Why does this work?

[1shot]

Brain damage time! I've been hurting my poor head on the following for sometime and could not find an answer in the "Stickies"..
Conventional wisdom gained from various sources and my own (limited) experience bears out that in general terms, we need a boolit larger than bore size to be effective.
For example: My Marlin rifle barrel slugs exactly .308" so why is it that I am able to safely fire a boolit sized .311 without the barrel bursting? Simple question for some, right? :veryconfu

Question#1.
WHY, does a boolit need to be so much larger (even one thou) than slugged diameter? I mean, if the rifling is crammed full of lead how much "fuller" can it get and what possible gain is to be had by doing so?

Question#2.
On firing/sizing in the barrel, where does all of this extra mass go? Does the boolit simply elongate, or somehow compress into itself on some level causing increased density and the phenomenon of increased effectiveness when .002" or better over bore size?

All I know right now is that it works- but I'd love to know why...
Cheers!
1Shot

[looseprojectile]

Conventional wisdom tells me that you have started a thread that will get many arguments.
My wisdom tells me that I will use any advice and ideas that work for me .
Soft lead boolits slide down the hole in the barrel ahead of the boolit far easier than bullets do.
Boolits are generally shot at less pressure and velocity than bullets because bullets are much harder and better withstand the forces of pressure and rotation.
Velocity is one thing, accuracy is a totally different animal.
It is my understanding that boolits do enlongate some and compress just a tad when fired. I just ignore those facts and adjust my load to where it shoots well.
When working up a load for a rifle I use a boolit that will fit nicely in a fired case from that rifle. Most of the time such a boolit will be several thousandths larger than the groove dimension of that rifle. No problem.
When you get an ideal combination of alloy hardness, lubricant, boolit size, powder selection and weight, with good velocity and accuracy you have arrived.
Then you will do the same thing to dozens of other guns. It never ends.

Oh and welcome to Cast Boolits.

Life is good

[MtGun44]

Solids do not increase in density when compressed, they are incompressible unless they
have voids. The lead pushes into the grooves and a bit ahead of and behind the boolit
ends. This is a VERY tiny amount of lead to displace. Lead is soft and maleable and is
easily deformed compared to the high pressures in the gun barrel. Also, lead alloys have
a very low coefficient of friction against lubricated steel. All normal auto crankshaft bearings
are lined with a tin-lead alloy called babbit which is there to provide low friction when the
steel and bearing are in contact at startup. Once the bearing is spinning at normal speed
a wedge of oil builds up and there is zero contact between the bearing and the steel.
I cannot prove that the coefficient of friction of a lead boolit alloy on a barrel interior is as
low as a oiled crankshaft, but it should be similar, esp with a thin coating of a oil-wax type
of lubricant from the bullet it self and from previous boolits.

If you have a .308 boolit the area of the base is 0.07451 square inches. If you have a light
load that develops only 1/3 the normal pressure of a full power factory round you will have
about 20,000 lbs of force on each square inch (20,000 psi) so the force pushing the boolit
down the barrel is just a tad under 1500 lbs. This is why the boolit moves SMARTLY down
the barrel and that is when your load is 1/3 of the normal factory loads which are safe to
shoot forever. The force on a .308 full power (pressure) factory load in most calibers would
be about 3 times this or 4500 lbs. The will push a copper alloy jacketed bullet down the
barrel fast enough to leave the end at very high velocity, like 3000 fps in a normal .30-06
with a 150 gr bullet.

Have you ever slugged a barrel? Try pushing a .311 boolit into your barrel some time, with
it lightly oiled and the barrel, too. You will be able to very easily tap it into the muzzle and
then you can push it down most barrels with a real hard push or very light taps with a piece
of wood or a small hammer. Once you have done this, examine the displaced lead which may
take some magnification, depending on your eyes and your age. You can see where it pushes
into the grooves and ahead and behind the boolit.

Hope that helps some.

[AZ-Stew]

#2 first...

Some of the force applied to the boolit by the barrel (yes, the barrel squeezes the boolit) us used to elongate the boolit, some of it smears boolit metal into the boolit's lube grooves, some of the force engraves the rifling lands into the boolit shank. The force is distributed and goes to the areas (volumes) of least resistance.

Additionally, the boolit moving down the barrel causes the barrel to expand slightly. Think of a snake eating a mouse. A PhD engineer I worked for 25 years ago taught me that any time a force is applied from one object to another, both objects deform in proportion to the force and to their flexibility. Since the boolit metal is softer than the barrel steel, the boolit is subject to more deformation than the barrel. But take my word for it, BOTH deform. The barrel returns to its original state after the boolit passes, and the boolit regains some of its diameter after it leaves the barrel.

The powder gas pressure applied to the base of the boolit spreads the boolit base (assuming the pressure pressure force is greater than the strength of the boolit base) until it fills the bore, thereby sealing it against gas blow-by which acts like a cutting torch (high temp gas under pressure concentrated on a small area). This reduces barrel leading.

