridge is fired, the bullet is driven into the rifling, if properly designed, the pressure doesnt exceed the elastic strength of the bullet, but, the effect of the time/pressure process and rifling does definitely exceed the ultimate compressive strength since the rifling remains on the spent bullet.

so the question is how does the chamber pressure not exceed the elastic limit in terms of the bullet not being deformed, yet, there is enough pressure exerted by the rifling to cause a deformation

??

i'm stumped !

sg

De41mag · « **Reply #1 on:** December 09, 2004, 07:19:25 PM »

ScatterGunner;

I can't answer that question but with your avator, the question seems kind of moot. I don't mean to be smart a$$ but I just had to respond. :shock: :shock:
Please foregive me. :wink:

Dennis

safetysheriff · « **Reply #2 on:** December 09, 2004, 07:19:47 PM »

you're in over your head :shock: take a drink, take two drinks, and it'll all feel better in the morning :lol:

want to know where they think deformation might take place because of the pressure vs time curve that they must be witnessing? in cartridges like the .243 where the base of the bullet is (significantly?) behind the shoulder/neck juncture in the brass case. the base of the bullet gets bulged out and then has to be squeezed back to a diameter that will enter the neck of the brass -- per the NRA 'Handloading' book. no, they're not entirely sure that it's happening, but they believe that it is a possibility.

once in a rifle bore the bullet is obviously contained and the jacket must apparently be strong enough to resist both deformation and centrifigal force -- in most cases. some bullets do blow up from centrifigal force before getting near the target. they just come apart in a small cloud....

take care,

SS'

De41mag · « **Reply #3 on:** December 09, 2004, 07:23:33 PM »

SS;

Hope you are feeling better?

Dennis

TimJ · « **Reply #4 on:** December 10, 2004, 05:13:09 AM »

I think it was either Jim Carmichel or in one of Ackley's books that I read they shot a .30 cal bullet through a .270 barrel. They had the .270 barrel specially chambered for a .30-06 just too see what would happen. They were a bit surprized that it shot just fine. If I remember right it did raise pressures a little but nothing blew up.
They did this with great care and it was a remotely fired gun, not something you would want to try with your own .270.

Not that this really answers your questions but I guess it shows just how much softer lead and copper are then steel.

Tim

Donaldo · « **Reply #5 on:** December 10, 2004, 05:26:25 AM »

It will be interesting to see what Fred has to say about this. I have kind of wondered myself about what happens to a bullet just as it starts to enter the rifling.

ScatterGunner · « **Reply #6 on:** December 10, 2004, 05:30:44 AM »

i was kinda trolling for fred.

it looks like there may be some elastic deformation of the bullet as the bullet engages the rifling. if the pressure on the base doesnt exceed the elastic pressure, then the rifling wouldn't cut into the copper jacket. although, there may be some other mechanism at work here i just dont know about.

sg

Fred M · « **Reply #7 on:** December 11, 2004, 12:22:01 PM »

SS.
Well I am not an interior ballistic expert. But here is what happens to a bullet and of course with some exceptions.. When you pull the trigger the primer explodes and drives the bullet into lead and/or throat which on the forward end is supposed to be the same as the bullet before the powder begins pressurization.

The bore at the case mouth or the throat is .0005" bigger than the bullet to get it started into the bore. At that point the bullet is contained and can't further expand or compress and is forced down the bore.

Nothing much happens to the bullet inside the case neck and the primer will not deform it. But if there is a lot of slop in front of the case neck deformation will take place as the powder pressure kicks in. The results of this can be seen on a target when compared to a no slop bore. All of this happens in a fraction of a millisecond.

When the bullet gets into the bore a swaging down takes place after being expanded, further destroying the integrity of the bullet in a poor bore/chamber.

The lands can not resist the gas pressure and the bullet is not constrained by the bore only by the lands. So not much deformation takes place if you want call the land engraving deformation. In a good bore a bullet pretty well stays the same shape as it was made. In spite of its low elastic limit.

