bearing surface length VS pressure

Rally good post, As to pressure you would need a transducer, to measure breach pressures.
Cpu at x 1,000 The only ones i know of are at the powder factorys labs
 
Well guys I think were wretched, nearly hopeless as Charles said but other than a big jump in pressure that you can see in primers or case expansion your banging your head against the wall. If you're getting the same velocities with the different lenght bullets, it has to be the barrel likes one or the other. You can sometimes add half grain of powder and not see a big difference on a target. But change lengths in bullets and watch what happens. Besides how much pressure starts to make a difference 1000 lbs, 5000 lbs you tell my. Cause I'm the the guy that goes by what he sees on paper.

Joe salt
 
Joe, there is a difference between testing and experimenting. The trophies & money you've got say you're a good tester. But suppose it turns out that .020 difference in bearing surface does not raise pressure, but it's something else giving you the result. You wouldn't care, as long as you could repeat it at will. There are lots of other things aside from pressure "it could be."

And if Kreiger barrels suddenly preferred the longer bullets, you'd find out, do what ever it took to be successful with your barrel, & wouldn't care why.

What works and why it works are two different things. With this particular post, I'm taking Al to be asking a "why." He's fully capable of doing his own testing.
 
Just trying to save you a lot of head aches, like I said I do one test at a time. Meaning Change one thing at a time! you will be amazed. And its about recognition, not money and trophies.

Joe Salt
 
Good article in this months precision shooting that gives a means to arrive at barrel friction that will answer your question. You might ask the authors as am sure their research touched on your question. Cheers.
 
old thread...
found this searching for the brg length thread.
i have two lots of sierra 30 cal 168 mk.
these were sold as blems..but are in fact seconds..not cosmetic blems.
dia is .3078...not 308
weight on one lot is 168.0 -.003( half of sierra's normal)
and bearing length in two groups is .502+.001/-.002, and 482+-.001...so right at .02 diff.
do not know if this is a big enought diff, but if i get a chance i'll shoot some side by side and report back.
mike in co
 
105 gr Match VLD Target, .243, 105 # 24429 VS 105 gr Match Hybrid Target, .243, 105, #24433

thanks

al
 
i am not a benchrest competitor, but strive to achieve one hole groups trying to understand the science involved in this hobby. bearing surface length directly affects the amt of friction between bullet and barrel...longer equals more friction, and slows the bullet( in fractions of a millisec). if you believe chris long's shock wave theory in determining accuracy, you'll realize two bullets weighing the same but of different bearing surface lengths propelled by the same amt of same powder will arrive at the muzzel at different times( fraction of a millisec). this arrival time and the position of the shock wave determines stability of the bullet at the moment of exit. anecdotal observation...i had a very accurate load but changed bullets (same weight) and seated the new ones at the same jump to the lands and the group went to hell. measured bearing surface and the newer bullets had a much shorter bearing surface. adjusted the seating depth and found the point of departure that allowed bullet to exit when shock wave was at the receiver and, yes, the one hole reappeared. muzzel velocity differences are probably so slight that we can't measure with our crude chronographs. one can really pull his hair out when you find bearing surface differences in one box of bullets, which is why i am using custom made much more often than mass produced bullets. had a box of a well know bullet maker that had bearing surface variation of .025 thous...you could hold two bullets side by side and see the difference. just my observations...thanks.
 
lpreddick said:
bearing surface length directly affects the amt of friction between bullet and barrel...longer equals more friction, and slows the bullet( in fractions of a millisec).
Click to expand...
Number please. Driving a single car over the Golden Gate bridge puts wear on it, too Question is, how much.

if you believe chris long's shock wave theory in determining accuracy, you'll realize two bullets weighing the same but of different bearing surface lengths propelled by the same amt of same powder will arrive at the muzzel at different times( fraction of a millisec).
Click to expand...
If that's what the theory predicts, what's the result of controlled testing, where variables other than bearing length are removed? Seems a fairly easy test to make, and wouldn't the military be interested in the quantifciation, for artillery rounds?


i had a very accurate load but changed bullets (same weight) and seated the new ones at the same jump to the lands and the group went to hell. measured bearing surface and the newer bullets had a much shorter bearing surface.
adjusted the seating depth and found the point of departure that allowed bullet to exit when shock wave was at the receiver and, yes, the one hole reappeared.
Click to expand...

Best I can follow, you had to change the bullet jump, then. Depending on just how much that change was...

And as far as that anecdotal story goes, I've had the opposite, with 142 grain Sierra 6.5 boattails, at least in the early days when they first came out. If you mixed the two sorts, the group was still OK, but not as good as if you didn't. But if you shot groups with the same "sort," neither one of them "went to hell" if you didn't change anything.

