Do bullets expand in the bore?

[peterotte]

A discussion on another forum has lead me to ask whether a bullet shank can expand, in the bore, under the effects of acceleration and inertia due to very high chamber pressure, enough to increase bullet drag in the bore?

It has been said that sometimes, incremental powder charge increases will produce an expected velocity increase to a point when the velocity increase becomes dramatic, no increase or even decreases. Is this caused by erratic combustion at the high end or bullets being 'bumped up' in the bore? (All bullets will undergo a slight elastic deformation under the effects of a force being applied to the base of the bullet in the bore).

 

[Six O'clock Tactical]

you could do the calcs for hoop stress and use the E for lead... or FMJ, whatever your bullet is. For hoop stress youll need thickness, radius and E. I forget the exact orientation of this calc right now... but out of it will come your maximum pressure prior to plastic deformation (or failure, frankly anything over the elastic curve with the standard 2% offset)

If chanber pressures exceed that pressure, then yes. you are deforming.

As for acceleration, youd need to take a stress block on the bullet somewhere and do an acceleration calc with inertia. once you figure out the magnitude of compression thrughout the bullet, compare it to E and if its greater, the bullet is in the plastic deformation range.

REMEMBER to use bullet MASS, not bullet weight, when doing inertia calcs, and you can probably just use gage pressure cause with pressures normally found in chambers I doubt that the atmospheric 14.7 matters much.

 

[snuffy]

To my way of thinking, most all lead core bullets slug up, or as you put it, bump up, during a normal load firing behind the bullet. Extreme force is applied to the base of the bullet before the front or point can move, causing it to shorten. This causes the bullet jacket to move sideways to better fit the bore.

The exception is the monometal all-copper bullets. A solid bullet can't deform as well as a cup&core bullet. Again, I'm not a college trained math major, just using common sense and 40 years of loading experience.

The phenomenon of chronograph readings slowing down, stopping, or even reversing is common when the powders burn rate has been exceeded. It's the best way to see that pressure maximums have been reached or exceeded,(without having a pressure gun). I've seen it myself, I stop adding powder when I see the gain decrease, whether I'm at book max or not.

As for the higher pressure causing more slugging of the bullet, I'm sure it does. This would increase drag, or friction in the bore.

 

[peterotte]

I was thinking that plastic deformation would not come into play as the bore supports the bullet. But the soft lead core way well deform permanently. Now I am wondering whether that would influence the accuracy of some bullets - as in groups that tighten up at higher velocity (and hence chamber pressure)?

P.S. I am waayyy to rusty to do the calc's myself. I was hoping to interest someone else into doing it. The calc's would be quite complicated. Someone with suitable software would be able to do it with ease.

Regards
Peter

 

[peterotte]

The phenomenon of chronograph readings slowing down, stopping, or even reversing is common when the powders burn rate has been exceeded. It's the best way to see that pressure maximums have been reached or exceeded,(without having a pressure gun). I've seen it myself, I stop adding powder when I see the gain decrease, whether I'm at book max or not.

So, not only do we have action strength, case strength and primer strength, we have bullet strength too, in setting the max safe operating pressure of a gun!

Would that be why my hornet groups with cast bullets starts to open up at higher velocities, even though the captured bullet shows no sign of flame cutting or lead stripping? (Capturing the bullet will 'hide' any bullet upset).

Regards
Peter

 

[ZeSpectre]

Well the entire point behind a Minié ball was that it would expand more tightly into the barrel. I don't know if modern rounds do that as well but it certainly is possible.

 

[The Bushmaster]

Lead hollow base bullets will expand when the powder is burned behind it.

 

[Art Eatman]

If the bullet is a smidgen less in diameter than the bore, it will be expanded to fill the bore--but that's all that will happen. If the bullet is exactly the same diameter as the bore, it can't do anything.

If the bullet is a smidgen larger than the bore, it will be slightly elongated.

But for all practical purposes, the bore diameter ain't goin' nowhere.

, Art

 

[Jim Watson]

A lead bullet will "slug up" to fit the barrel and fill the grooves when hit by the sudden ignition of black powder. Doesn't work with progressive smokeless, this is not a steady state phenomenon for which calculated effects make my head hurt.

 

[1911NM]

IIRC, the term is obdurate.

 

[Jim Watson]

Ectually, 1911NM old chap, if you are going to be pedantic about it, the term is obturate. Obdurate means stubborn.

 

[GP100man]

you can down load store bought boolits & theyll lead the heck out of your barrel ,then apply more pressure(powder) & the leading will decrease to a point!!!! been there done it to many times.


GP100man

 

[esheato]

Close...it's actually obturate.

 

[brickeyee]

Calculate the acceleration on the bullet and see how much internal force is being generated.
You are usually well into the plastic deformation zone of the lead core.

 

[moooose102]

i'll just go by the reloading books and not worry about it. my dad always said "dont sweat the small stuff- life is to short". and he was right, he died at 58. and i'm 51!

 

[1911NM]

ok, ok...I had a stuffy nose that day...just came out sounding wrong. I stand humbly corrected.

 

[peterotte]

Thanks for that brickeyee . I would assume that bullet drag would start to increase somewhat once the strength limit of the jacket has been exceeded. This is very interesting! I had never thought about it before - even though I knew about cast bullets obturating in a revolver. I just loaded up and when shooting!

 

[243winxb]

http://en.wikipedia.org/wiki/Obturate Obturate, i always thought pure lead in shotgun slugs obturate much faster than cast alloy bullets. Do cast bullet really obturate to fill the bore? Maybe if the are under grove diameter to start with. I have recovered many cast alloy bullet, the base of the bullet looks deformed on some, not because of obturation, but because the lands have forced the correctly sized cast bullets metal to flow, from the pressure of the lands, to the base of the bullet. If a cast alloy bullet is wearing a gas check, then the pressure/velocity can go higher. Just how i see it.

 

[ranger335v]

I never knew just how much a lead bullet would obturate until I dug some fired SWC/GC 250 gr. moderately hard cast .429" bullets out of an old sawdust pile to see how deep they penatrated and how they looked. Much to my surprise they looked very much like a long wadcutter! The truncated cone points had set back so far that at first I didn't realise I had my own bullets in my hand! That experience led me to doubt that precise sizing of case bullets is really all that important.

Yes, it also happens to some jacketed/lead core rifle bullets, but not all and not so dramatically. It won't happen at all with solid copper bullets nor some other "premium" heavy jacketed bullets such as Nosler Partitions or Soroccos, so it is imperative that they properly match bore diameter to shoot well.

 

[dagger dog]

I think the laymans term is upset, to thicken and shorten. Obturate from obtrude, to thrust out, or extrude.