Accuracy & Precision vs. Bullet Jump

[rkittine]

Neck Tension, Jump, Touch or Jam and every combination of powder, primer and brass can effect pressure and other areas and they may be different for different barrels, twists, number of groves etc. You could spend you entire life (and burn out a lot of barrels) trying to find THEE perfect combination, but then you have to be able to actually shoot it well enough so that you are not causing the variation and be in the right conditions.

Fortunately I have access to an indoor range, that is fine for center fire up to 100 yards. On days that it is closed, the fans can be left off and some of the variations eliminated, like wind.

What is interesting is that shooting 1,000 yard, I weight each charge, while many 100 yard benchrest shooters, not only load / tune on the bench, but use a powder measure to throw the powder charges.

After three times at the Williamsport 1000 Yard Bench Rest School, I have replaced most ladder testing with just looking for reduction in velocity variations, which for sure will effect vertical spread.

Bob

 

[taliv]

When you guys solve this mystery and are ready to move on to the next one...

tell my why fouling from different powders affects accuracy


Try this test. Get two accurate loads with different powders. Foul the barrel in with one. Now switch to the other load. Does it shoot tight? Or do you have to clean the barrel to remove the powder fouling and then foul it in with the second powder before it shoots?

 

[d2wing]

I don't think it is as simple as bullet jump which is why it doesn't always work. Changing seating depth may affect ignition or burning rates, bullet alignment trueness of lands, bullet etc. I would think the biggest factor would be how the bullet leaves the barrel. Trueness to the bullet base and muzzle. But I don't know any of the answers.

 

[Thomasss]

taliv, I have tried your test with two different powders and have found consistently with different powder combinations, I have to clean the barrel and remove the powder fouling and then foul it with the second powder before I can obtain reasonable groups.
But I have another question.....everyone tells me when I shoot a boat-tail bullets it will not achieve its best accuracy unless it travels over 300 yards, yet my best groups have always been with boat-tail bullets at or under 100 yards. What gives? Prejudice, Old wives tails, or ?

 

[JFrank]

If you or anyone else could provide test results showing a BTHP shoots smaller at longer range than short range, that would be excellent.. And probably save a lot of discussion

 

[Nature Boy]

tell my why fouling from different powders affects accuracy

I don’t know why it does, but it does

 

[taliv]

If you or anyone else could provide test results showing a BTHP shoots smaller at longer range than short range, that would be excellent.. And probably save a lot of discussion

I suspect they have misunderstood a popular belief that flat base are more accurate at close range than boat tail and vice Versa at longer ranges.
I don’t know if it’s true because I have almost zero experience with flat base bullets. But rumor has it they are popular in short rest benchrest. Maybe that’s because it’s easier to manufacture a flat base bullet consistently instead of some aerodynamic difference.

And at longer ranges obviously the higher bc of a boat tail will be a huge advantage.

I don’t think it’s true that boat tails need 300 yards to get more accurate.

 

[Howa 9700]

Long time shooter, but one who is fairly new to the technical aspects of reloading. One of the concepts new to me is this notion of bullet seating depth. Was introduced to it a few years back when a friend offered to load some rounds for me, but wanted the rifle on hand so he could get bullet seating depth right. Now that I"m going to be reloading my own, I've started looking into the concept. I'm finding answers to some long term questions of mine about a lot of things related to group spreads. Answers that make sense.

Always a risk of somebody (like me) spouting off on topics they know nothing about, and I'll accept that ding if it comes to it. Having said that, I'll go on to admit that 2 weeks ago, I had never heard of Eric Cortina. But I find his explanations and reasoning of most things technical to be pretty good. He certainly has the chops to back it up.

On the concept of seating depth, he flat out admits he does not know nor does he care what the exact number is. But he does use an easy way to find out. He finds a good load, then fine tunes it by shooting variations until he finds a group that works, then sticks with it.

Basic process is this: load dummy case (no powder or primer) with snug but moveable bullet, then chamber the round to jam bullet in the lands. Enough to mark it, but not stick it. Bullet will slide in place, and when you pull it out, you are left with OACL to the jams. Back that out 2 thousands and that is your starting point. Then take a whole stack of otherwise identical loads and start seating groups of 3 to 5 rounds each about 3 thousands deeper. Maybe as many as 15 or more groups. By the end, you may have a lot of jump.....maybe not to min case length, but a lot of jump just the same.

When the spread of these groups is graphed, (Spread on the Vertical Axis Y, distance from the jams X), a set of waves will form.

Groups will appear as O O O o . . o O O O o . x . . . o O O o . . o O O O etc.


Where O = big group, o = smaller, . = smallest group

Measure the case length that gave you ( . . . .) and that is where you need to be.

The theory is this is a timing issue. That bullet seating depth "times" that bullet to that charge of powder and that primer, so the bullet travels with the shock wave created by the powder burn. Synchs it up to the harmonics of the barrel. In the case of example above (X) marks the spot. Since this distance is almost impossible to know or predict, let the rifle tell you where it needs to be.