Even bullet that are driven at horrendous speed will leave the barrel as is but the air friction heat during the first few yards will blow it up, because the molten lead will not hold it together. The time a bullet is in the bore is not long enough to liqidize the lead in the core.

safetysheriff · « **Reply #8 on:** December 11, 2004, 01:01:30 PM »

Quote from: Fred M

SS.
Even bullet that are driven at horrendous speed will leave the barrel as is but the friction heat during the first few yards will blow it up, because the molten lead will not hold it together. The time a bullet is in the bore is not long enough to liqidize the lead in the core.

Fred,

I don't think the lead is molten -- though it might be; but I think the rate of twist and forward velocity is what blows up those bullets. You can see some complaints about them on www.long-range.com some of the time. the Swift, the .22-250, and some other hot-rocks would be good candidates to blow-up bullets at high rotational velocities. The same bullets at lower revolutions per second will hold together.

Dennis,

All is well, thanks. Hope you are, too.

SS'

Hildy · « **Reply #9 on:** December 11, 2004, 01:15:51 PM »

Hmmmmmmm......

Perhaps I will look down the barrel of my gun next time I fire it to see for myself what happens. I'll let you guys know the results :roll: :roll:

^^^^Just Kidding^^^^Although it isn't really a thing to kid around about :shock:

I have heard of cases where the bullet will come apart in mid air from high twist and fast velocity guns. I believe it was a 243AI in which a guy tried to fire 55 grain Nosler BTs at extreme velocities (4000+fps). Well, when he fired it looked like a shotgun blast to his paper target at only 25 yards.

Yet, I have seen cases where bullets will stay together when traveling in excess of 5000fps. The 22-250AI, 220 Swift, 22-243 Middlestead, and 22 Eargesplitten Loudenboomer will all fire the low grain bullets in excess of 5K. On www.reloadersnest.com there is reloading data for the 22-243 where the guy throws a 30g Berger MEF at about 5250fps and managed groups to fall under 1 inch at 100y. By no means can this info be true but it might be.

We need bullets made of depleted Uranium :-D

Hildy

Fred M · « **Reply #10 on:** December 11, 2004, 01:50:31 PM »

Hildy

This is one reason I like a heavier bullet, slower speed with the least amount of powder. At long range they shoot flatter and are much easier on barrels an throats like these wee high pressure blow torches. Some one used a .378Wby case and necked it down to .257" the barrel was gone before he could actually develope a good load.

A solid bronze bullet will stand the velocity, but 125gr of powder will burn the barrel to shreds in no time. Of course some steel alloys will stand heat better than others but non are immune to that kind of heat.

Hildy · « **Reply #11 on:** December 11, 2004, 02:26:18 PM »

Fred,

Yes, I will agree with you on the barrel torching, but I sure do enjoy watching a groundhog do flips with some of those faster calibers.

The 22 Eargesplitten Loudenboomer which was developed by PO Ackley back in the 50s or 60s was made by necking a 378 Weatherby Mag case to .224 diameter. Over 100 grains of powder to launch a 30-80 grain bullet? I dont think so! That was its main objective though as it would throw 50g bullets faster than 5000fps. I would LOVE to see the chamber pressure on that one. :-D

Hildy

Fred M · « **Reply #12 on:** December 11, 2004, 02:48:16 PM »

Hildy
I saw a lot od Groundhogs do flips with a 22-250 with 55gr bullets that is all I ever used in that rifle. One load one bullet works great. The 25 Hunter with a 75gr V-max will do a flip on a yote as far as I can hit him.
Uses no more powder than a 22-250. It does 3430 ft/sec.

bajabill · « **Reply #13 on:** December 11, 2004, 04:49:30 PM »

the compressive stress on the base of the bullet will be less than the stress at the surface interfacing with the rifling, because there is more area at the base than at the rifling. Also, the compressive stress causes the bullet to try to bulge, but the barrel resists this and the bullet is undergoing a hydrostatic pressure at the rear of the bullet (it can deform toward the front though). Under hydrostatic pressure, there will be no deformation, poissons ratio kicks in. Also, any permanent deformation is the result of plasticity, not elastic deformation.

Fred M · « **Reply #14 on:** December 11, 2004, 05:35:56 PM »

bajabill

Quote

Also, any permanent deformation is the result of plasticity, not elastic deformation.

You are right, my expanation suffers thought.