How do you straighten runout?

[Dgutter]

So I've been following this thread intently in hopes to find some new points of view to help shape my own and to generally become educated by people more knowledgable than myself. However, I think through the whole process of what happens from the time the trigger is pulled til the bullet leaves the barrel and all that could affect its accuracy and I'm curious about a couple points in there that nobody has spoke of.

1. Whether or not the neck, shoulder or bullet is centered in the bore....how bout the case head itself. I don't think I have found a shell yet where the primer was struck dead nuts center. Which leads me to conclude the case head is not being held on center with the bore. That gives me reason to believe that if the bullet is centered in the bore but the case head is off....the bullet is actually crooked. ????? Any thoughts? Does my bolt head need work or firing pin off center???

2. How about whether or not the bullet is against the lands or has a jump? If it has a jump and the extractor holds the case to one side of the chamber then does that mean the bullet is going to be crooked as it enters the lands? And if its jammed against the lands should the bullet "self-center" itself aside from what was mentioned above in #1? Any thoughts on this as well???

 

[Gone Ballistic]

In answer to your first question, I would say that your firing pin is off center which, by the way, is the case on many rifles. You didn't specifically state what type of action your firearm is. Most lever, semi-auto and pump actions are generally somewhat off center. If it's a bolt action it alsocould be somewhat off center. It shouldn't be a problem as long as your head space is within proper tolerances and your bolt face is square to the cartridge base.
The second answer is that the case is held tightly against the bolt face when the round is fires, so the extractor has nothing to do with anything other than extracting the fired round. If your chamber was out-of-round it might cause case alignment problems with the neck but usually you would end up with a case splitting open. The same would happen if your headspace was too great.

 

[Gone Ballistic]

Dgutter,
I'm sorry, I missed answering a part of your second question. Unless you have been reloading your own ammo, your bullets are definately "jumping" forward to your lands and grooves. Weatherbys are made to shoot all bullets like that. It's called freebore. Most reloaders can sometimes enhance their groups by backing the bullet off the lands and grooves by a thousandth or two. Some even load with the bullet touching. Normally, if you have a standard size box type magazine, you have to load to SAAMI standard length or they won't feed into the chamber from the magazine. Those that load the previous way I mentioned usually have custom made guns with lengthened magazines or shoot single shot.

 

[Dgutter]

In answer to your first question, I would say that your firing pin is off center which, by the way, is the case on many rifles. You didn't specifically state what type of action your firearm is. Most lever, semi-auto and pump actions are generally somewhat off center. If it's a bolt action it alsocould be somewhat off center. It shouldn't be a problem as long as your head space is within proper tolerances and your bolt face is square to the cartridge base.
The second answer is that the case is held tightly against the bolt face when the round is fires, so the extractor has nothing to do with anything other than extracting the fired round. If your chamber was out-of-round it might cause case alignment problems with the neck but usually you would end up with a case splitting open. The same would happen if your headspace was too great.

Good point, I shoot a Savage bolt action which I have been reloading for. It's a factory rifle and essentially has not been altered in regards to action or barrel. I guess I may have a slight misconception as to what happens the moment the firing pin strikes the primer. Let's see if I can try to explain my thought process and anyone feel free to correct me as this is quite educational for me. K here it goes....
Once the firing pin strikes the primer, the primer ignites thus setting off the powder within the case. The pressure within the case begins to increase and as a result the case begins to expand. The bullet is then exits the cartridge and engages the L&Gs. The pressure inside the case and space behind the bullet increases at such a rate that the case is pushed back against the bolt face and the case walls engage the chamber creating a seal. The increasing pressure following sends the bullet down the bore. Once the bullet exits the pressure instantly drops and the case slightly retract due to the modulus of elasticity of the brass.
So from my thinking the bullet would actually leave the case before the case is seated firmly against the bolt face. Is this true or are my mechanics off? I feel the only way it could seat firmly against the bolt face before the bullet leaves the case is if the bullet were seated against the lands??? yes/no/maybe so???

Please anyone feel free to jump in and correct me.
BTW if you feel that this should be its own thread let me know and I would be happy to move it.

 

[ken snyder]

The closest to zero head space is to fireform with the bullet slightly stuffed, the closest brass to chamber fit is to neck size only. The brass will always be smaller than the chamber (or it wont chamber) The brass will always drop, the more it is sized down the more it will drop. the brass will always center itself off the largest radious that is more than 180º. As said earlier unless you go custom the best a person gets is what they have. Use good brass and dies and do your best.