He goes on to say he can even improve factory ammo by doing the same thing. Just start seating groups of bullets progressively deeper and watch what happens.

If this is true, it explains a lot. The theory is to reverse some common held practices. Rather than seating bullet to constant length and changing powder charges, keep a workable powder charge constant and change seating depth.

BTW, this was also backed up on a video I watched that involved an engineer with Nosler......he basically said the same thing, but in far fewer words.

FWIW.

And again, consider this more in context of a question that statement of fact. It's out there and I just want to know if anyone has tested and can confirm or rebut the theory.

 

[taliv]

i continue to believe when the primer explodes, it pushes the bullet into the lands, where it stops until the powder burns enough to raise the pressure and move it forward again. given that, i don't buy his theory

 

[JFrank]

I let the barrel tell me to either chase the lands or stand pat with a particular depth.
Notice here a perfect scenario where the depths change also the results are quite distinctive in fact create a diagnostic tree. My optimum seating depth is .018 off my starting point.
Later (about a 1000 rounds and to Eric's point) that same barrel shot like Crap chasing so I just stayed at 1.798 BTO which was about .030 and who cares it shoots small..

 

[JFrank]

I suspect they have misunderstood a popular belief that flat base are more accurate at close range than boat tail and vice Versa at longer ranges.
I don’t know if it’s true because I have almost zero experience with flat base bullets. But rumor has it they are popular in short rest benchrest. Maybe that’s because it’s easier to manufacture a flat base bullet consistently instead of some aerodynamic difference.

And at longer ranges obviously the higher bc of a boat tail will be a huge advantage.

I don’t think it’s true that boat tails need 300 yards to get more accurate.

Me neither.
I always heard it was the sleeping bullet thing..

 

[Walkalong]

i continue to believe when the primer explodes, it pushes the bullet into the lands, where it stops until the powder burns enough to raise the pressure and move it forward again. given that, i don't buy his theory

I believe you are correct, unless neck tension is heavy, or the bullet is crimped into a cannelure, and it would be easy to test, I'll try it soon.

I know a primer can drive a .45 ACP bullet out of the case, leaving it in the barrel, I've done that.

 

[High Plains]

So, I'm curious as to how many folks are routinely and consistently achieving sub-MOA, 5-shot groups with long bullet jumps? For the purpose of this discussion, I consider a "long jump" to be something 0.050" or greater. Who is achieving sub-MOA results with long bullet jumps and what jump are you using?

Shooting off of sticks and sitting on the ground, my 300 Weatherby Vanguard with 180 grain Hornady Interlock boat tails made a 5-shot group I could cover with a quarter. Bullets were seated to the maximum length that would feed from the magazine. I fired about every 2-3 minutes so the barrel would cool. All I was really after was consistent muzzle velocity readings but the rifle has never failed me on accuracy.

 

[Demi-human]

All I know is I have to jump...

On my right foot twice, then clap before sitting down.
Then I rub my hands together flat and blow on my fingers.

Assuming, of course, the loudest chicken was “silenced” at the proper time the day before, and was delicious with sliced and seasoned potatoes. (Sliced North to South! Obviously!)

That is how I get my best groups.

i continue to believe when the primer explodes, it pushes the bullet into the lands, where it stops until the powder burns enough to raise the pressure and move it forward again.

I do as well. I heard or read somewhere an average primer is worth about 3-5,000 psi, depending on case size.
It stands to reason a bullet seated with a mere 50lbs of force, with a greater jump length to gain momentum, would only be pushed further into the lands before stopping and then blasted by powder pressure to move again.

But, I don’t know if my bullets that are seated to touching are actually forced into the lands and then stopped, or if they stay put until the powder ignites.

What we need is a high speed X-Ray video camera...

Honestly, the mystery is fun. Even if we found out the primer answer, some barrels would still shoot better some ways, and other will be opposite just to foil the trend.

 

[rkittine]

Some projectiles tend to spiral while gaining stability and that spiral is sometimes wider at shorter distances. We have to look at pure groups by MOA and not necessarily by size. The group at 100 yards might be say .5 MOA while the group at 300 Yards might be .3 MOA and still about the same physical size though actually tighter. If I was home I would post some targets - 6mmBR Norma 107 Grain BT, Closed Tip Sierra's. Smaller physical group at 200 yards compare to the same, under the same conditions at 100 and still lower MNOA at 300. My 6PC falls apart after 200 yards.

Bob

 

[Varminterror]

Some projectiles tend to spiral while gaining stability and that spiral is sometimes wider at shorter distances.

Could you please provide empirical provenance for this claim? You’ll be a wealthier man if you can.

 

[lysanderxiii]

i continue to believe when the primer explodes, it pushes the bullet into the lands, where it stops until the powder burns enough to raise the pressure and move it forward again. given that, i don't buy his theory

Yes, and no.

