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shot start presure, bullet jump, complete combustion
Discussion regarding the reloading of ammunition and tuning of loads for accuracy
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slimjim
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PostPosted: Wed Sep 15, 2010 4:27 pm    Post subject: Re: shot start presure, bullet jump, complete combustion Reply with quote

Stovey, when you first started loading El-Kabong, you looked at varying bullet seating depths. Did you get any interesting results or was that with a different bullet than you are using now?

BTW, like chambered said, Barnes bullets need the jump to avoid pressure build-up. I found it pretty easy to find the sweet spot with my Barnes TSXs.

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PostPosted: Wed Sep 15, 2010 4:45 pm    Post subject: Re: shot start presure, bullet jump, complete combustion Reply with quote

I believe that too much attention is given to neck tension in regards to proper combustion.
I have seen guns shoot well with a factory crimp such as your 150's and I've seen them shoot well with bullets barely seated in the neck and/or neck sizing done to only a portion of the neck. Don't get me wrong here, the amount neck tension can definitely have cause and effect. I just believe it's a separate issue that can influence the combustion process but isn't necessarily a part of the process.
I also believe that a consistent burn is more important than a efficient one although I'd prefer they go hand in hand.

The physics that gelan has eluded too are staggering !!! Shock waves, gas expansion, expansion ratio, pressure gradient, heat and loss of it all play a role into making them bullets go in the same hole all the time.

The only tool I have when it comes to this is experimentation. For that I sadly fall prey to my own empirical findings. I can only wish now that I would have studied and understood physics in my school days.

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slimjim
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PostPosted: Wed Sep 15, 2010 6:07 pm    Post subject: Re: shot start presure, bullet jump, complete combustion Reply with quote

chambered221 wrote:
I can only wish now that I would have studied and understood physics in my school days.

It is never too late to learn! You keep pulling us forward!!
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gelandangan
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PostPosted: Wed Sep 15, 2010 9:01 pm    Post subject: Re: shot start presure, bullet jump, complete combustion Reply with quote

I concur that there are difference of opinions among shooters and handloaders.
I am definitely not here to say that I am right and you are wrong, nor that I am entirely correct.

Simply I wish for a good two way discussion to further my knowledge of this interesting sport that I love.
I love comments that dissect my findings pointing out my mis interpretation of the law of physics,
or my blatant generalization of some important facts.
Especially those who can help me to understand the real basic physic behind the microsecond combustion of gunpowder.

Those who TELL me that the answer to the universe is "42", without giving me the provable and repeatable arguments, peeves me.
I am not a sheep and I don't accept that something is true, just because some ancient books written a few thousand of years ago said so.

------------------------------

Ok thats out of the way, lets get back to the discussion.

There are a lot of confusion about the amount of jump the bullet should experience BEFORE it meet the restriction of the barrel grooves.

IMHO, the real need of this jump is to built a momentum on the projectile so that it would be easier to get into the barrel groove.
The bullet is to be swaged at the beginning of the groove and this require a lot of energy.
(Viz. you need to build up a swing when you try to hammer something,
that swing can be a whole 180 degree arc slam or a short 10 degree arc tap)
Any starting momentum would help the swaging action.

Some bench rest shooters want to seat the bullet very close to the rifling (some are actually touching),
that is fine too due to the fact that most benchrest bullet is of a parabolic ogive type.
Having a parabolic ogive would mean that the actual swaging is done gradually instead of abruptly.
Either way they facilitate the momentum building of the projectile.

What is the difference between distances of jump you may ask.

In a hunting rifle, the neck portion of the chamber are made to accommodate the cartridges in a hurry,
and said cartridges or barrel may be a tad dirty etc, thus they are of a wider tolerance design.
It is normally the opposite circumstance happening in bench rest, where each cartridges are feed slowly and softly into action.
Bench rest barrels, usually have neck size smaller than hunting barrels and usually are with much narrower tolerances.
The neck tolerances would reflect the differences in velocity variants of the projectile, thus may reflect ultimately in its accuracy.

Choices of Projectiles between the two group of shooting also make difference in the final result.
Bench rest shooters spends higher amount of money to get higher consistency projectiles,
with having smallest group of impact as their priority.
Hunters need to balance between the actual accuracy and terminal performances (mushrooming, tumbling, exploding etc)
with priority placed in the fastest killing or largest side effect (eg blood trails) on the target.
These is the reason why Sierra bullets do not recommend their very nice and consistent SMK range for hunting.