 

[rscott5028]

[...]The bullet is then exits the cartridge and engages the L&Gs. The pressure inside the case and space behind the bullet increases at such a rate that the case is pushed back against the bolt face and the case walls engage the chamber creating a seal. [...]

I'm no physics expert.

But, I just don't see how the bullet could move forward until the case is pressed back against the bolt head or at least has significant grip on the chamber.

If you've ever seen a cartridge explode in a fire, you'll notice that the brass goes flying and the bullet pretty much sits there.

That's just my thinking. But, I've been wrong before.

-- richard

 

[Dgutter]

Richard,

I'm definitely not trying to argue. I'm just looking for the extra pieces to the puzzle that I'm missing. I do understand your point about the case being pushed back as the bullet "stands still".
However, what I'm thinking is that the pressure in the case acts on all inside surfaces equally including the bullet. With light neck tension as most reloaders use the bullet would (imo) have to release from the case. Now at this point i suppose the "back pressure" created by the bullets mass vs case mass could be the driving force that pushes the case against the bolt face. But even then the bullet wouldn't be in perfect alignment with the bore as it "released" from the neck tension before hand.
Idk??? I could be wrong as well and in my attempt to explain my thought process I was hoping to be proven wrong (or right) through yourself or many of the other knowledgable folks here. Maybe I should crunch the numbers myself...but that just takes so much time.

 

[Bart B]

In all the rifle's I've used, here's how the case fits the chambers:

1. Extractors all push the case sideways against the lip of the bolt face. The bolt face diameter's larger than the back end of the chamber. So when a round's chambered, the extractor pushes the case against the chamber wall.

2. The clearance between bolt face and extractory lip is always more than case rim thickness. There's no way any extractor I know of to hold the case head against the bolt face. There has to be ample clearance between bolt face and extractor lip to allow the thickenst rimmed case to easily slide in when the round's loaded. (Go measure your own rifle bolts and rim thickness to see what the difference is.) After the round's chambered, there would have to be some force pulling the extractor back such that it pulls the case back against the bolt face. I don't know of any extractors that pull the case hard against the bolt face hard enough to keep it there when 25 to 35 pounds of force smack the primer.

3. All my rimless bottleneck cases from 22 through 30 caliber have always had their shoulders set back a thousandth or more when the firing pin strikes the primer. How much depends on firing pin spring strength, firing pin protrusion from bolt face, firing pin tip shape, primer cup hardness, shoulder area and angle, and the friction coefficient between case and chamber shoulder. Loads with 10% or more reduced powder charges typically end up leaving the primer pushed out of the case and case headspace is less after firing than before; proof the case fired with the case head forward of the bolt face. I've measured too many such primed cases without bullet and powder to see how much their shoulders get set back by firing pin impact. I was amazed when I first measured them. So there's no way my extractors were holding the case head against the bolt face.

4. When a rimless bottleneck case is struck by a firing pin, it gets driven forward into the chamber an amount equal to chamber headspace minus case headspace. The force sets the shoulder back lengthing the case neck. And because case and chamber shoulders have the same angle, this perfectly centers the front of the case in the chamber. When the round fires, the case head is a few thousandths further away from the bolt face and the case expands to fit the chamber starting at the thinnest part of the case first at its front end. The primer is also pushed back out of its pocket. As pressure builds and the bullet starts down the bore, the case expands more pushing the back of the case against the bolt face seating the primer as well as pushing the case against the chamber wall. As the case expands fully against the chamber wall, it draws brass back out of the neck and shoulder area shortening the case. All such cases are typically shorter after firing's complete. They lengthen when resized; a little with neck only, more with full length sizing.

5. Headspace is the distance between the bolt face and the chamber point that limits case movement forward in the chamber; according to SAAMI standards/specifications/glossary. Head clearance is the difference between chamber headspace and the same measurement on the case; the space between the bolt face and case head when the round fires. Let's not get these two terms confused.

 

[Dgutter]

After more research I think I have finally answered my own question. Read an article on the website that explained it quite well. Thought anyone reading my insane question may also benefit from this. In a way I kind of feel like an idiot not realizing this myself. Anyhow here's the quote:

"If you are single feeding only, try to get the bullet as close to the lands as possible. However, I don't want the bullet to fully engrave. If the bullet is loaded with little to no runout, having it on the lands is not necessary for bug-hole accuracy. The key is ensuring that the bullet will fully engrave in the lands WELL BEFORE the bullet leaves the neck. If the bullet has to make an unsupported jump, no matter how small, sub MOA accuracy will be very tough to achieve."