Q #1...

If you launch a bore-diameter boolit slightly off-angle, you open a gap between the boolit and bore, through which gasses can flow causing leading. A larger diameter boolit (by .001 -.002-3) provides a better seal and reduces boolit yaw, providing better accuracy.

Regards,

Stew

[303Guy]

Solids do not increase in density when compressed, they are incompressible unless they have voids.Ah but they do have voids - at the crystaline level and they can and do compress. High stress can cause atoms in the crystaline structure to realign and form a different crystal while chrystals themselves can re-align to close voids or compact more densely, but for the most part, malleable metals will simply displace when 'squashed'.

Barrels do indeed deform with the passage of the boolit by by a miniscule amount. however, the barrel will deform by a substancial amount behind the boolit due to the gas pressure. Perhaps this expansion, which will progress slightly ahead of the base of the boolit needs to be compensated for? But very importantly is throat fit to align the boolit with the bore.

[runfiverun]

they don't have to be bigger my 358 win gets 358 boolits my 223 rem gets 224 boolits and my remington 8mm gets 323 sized boolits.
my 308 however likes 310 sized boolits with 301 noses.
squeezing down 20 bhn boolits .002 is only gonna raise your pressures about 7-8 hundred psi.
you can do it easily in a press and i seriously doubt you are creating 25k worth of pressure.

[Marlin Hunter]

Brain damage time! I've been hurting my poor head on the following for sometime and could not find an answer in the "Stickies"..
Conventional wisdom gained from various sources and my own (limited) experience bears out that in general terms, we need a boolit larger than bore size to be effective.

Question#1.
WHY, does a boolit need to be so much larger (even one thou) than slugged diameter? I mean, if the rifling is crammed full of lead how much "fuller" can it get and what possible gain is to be had by doing so?




...and...
If a boolit obturates, why do they need to be over-sized?

[longbow]

First, even a relatively light load develops more pressure than the yield strength of the strongest lead alloy.

The highest yield strength lead alloy you are likely to find is about 12,500 PSI for heat treated lead/antimony alloy so when you are shooting loads that generate 20,000+ PSI the lead will lose the battle and extrude without causing undue pressures.

question #1: Not only does lead have some elasticity, barrels are not perfect. My view is that as the lands engrave the boolit they cut and compress, so a groove diameter boolit may be squished slightly under groove diameter and all it takes is a few tenths of a thou to allow 20,000+ PSI gas to scoot by and do damage to the boolit. Also, since barrels are not perfect, my feeling (opinion) is that a boolit slightly larger than groove diameter stands a better chance of sealing because it will swage more evenly to suit the barrel and the remaining elasticity may help accomodate barrel imperfections.

Question #2: Yes the boolit does compress a very small amount increasing density but mostly it will elongate and/or metal will be displaced into lube grooves, off the base, etc. Again there is a small amout of elasticity as well so a little (very little) bounce back.

I find that recovered plain base bullets groove diameter or even 0.001" over groove diameter show gas cutting in my .44 mag Marlin, .308 Win and .303 British. When at least 0.002" over groove diameter with same loads there is little or no gas cutting.

Whatever the exact reason, the larger boolit seals better.

That's my story and I'm sticking to it.

Longbow

[lwknight]

Ah but they do have voids - at the crystaline level and they can and do compress. High stress can cause atoms in the crystaline structure to realign and form a different crystal while chrystals themselves can re-align to close voids or compact more densely, but for the most part, malleable metals will simply displace when 'squashed'.

Barrels do indeed deform with the passage of the boolit by by a miniscule amount. however, the barrel will deform by a substancial amount behind the boolit due to the gas pressure. Perhaps this expansion, which will progress slightly ahead of the base of the boolit needs to be compensated for? But very importantly is throat fit to align the boolit with the bore.

You are correct about the compression of solids. It will not likely happen in the barrel because of barrel squeeze or even from chamber pressure but can occur on impact if the boolit is in such a position as not to simply liquify and splatter.

Example: A boolit shot into hard clay when recovered will be noticeably harder than it was when it was fired. I mean a lot harder too.

The boolit in the bore simply elongates. Thats all.

[Bret4207]

...and...
If a boolit obturates, why do they need to be over-sized?

Because they don't all obturate to the same degree. You smack an undersize for the throat boolit really hard and you'll get more relative obturation than you do gently ejecting a throat fitting boolit with a different charge. Fit the boolit to the gun and obturation is not needed or wanted.

Why do they need to be oversize? Define "oversize". IMO my "oversize" and someone else may differ. I prefer a boolit around .002 over groove size in SOME guns. In others most anything at least groove size works and in others a very narrow range of sizing variance seems to be required. Change alloy or boolit design and the requirements change. Ie- boolit fit.