Having said the above,
the further the jump, the higher the shock the projectile is subjected to due to the higher velocity it acquired during the free flight.
This would cause the projectile to be "bruised" by skidding in the first few inches instead of smoothly spinning.
Of course the end result could be an unbalanced projectile that ultimately be not accurate (read single hole 0.001 inch groups).
Thus seating projectile very close to the rifling, will ensure the projectile starts with a slow smooth rotation in the rifling.

However,
IF there is an amount of discrepancy in the system outside the norm,
say, the projectile happened to have a little harder jacket or core material (minute impurity may harden a lot)
if the projectile is seated right into the groove, it may increase the pressure on the chamber greatly.
As the result of this, it started a chain reactions
it also increase the rate of burn of the powder due to the higher pressure experienced by the powder.
And followed by less of energy available to push the projectile down the barrel..
And thus lower muzzle velocity etc..

It is a matter of lesser between two evil,
do you ensure the bullet not to have bruises before it even got out of the barrel,
or do you ensure that they get more consistent swaging but with a bruised projectile?

What I am trying to say above is that:
Having a higher jump space, ensures that the projectile built up enough momentum for the initial swage,
thus giving you a more consistent result while using cheaper wider tolerance equipment.

Interesting huh!

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stovepipe
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PostPosted: Thu Sep 16, 2010 9:13 am    Post subject: Re: shot start presure, bullet jump, complete combustion Reply with quote

I really liked your info on the chamber differences between production rifles and custom BR shooters. Very interesting and I hadn't thought of that.
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chambered221
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PostPosted: Thu Sep 16, 2010 1:06 pm    Post subject: Re: shot start presure, bullet jump, complete combustion Reply with quote

chambered221 wrote:
When ever this subject gets brought up someone usually throws in the fact of how benchrest shooters do it a certain way. Although true I like to remind them that they are also dealing with cartridges that were designed with all that in mind.

Stovey, That's pretty much what I was referring to.......gelan did a better job of getting the point across though !!!

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stovepipe
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PostPosted: Thu Sep 16, 2010 1:19 pm    Post subject: Re: shot start presure, bullet jump, complete combustion Reply with quote

Slim- so far with the H4831/MK's the closer the better. Although one load shot well .030" OTL but it was stouter than I'd like to use for gong shooting all day. The pressure was good and well within limits but it hit pretty stoutly compared to a grain less that grouped well .010" OTL.

Go some RE22 loads ready to test soon too..

221- I think most if not all these posts here have merrit. A good discussion so far. This topic should get an award for staying on topic as long as it has, too.
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stovepipe
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PostPosted: Thu Sep 16, 2010 1:40 pm    Post subject: Re: shot start presure, bullet jump, complete combustion Reply with quote

And another thing....

I do a lot of screwing around on this site, jokes, etc... But, I appreciate sincere discussion like this as well, esp when I dont fully grasp some of the physics involved or an aspect of a given topic.

I very much appreciate other's POV and respect their experience.

Thank you.
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slimjim
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PostPosted: Thu Sep 16, 2010 4:40 pm    Post subject: Re: shot start presure, bullet jump, complete combustion Reply with quote

stovepipe wrote:
I really liked your info on the chamber differences between production rifles and custom BR shooters.

The one day I admired a BR rifle on the range and the owner let me shoot it was an interesting experience. He reloaded on site and seated his bullets only .030" into the case neck (.30-cal BR). Case was full of powder but no compression. Best 5-shot group I have every shot - one ragged hole but he still called every one of my impacts based on slight wind changes. His shots were one single hole.
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slimjim
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PostPosted: Thu Sep 16, 2010 4:50 pm    Post subject: Re: shot start presure, bullet jump, complete combustion Reply with quote

stovepipe wrote:
so far with the H4831/MK's the closer the better. Although one load shot well .030" OTL

My Berger 150gr VLDs did best .010 OTL. My Barnes 130gr TSX did best .060 OTL in the same rifle. I always find the best powder load via OCW first then work bullet depth for that powder charge to find the sweet spot. Oops, now that I think of it, when I bought a box of the Hornady 130gr GMX just for grins, I just took a guess at a powder charge (-.5gr less than the 130gr TSX) and loaded up some just to see how they shot. I didn't even adjust bullet depth - just took what the die gave me. That was the best group I've shot out of this particular rifle - any of my rifles! I didn't bother to adjust powder or depth - just kept shooting it. I was lucky (or GMXs are that good) and I didn't want to mess with success.


Last edited by slimjim on Fri Sep 17, 2010 3:42 am; edited 1 time in total
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44marty
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PostPosted: Thu Sep 16, 2010 6:07 pm    Post subject: Re: shot start presure, bullet jump, complete combustion Reply with quote

Thanks, Gelan - That's a very good logical discussion of bullet styles, chambering and objectives of hunting VS precision target rifles.