Makes sense to me considering a bullet should be seated in the neck considerably more than the distance it is from the lands. I apologize for the long discussion I created.

 

[Bart B]

"If you are single feeding only, try to get the bullet as close to the lands as possible. However, I don't want the bullet to fully engrave. If the bullet is loaded with little to no runout, having it on the lands is not necessary for bug-hole accuracy. The key is ensuring that the bullet will fully engrave in the lands WELL BEFORE the bullet leaves the neck. If the bullet has to make an unsupported jump, no matter how small, sub MOA accuracy will be very tough to achieve."

If this is really true, how did all those 'smiths rebuilding match grade 7.62 NATO M1's and M14NM's get 2/3rds MOA accuracy at 600 yards with them using good commercial .308 Win. match ammo with bullet runout up to 3 thousandths and charge weight spread of 3/10ths grain with new, unfired cases? They'd shoot about 1/3 MOA or better at 100 yards.......... All this with all those moving parts having to go back to exactly the same place after each shot, mil spec chambers and several thousandths of bullet jump. Amazing, I think.

 

[rscott5028]

I'll defer to the experts on internal ballistics. But, even the experts tend to dumb it down for the rest of us...
Internal Ballistics - Hornady Manufacturing, Inc

I'm not even sure that there's a single right answer for your question/theory since it may depend on seating depth, headspace, case head diameter, extractor/boltface, runout, and many other factors.

Regardless, I think most of us understand that there are many variables. Some can be estimated, anticipated, and/or measured. But, they all add and/or subtract from eachother thus making every rifle somewhat unique and requiring load development for nitty gritty fine tuning.

I've seen many resources that indicate that .003" TIR or less is not going to adversely affect accuracy/repeatability. So, I work towards .002" or less while concentrating on other variables such as headspace and neck tension, etc...

It's probably easier to avoid all of the things that will undoutedly decrease accuracy. ...like making crooked brass

-- richard

 

[Dgutter]

If this is really true, how did all those 'smiths rebuilding match grade 7.62 NATO M1's and M14NM's get 2/3rds MOA accuracy at 600 yards with them using good commercial .308 Win. match ammo with bullet runout up to 3 thousandths and charge weight spread of 3/10ths grain with new, unfired cases? They'd shoot about 1/3 MOA or better at 100 yards.......... All this with all those moving parts having to go back to exactly the same place after each shot, mil spec chambers and several thousandths of bullet jump. Amazing, I think.

I agree, it is impressive. I did leave a part out of that article as to not quote the entire thing. But the author did say he wasn't extremely concerned of runout up to 4 thousandths and charge weight spread of +/- .2 grain. The part of what I quoted though that interested me most was that the distance the bullet is seated off the lands is less than that of the seating depth within the neck. That way as the bullet engages the lands it is still supported by the case neck. This can easily be achieved with what you stated above. So in a way I think the statements do agree.

 

[rscott5028]

After more research I think I have finally answered my own question. Read an article on the website that explained it quite well. Thought anyone reading my insane question may also benefit from this. In a way I kind of feel like an idiot not realizing this myself. Anyhow here's the quote:

"If you are single feeding only, try to get the bullet as close to the lands as possible. However, I don't want the bullet to fully engrave. If the bullet is loaded with little to no runout, having it on the lands is not necessary for bug-hole accuracy. The key is ensuring that the bullet will fully engrave in the lands WELL BEFORE the bullet leaves the neck. If the bullet has to make an unsupported jump, no matter how small, sub MOA accuracy will be very tough to achieve."

Makes sense to me considering a bullet should be seated in the neck considerably more than the distance it is from the lands. I apologize for the long discussion I created.

Please share the link to the article you quoted...

 

[ken snyder]

Fun thread! The cartridge is driven forward by the pin strike. After that it is a physics race of inertia and friction as to what moves and how far. The best that can be done is to fire form slightly stuffed. The reason to me is that at some point a difference wont be made out of something that can't make a difference. No matter what we do there will always be a little over .001 clearance and I would hope quite a bit more than that aruond the neck!