shot start presure, bullet jump, complete combustion

stovepipe

(posted on RRA but deleted and moved here for discussion/critique)

There's a million variables between given rifles but I think I'm starting to see something that makes sense to my pointed little head and I think it might be the jump and the effect it has on the combustion of the charge.

I'm thinking factory loads (glued or crimped w/ a lot of neck tension, etc) and non-canalured reloads moved close to the lands (bullet acts like a plug in a bottle) have a better shot start pressure/allows the charge to combust more uniformly and consistently w/ less blowby, bullet scouring etc.

I aint sure I'm explaining this right but it makes sense to me that the better the seal on the case/chamber the more uniform the combustion of the powder will be hence better accuracy/consistency if more of the charge can burn behind the bullet and not as much sneak past.

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.

Any of that make any sense or need tweaking? Or am I way off base here and missing the actual physics. I'm not sure but I think I just had an epiphany. Shocked

gelandangan

IMO -
Depending on the speed of powder combustions,
there may or may not be advantage in increasing start pressure in the chamber.

Viz,
on SLOW burning powder, it is best to ensure that ALL powder have at least started to burn before they are being dispersed down a long barrel.
Short of discussing the physics of internal combustion, we know that powders burns best when contained in space and thus high pressure.

As the projectile moves down range in the barrel, pressure decreases;
even more so if the movement of the projectile is faster than the positive addition to the pressure within the chamber due to powder combustion.
This may effect in slowing the combustion to the point that they may have incomplete burns.
To counter this effect, ensuring that each grain of powder have been subjected to their minimum required pressure,
thus combusted adequately prior to their dispersion into the relatively huge space within the barrel,
would ensure a higher consistency of the bullet performance.

Quick burning powders usually completely burned out within a few inches of projectile travel,
regardless of the constriction of projectile movements (either due to heavy crimping or groove forming/friction)
Thus position or constriction of the projectile may have less effect to their final velocity than the effect of "long" barrel friction.
This hold particularly true if you think about the working of a revolver, with the gap between the barrel and cylinder,
massively lowering the pressure within the barrel, an yet able to maintain good accuracy and consistency.

What I am trying to say above is that yes I think your finding is valid when the loads is made up of "slower" rifle powders and being used in long (> 8 inch) barreled gun.
And, it may not hold true if the load is made of "faster" powder and/ or shot in short barrel.

PS. I LOVE this kind of discussion.
Thanks for bringing the subject up Stovie!

slimjim · Last edited by slimjim on Mon Sep 13, 2010 4:37 pm; edited 2 times in total

Stovey, your starting this discussion again? You started the last one on this.

www.huntingnut.com/ind...p&start=15

Though there may be good reason to crimp bullets for revolvers, lever actions, or rifles that bounce their bullets around a lot or get bounced around in transport - I don't think crimping is crucial to powder burn and accuracy in a long barrel rifle. Long range shooters using .45-70 recommended to me not to crimp and not to even neck size. Just seat the bullets into unsized cases. With a faster burning powder, I get complete powder burn.

After the last discussion (post above), I went to one of my shooting buddies who has won several long-range shooting competitions with .223 and 6.5-.284 and asked if he crimped. No he didn’t. I’ve wondered about the seeming large velocity spread on some of my loads with IMR4350 and Barnes bullets but then chrono’d my buddies .270 and he gets similar spreads with factory ammo. His gun spit flame just like mine did with crimped factory ammo.

Last, with Marty’s recommendation, I got some IMR4064 and sure enough, my SD on velocity spread went into single digits – no crimping, just a more consistent burning powder. IMO, Crimping may be one of those factors that helps a particular gun shoot better but I don’t think it is essential to complete combustion, etc.

chambered221

Yea....what he said !!! Very Happy

Keep in mind that by starting the bullet closer to the lands you have less of a chance of it entering the barrel off center. This may or may not play into the combustion theory. Then again some bullets need that jump. Once again it all boils down to experimentation.

Do it long enough and you'll start to see commonalities in what works and what don't for certain components.

In the mean time..........If you really enjoy ballistics and want to understand them better get Robert Rinkers book "Understanding Firearm Ballistics"

slimjim

Oh, without a crimp, I can't get my bullets out of my .270 cases with an impact puller. I don't think a little crimp on the end of the case neck is going to change the amount of the neck tension much.

stovepipe

slimjim wrote:

Stovey, your starting this discussion again? You started the last one on this.