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wncchester
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PostPosted: Fri Sep 17, 2010 10:26 am    Post subject: Re: shot start presure, bullet jump, complete combustion Reply with quote

"My Mossy sure liked it when I moved the bullet up on the lands w/ my hand loads to cut the jump down on them SMK's and that long distance back to the ogive."

Obviously, more consistant power burn will produce more consistancy of both velocity and accuracy potential.

Seating to the lands of your Mossy, or any other rifle, increases bullet start resistance, which increases start pressure and that may also increase burn consistancy, depending on several other variables including bullet weight and, to a small degree, a crimp. Increasing the intitial pressure can also be accomplished with faster powders and primer changes.

Any burn pressure that exceeds the yield strenght of the brass is going to fully seal the case to the chamber wall and that pressure is much lower than normal chamber pressures. Any slight smokey residue on a neck and shoulder is trivial and can occur after the bullet exits the muzzle.
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Aloysius
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PostPosted: Fri Sep 17, 2010 11:35 am    Post subject: Re: shot start presure, bullet jump, complete combustion Reply with quote

Gelan, while reading your essay on this matter I got some strange ideas. I put your point of view in the cylinder of my revolver and saw a very big jump and I started wondering why I've never heard of someone starting the rifling already inside the cylinder of a wheelgun? Theoretically some slow twist in the cylinder could improve accuraty (think about these competition black powder guns with increasing twistrates inside the barrel)
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gelandangan
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PostPosted: Fri Sep 17, 2010 5:17 pm    Post subject: Re: shot start presure, bullet jump, complete combustion Reply with quote

Aloysius wrote:
Gelan, while reading your essay on this matter I got some strange ideas. I put your point of view in the cylinder of my revolver and saw a very big jump and I started wondering why I've never heard of someone starting the rifling already inside the cylinder of a wheelgun? Theoretically some slow twist in the cylinder could improve accuraty (think about these competition black powder guns with increasing twistrates inside the barrel)

Aloysius,
Let me separate your thoughts in two parts:

1. there is a very big jump taken by revolvers bullet before it get to the rifling.

If you observe carefully, the actual "free air jump" in a cylinder is in fact very small,
the revolver cylinders are chambered and headspaced like any other gun,
there are usually a sort of "forcing cone" just on the downstream side of the cylinder before the exit.
In the cases where you are using such as 38 SPL in a 357 MAG cylinder, the jump do indeed became larger.
However, the swaging of the projectile is already started during its pass through the forcing cone.
An thus it just get a longer swaging distance instead of a short abrupt swage distance.

2.Slow starter twist could improve accuracy.

Here the opinion varies.
If you think of it, grooves in the barrel act in two way,
Firstly it guide and force the projectile to rotate and thus create gyroscopic inertia that help it to straighten the path of its trajectory.
Secondly, it also swage grooves in the projectile so it will indeed follow the twist of the rifling.

In MY opinion gaining (increasing) twist will NOT improve the accuracy nor giving any improvement to the projectile flight,
in fact I think it is exact opposite will happen, the MV is slower, the ME is lower and a really bruised projectile will affect the projectile impact location.

Here are my reasons:
if the groove already started in a certain angle, it will need a great deal of energy to change the groove angle to something else
because the surface area of the groove has increase (the actual groove has formed).
Picture this, you are driving in an unsealed country road, where a lot of vehicle has passed through it and creates wheel grooves parallel to the road.
It would be easy to follow the groove direction, and it would be hard to move out of this grove.

During the start of the projectile travel into the rifling, the energy released in the chamber are in or near to their peak,
swaging the projectile during this period of peak energy is logical.
Downrange, away from the chamber, the pressure drops and energy available is much lower,
thus it does not seem to be logical to use this dwindling energy to swage the projectile into another grove angle.

Cool

Now as to your muse about why no one start the rifling inside the cylinder, well I am not sure,
but I think it is much harder to ensure that the groove from 6 barrels to match exactly the groove of the barrel under field conditions.
Theoretically this can be done, but the complexity of the manufacturing and maintenance during its life would make it very much uneconomical.
Mind you we are talking of item that rotate in the high tens of thousand to low hundred of thousand RPM,
even a little discrepancy can cause catastrophic failure.

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fnuser
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PostPosted: Fri Sep 17, 2010 5:51 pm    Post subject: Re: shot start presure, bullet jump, complete combustion Reply with quote

have you ever recovered a projectile from a gain twist barrel?

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