Max pressure coinciding with max accuracy.

F

Fretka

Guest
At what point does max accuracy coincide with max pressure? At first thought we might brush this question off by saying "at the muzzle" but perhaps complete burn of the charge might occur just short of bullet exit from muzzle. Certainly this is adjusted with burn rate and/or bbl length. I guess the question might be the point where the bullet inertia changes from positive to negative should be inside or outside the bbl.

Any thoughts?



Fretka
 
Do you mean max pressure vs max accuracy or max pressure vs max velocity? i.e. at what point in an infinite length barrel does the bullet start slowing down? We all know in real life the bullet starts slowing down at the muzzle.

I've never gotten max accuracy from a max load.
 
At most times there exists greater gas pressure behind the bullet which continues to accelerate the bullet down the barrel. But with a certain pressure curve and bullet weight there may exist a point where actual max speed may occur BEFORE the bullet exits the muzzle. this condition would be more likely to occur due to a number of conditions:

A very long barrel
A small charge of fast burning propellant
A bullet which does not seal well or quickly, allowing working pressure to escape.
A bullet which is of a very tight fit causing high internal friction

So if this condition does exist then there must be a point where the bullet changes from accelerating to static to loss of speed (would likely occur towards the muzzle end) causing a potentially unstable condition over a very short time-span.

So far so good? For my "experiment" lets call max pressure the same as max velocity.

So we now have a bullet that undergoes a change from pressure driving it from behind to friction slowing it down and I'll call that occurrence a "phase-change
for lack of a better name. If this does indeed happen then it will be a VERY short transition (both time and space) and if it occurs at a crucial point (such as the bullet partially emerging from the barrel) then we might have an explanation for non-linearity or maybe a dropped shot or other unknowns.

In a way this would behave like a tight spot in the bore but be due to flame front speeds as it interacts with the propellant.
 
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It should be possible to , theoretically, determine this burnout point with a ballistic program like QuickLOAD by setting up different powder charges and changing the barrel length in the program. This could certainly be factually determined by taking long barrels and cutting them off inch by inch while shooting over a chronograph.

I build some long range hunting and varmint rifles and do several "what ifs" with different powders and charge weights. You'd be surprised on something like a 22 BR and AA2520 powder how much difference 4" of barrel makes even in the 28-32" range of barrel length. That 4" of barrel changes the velocity 100 fps, while 4" of barrel in the 22-26" range changed the velocity 150 fps...but the bullet appears to still be accelerating at 32" of barrel. On a 22 rimfire the burnout of target ammo is supposed to be in the 16" or so barrel length.

.
 
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Thanks for the info Jerry!

I approach shooting by looking for the physics behind it. Some issues we have control over but some such as primers we have less control than I would like.

I once made holograms of an aluminum round-stock, about an inch in dia. and then made an interferogram so I could watch the bending that occured by the weight of one dime placed in the middle of a 12" span. Very little bending of course but there was deflection of about 6 wavelengths of green light (6320 angstroms). The problem was I could not capture a live fire condition so that only showed me where the max. nodes occurred in a static condition. Fun stuff when mother nature lifts her petticoats so to speak!

I guess I would like to know where this burnout point is with loads/caliber to see if that can have an effect on accuracy. Can anyone tell me a good internal ballistics web source?

Fretka

Also, your post makes me wonder if we might benefit from the lowest gas pressure release as the bullet exits the muzzle as possible. Yes, crowning can help but more better!

It just occurred that all this stuff I'm thinking about could be greatly affected by bullet seating standoff. Off the top I can't see how any standoff would be good due to working gas pressure loss and maybe some gas cutting that could occur prior to full bullet engagement with the lands.

OTOH standoff could effect this burnout location thus being a useful tool for tuning the gas pressure/bullet location in the bbl.

Never ends!
 
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Surely someone has tried electric primers or priming...Yeah? Running a tungsten or a carbon loop into the case to start the burn. The only thing I like about the idea is you get absolutely repeatable ignition, might as well benefit from new clean burn engine tech as the cartridge/propellant shares much with spark plug/fuel vapor in your 409" Chevy. Sure reduce lock-time as well.

Sorry for the ramble...........

Bruce
 
Fretka said:
Surely someone has tried electric primers or priming...Yeah? Running a tungsten or a carbon loop into the case to start the burn. The only thing I like about the idea is you get absolutely repeatable ignition, might as well benefit from new clean burn engine tech as the cartridge/propellant shares much with spark plug/fuel vapor in your 409" Chevy. Sure reduce lock-time as well.