Hmm....OK. Sorta forgot about that one...diff topic, no? Hey- I've hit my head a lot.
I wanted a stand alone to get some input on the goings on when the hammer drops and the bullet starts to enter the bore. It's pretty neat science and I love knowing what makes things tick and the variables involved.

This was a good suggestion and I'm gonna check it out.
Robert Rinkers book "Understanding Firearm Ballistics.

Sorry for beating a dead horse, guys. Embarassed

Dawgdad

Lets throw annealing brass in there for consistnecy of neck tension too. lots of factors can affect how the brass holds the bullet and then opens to seal the chamber. Sooty cases tell me I m getting too many reloads and need new brass or to anneal the ones I have to make them less work hardened.

gelandangan

I gather a metric discussion of the subject basing on physics theory is not very interesting to some.

I think those who claim that they do not see differences in whether to crimp or not,
have valid point when they stated their opinion based on their empirical findings.
It seems to me that those findings are limited to a particular gun in particular condition.
I usually deal with reasons, NOT with a claim made by a good friend of mine that happen to have seen the elephant.

The Earth is flat because all I see is flat land type feeling is rife in this society.
But of course, mankind cannot fly nor can we talk to others on the other side of the planet.
I definitely cannot disprove those who have seen ghost or can read the future either.

So yet again another interesting topic meet a sad ending.

slimjim

stovepipe wrote:

I'm gonna check it out. Robert Rinkers book "Understanding Firearm Ballistics.

Stovey book mark this topic let us know what you learn.

slimjim

gelandangan wrote:

So yet again another interesting topic meet a sad ending.

Ok, my bad. At least my memory is still working. Didn't mean to throw a wet blanket on the topic.

stovepipe

chambered221 wrote:

Keep in mind that by starting the bullet closer to the lands you have less of a chance of it entering the barrel off center. This may or may not play into the combustion theory. Then again some bullets need that jump. Once again it all boils down to experimentation.

Let's expand a little on this. If the jump were .050" or .010" and all thing being equal what diff would that make in accuracy if it were askew to begin with, other than the obvious the gun might like it this way....?

I also dont understand why a bullet would need a "jump" to be more accurate. I'm NOT an expert and granted no two guns/loads are alike, but it would seem to me "a" bullet would "like" to start it's journey being up on the lands.
I'm likening moving the bullet to the lands like a propper sized cork in a bottle. It lets pressure build more and evenly w/less blowby. With a better seal in the chamber, given load would be off to a better start, no?

Or does the bullet not act so much like a seal for the chamber and perhaps I'm likening this process too much to an internal combustion engine (better seal = better combustion et. al.)?

stovepipe

gelandangan wrote:

IMO -
Depending on the speed of powder combustions,
there may or may not be advantage in increasing start pressure in the chamber.

What I am trying to say above is that yes I think your finding is valid when the loads is made up of "slower" rifle powders and being used in long (> 8 inch) barreled gun.
And, it may not hold true if the load is made of "faster" powder and/ or shot in short barrel.

PS. I LOVE this kind of discussion.
Thanks for bringing the subject up Stovie!

Great minds think alike! Yer welcome sir. Cool

And don't let the "wet blankets" discourage you. Wink

chambered221

There is no doubt that experimentation has proven you should move your bullets in and out to find the position they like to start from !!!

Set a bullet to deep and it's possible it will not enter the barrel square to it's axis. Thus leaving the muzzle with the same off centered force. Set it to far into the lands and the result may be not enough neck tension to allow for proper combustion.
Light for caliber bullets are all most impossible to start at or near the lands.
Some guns have way more throat than need be.
Some bullets such as Barnes don't have give to them like the lead core bullets do.
These are just a few of the variables that you encounter with this subject matter.

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.

I do appreciate gelans thoughts on the subject and wish him to reconsider and give further comment !!!

stovepipe

And I as well-

But.....what are your thoughts on combustion efficientcy when the bullet makes a better seal on the end of the case/chamber?

Does my point hold any merrit or?.... 150gr factory hunt loads jump a country mile in my 300 yet they will clover leaf or better @ 100y if I hold well. They have a ton of neck tension/adheasion.

My 200gr handloads like being just OTL. Very little neck tension on these comparitivley speaking. I could only duplicate that type of neck tension with a crimp (0 cannalure on the MK) and an adheasive.

I'm thinking the only two things in common there is the powders are combusting well (and all that goes with same) creating an efficient burn and pressure starting the bullet on it's way w/ less undesired effects.

slimjim

I saw Robert Rinkers book "Understanding Firearm Ballistics" in Cabela's today. I should have picked it up for a reference! I'd really prefer a digital copy.