In short, your anecdotal evidence don't fit my anecdotal evidence, so where are we?
 
Ipreddick: That is what I've said, same load same jump, the groups went to hell. All that was different was the bearing surface .030 difference. Thats the reason I only change one thing at a time then I know what did what.

Joe Salt
 
Joe Salt said:
Ipreddick: That is what I've said, same load same jump, the groups went to hell. All that was different was the bearing surface .030 difference. Thats the reason I only change one thing at a time then I know what did what.

Joe Salt
Click to expand...
Joe that helps tremendously. Far as I'm concerned, thirty thousands is a lot. One or two thou is not.

FWIW
 
Lou, could you explain a little more about the phrase "lead line" and what you mean by "approaching it" or "passing it?"
 
Some consider bearing surface to be that cylindrical portion of the bullet that is near bore diameter. I consider bearing surface to be all the bullet rear of the land diameter (flat base bullet). Lou's words caused me to realize that changing the lead line in a bullet is effectively changing seating depth - all else being equal. My thinking here is the resistance increases when the lands reach the lead line and thus the pressure curve would differ from lead line to lead line - all else being equal. On the other hand, a sectioned bullet will almost always show the lead line forward of the land diameter - so scratch all that..... I suppose my only contribution is that the bullet "bears" starting at the land diameter.

I'd better leave the "approaching" and "passing" explanation to Lou...
 
I believe that what Lou is referring to is a specific dimension from the base of a bullet to the top of the seated core, that has been experimentally determined to give the broadest possible tune, and that as this dimension was increased or decreased, that the breadth of tune diminished to a noticeable degree. Additionally this preferred dimension is undoubtedly specific to the overall shape of a bullet made from a specific pointing die, with a particular length jacket.

Lou shot thousands of rounds in the best possible conditions, with the highest quality equipment to help develop the newest Berger short range benchrest bullet. Letting the location of the lead line determine a bullet's weight, rather than having the desired weight determine the location of the lead line, seems to me to be new idea that may have potential extending well beyond this one bullet.

Could it be the relation between the center of pressure and center of mass that is being optimized? Do some bullets "go to sleep" faster than others? Can existing designs be improved by slight weight adjustments that relocate their lead lines? IMO, this could get even more interesting than it already is.
 
Now I take the lead line as being, how long a bearing surface can you have before everything starts going the other way. So I think with different bullets Of different design and the lengths if they are not same will not perform the same. Thats why you have to find the bullet that is right for your rifle. Am I rambling, or does that make sense. I know what I mean anyway! One dam bullets going to work better than the other whether it be because of shape or length. The lot# I have Works so thats the length I'll look for.

Joe Salt
.
 
Just to make sure everyone knows we have both point-blank (100 to 200 yards) shooters, and 1,000 yard shooters involved in this tread. Gravity being what it is, pressure -- hence velocity -- varies in importance. It's the "square" in 32 ft/sec^2 that's at play here.
 
Ok Charles we'll make a duck a duck. I'm shooting 1000 yards! So I'm going to get bigger variations, in groups. Hay maybe thats why my scores are better also, there spinning better. If I go to the world open early I'll try some that have a .015 difference on paper and let you know what happens!

Joe Salt
 
This has been fun to watch.

Does differences in bearing surface have an affect pressure. I doubt we could determine it one way or the other once the bullet has entered the rifling without a pressure barrel in a tunnel somewhere. Even then there would so many variables the results would be questionable and I'm sure very minor. The largest variation is the pressure required to push the bullet out of the case into the rifling. More to the point it's the duration of the required pressure to fully engrave the bullet in the rifling that affects the pressure curve. How do I know. I've taken sections of barrels, some the same caliber different manufacturers, partial chambers, threaded them to fit a reloading press and pushed bullets into the rifling. Bullets of very differing lengths. Once in the barrel it took about the same swing with a hammer to drive the bullets back out. Sometimes fewer swings sometimes more swings but within a caliber friction was about the same. Seems like another discussion looking for a solution to a problem that doesn't exist or is ill defined.

Dave
 
Dave Ill defined, I know I'm seeing a difference on paper but can't say for sure its the twist, pressure, is my barrel bigger inside. I know I'm running more powder than most using 300 wsm, so all I've done is find the right bullet for the job. The barrel is a Krieger 1-10 28" velocity 2855 fps. with 63.2 gr.H-4350 and it will go to 65.5 without any pressure. It likes what it likes.

Joe Salt
 
You must log in or register to reply here.
Back
Top