Sorry for the ramble...........

Bruce
Click to expand...

I believe Remington tried this years ago. Didn't seem to catch on.
 
Well there ya go,another beautiful theory slain by an ugly fact.
 
Elmer Keith tried primer tubes in his .333OKH round to ignite the charge from the front, back in the 30's and 40's. He needed a large case capacity to have room for the tube. He got higher velocitys with lower pressures. This was tried in .50BMG during WW2 and ended up being used in 20mm. This is still used in tank rounds. The idea is that the powder stays in the case and burns instead of being pushed down the barrel by the primer. For more info see his "Gun Notes".
 
Thanks MilGunsmith,

It seems to me that with all the research going into auto combustion chamber burning (flame-front tech) driven by pollution economy goals we could learn something applicable to our own niche in the world.

As you motor-types know the latest deal is direct injection with the ability to control the flame kernal in an active way. We strive for repeatability and have come a long way with propellant being clustered close to the primer and somewhat contained but How repeatable is the primers performance?

I last shot bench back in 1977 and really very little has changed (excepting the loss of the 6x47)

It seems to me we have pretty much become static in regards to small arms internal ballistics.
 
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Fretka you got me thinking along with other posts on this thread. I think it is rare you have the most accurate load at max. velocity. I went to my QuickLOAD program and plugged in a known accurate load in my 6 br. rifle. 26" barrel, 30gr h322, 70 gr Sierra Match King, there is no section for primmer.COL was 2.175....jumped .010.

The numbers: Chamber psi 50545--3308 fps--99.67% powder burnt---chamber pressure at bullet exit is 8488 psi. NOTE: still pressure with powder not 100% burnt. I plugged by inch and with
a 34.5" barrel you would have 100% powder burnt. Several people wondered if max. velocity was attained in the barrel...the program if correct/accurate say's no as long as you have pressure.
I ran the numbers out using a 50" barrel. Velocity jumped to 3696 fps---100% powder burnt and the 3833 psi.

I plugged in same load with a 200" barrel 683 psi/4271 fps----381" barrel flat line psi/4464 fps----400" barrel flat line/4477 fps. Again if program is correct/accurate you still have an increase in
velocity even with no chamber pressure. Program limited me to 400" so data ran out.

MY conclusion is and alway's was/is. Max pressure and Max velocity and Max accuracy is highly unlikely...... DO YOU REALLY WANT TO STUFF A CASE FULL OF POWDER TO FIND OUT WHAT MAX PRESSURE IS...OUGH!! Have fun..

Brad
 
That really does make sense as there is so little friction once the bullet has imprinted the rifling that very little positive pressure is needed to overcome internal friction. Moly bullets even less so.

I guess then what becomes the best conditions to occur during bullet exiting the muzzle? Some pressure vs. bbl. diameter, some bbl. length and pressure vs. vibration node/internode? Will gas cutting occur at throat and muzzle.

This rambling is what some call mental experiments (or illness!) and is fun way to understand just what is really occurring.
Does anyone have any thoughts on gas cutting?


Fretka (a Polish ferret)
 
Fretka said:
That really does make sense as there is so little friction once the bullet has imprinted the rifling that very little positive pressure is needed to overcome internal friction. Moly bullets even less so.
Click to expand...


I think the increase in velocity you see in Brad’s examples is simply the longer barrel providing a longer distance and time for the pressure gases to act on the bullet.

Ken
 
Many of the top shooters who use N133 are shooting at over 3400 fps out of a 21 1/2 or so inch barrel.

I haven't done the math but I suspect that is in the 70,000 psi range.

The only drawback to shooting in this window is brass life gets a little shorter. But that seems to be where the extreme agging capability is with 133.
 
Shooting hot!!!

jackie schmidt said:
Many of the top shooters who use N133 are shooting at over 3400 fps out of a 21 1/2 or so inch barrel.

I haven't done the math but I suspect that is in the 70,000 psi range.

The only drawback to shooting in this window is brass life gets a little shorter. But that seems to be where the extreme agging capability is with 133.
Click to expand...

Hey Jackie,

I remember a comment you made to a competitor asking you about shooting hot and short case life at matches.

"I didn't come here to save brass" :cool:

V
 
:cool:
A lot of really good info here , and a bit of food for thought , as well.

Thanks all.
 
It seems to me that your crossover point is not going to happen in any normal caliber and barrel length situation, which ultimately reduces the potential utility of the concept.
 
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