Primer burn is not instantaneous, and the primer has its own little pressure pulse. So, the peak primer pressure occurs some time after the propellant is ignited, the two pressures together push the bullet into the rifling. Since start pressure (the pressure required to overcome the neck tension) is a lot less than the engraving pressure (the pressure required to engrave the rifling onto the projectile), there will be a slight slowing between the projectile moving into contact with the lands and the bullet moving down the barrel.

EDIT:

Here is a graph of an M855 cartridge firing. The solid black line is the chamber pressure, the solid red line is the projectile's acceleration, the broken red line in the position of the flame front inside the case.


T = 0 was defined as the ignition of the propellant charge, which coincides with the initial rise in pressure and movement of the flame front, which indicates that as soon as the primer starts to burn, it starts some of the propellant burning as well. You can see that the projectile does not move for about 0.0075 ms, then shoots forward some short distance (to the lands), then slows down, but does not stop completely.

 

[Varminterror]

i continue to believe when the primer explodes, it pushes the bullet into the lands, where it stops until the powder burns enough to raise the pressure and move it forward again.

I generally would agree.

The only counterpoint which remains to make my brain itch is a mis-loaded round I struck at the PRS Midwest Regional Finale three years ago - 105 Hybrid in Hornady brass mandrelled to 2thou neck tension, neck lubed, originally seated 8thou off of the lands, but by that time the barrel had eroded to ~140thou jump. BR2, but no powder. I thought it was simply a dud round. I attempted to restrike it after the match, still no bang. Shaky shake, no rattle. Hmm. Weighed it at home. ~40grns lighter than the rest of my lot of ammo. No powder. So effectively, I inadvertently performed an N=1 experiment to test this theory. Measuring BTO, it had not displaced the bullet even a measurable thousandth.

So I’m prone to believe the primer CAN and typically DOES move the bullet, but apparently it isn’t an absolute certainty, even in the absence of crimp or heavy neck tension.

 

[taliv]

lys, that's an interesting graph. can you link to the source?

varmint, that's interesting but not entirely apples to apples. in the event one failed to put powder in the case, there would be a large volume for the gas from the primer to expand into. to actually test, it seems like you'd want to fill the case with an inert material in the rough shape of powder granules. if you're above 90% case capacity, which would be the case with most ammo, the pressure inside the case would be much higher than at essentially 10% case capacity, or whatever room the base of your bullet takes up.

 

[rkittine]

"A well behaved bullet will begin it's flight on a center line and move up in the beginning of the arc as it heads down range. As the spin rate declines over distance, at a slower rate then the projectiles decay in velocity, the projectiles gyroscopic stability will increase" Many studies have been done on this, especially with regard to long range shooting.

 

[taliv]

Could you please provide empirical provenance for this claim? You’ll be a wealthier man if you can.

I assume he meant wobble about an axis like a poorly thrown football, not do loop de loops

 

[Varminterror]

I assume he meant wobble about an axis like a poorly thrown football, not do loop de loops

Were it not for the subsequent claims - directly stating smaller angular dispersion are longer range - I might have also given the benefit of the doubt. But the bolded section of the quote below is pretty hard to misinterpret.

Some projectiles tend to spiral while gaining stability and that spiral is sometimes wider at shorter distances. We have to look at pure groups by MOA and not necessarily by size. The group at 100 yards might be say .5 MOA while the group at 300 Yards might be .3 MOA and still about the same physical size though actually tighter.

This just doesn’t happen anywhere except the internet.

 

[Howa 9700]

A couple observations and a question. On observations, have often wondered / marveled......that for a bullet to get moving, not only has to first overcome the inertia of being at rest, but then when it hits the lands, has to also overcome more inertia and start spinning. Have often wondered if bullet has to initially skid a bit, like an airplane tire during landing.

But as for the question........where does twist rate of barrel fit in? If a bullet begins to twist the moment it hits the lands......why does twist rate even matter in this gyroscopic scheme of things?

 

[Varminterror]

Have often wondered if bullet has to initially skid a bit, like an airplane tire during landing.

Barring poor fit, no, there’s no “skid.” Recovered bullets would display remnants - witness marks - at the front of the bearing surface if the bullet had skid over the leade, but we can see that these witness marks are not present. There’s no force which would repair and disguise such a mark, so we know if it happened, we’d see it, and since we don’t see it, we know it doesn’t happen.

Convincing the bullet to spin (overcoming its angular inertia) is easier than convincing it to move and especially easier than convincing it to engrave itself upon the rifling. Properly fit for the bore (vast majority of cases), bullets don’t skid over the early rifling.

 

[Varminterror]

If a bullet begins to twist the moment it hits the lands......why does twist rate even matter in this gyroscopic scheme of things?

Spinning slowly doesn’t stabilize as much as spinning quickly.

What question are you really trying to ask here? Because this question seems too simple to need defining.