Bore rider boolits 101 needed

[DrCaveman]

As I am gaining momentum in my 30-30 loading of cast boolits (it has gone from total garbage to passable) I realize my knowledge base is very lacking regarding "bore riding" boolit designs. Looking for wisdom from you guys.

What I understand so far (and may be wrong, please correct if so) is:

-bore riders have a reduced diameter in front of the front driving band

That's about it. Here are my questions:

-why such a boolit design? To reduce friction traveling down the bore?
-how do we know the diameter of bore riding section based off (commercial) mold specs? And shouldn't it match the bore for best success?
-mold diameter is based on groove diameter right? So refer again to previous question.
-which 30 cal Lee mold designs are 'bore riders'? I guess I don't know enough to tell from website.
-the only boolit I can think of which doesn't taper down on the front is a wadcutter. So since some portion of the boolit nose will almost surely be the diameter of the bore (except maybe semiwadcutters with abrupt, distinct shoulders) would that make most boolit shapes 'partial bore riders'

Last, and the least academic of the questions is:
-do you all concur that bore riders are bad for micro groove marlin 30-30?
-has anyone used bore riders with a tikka t3 30-06?

I know that I should just shoot and see. I have done that, and had pretty good success with the 30-06 with my cast loads. However looking back through my notes I realized that the lee TL-312-160 2r gave me my best 100 yd groups. That one does NOT appear to be a bore rider. Haven't tried it in the marlin because of pointed nose.

And I am sorry if I have flip-flopped bore and groove in the above description. If I have done so, then it means I am WAY more confused than I thought regarding bore-riders.

Thanks

[fouronesix]

First, the cartridge OAL has to be able to cycle through a repeater. Next, weight usually has something to do with length of the bullet. And, usually the base of the bullet shouldn't extend too far into the body of the case. All those things working together sometimes call for a bore riding nose. If that nose is hung out too far and is unsupported during it's acceleration down the bore chances are pretty good that the nose will randomly slump to the side of the bore thus ruining accuracy. The bore rider portion should be a snug fit into the bore when the cartridge is chambered. In single shots and some bolt guns the resistance of the nose engraving into the lands is easily overcome. It can be very problematic in semi autos and can cause balky chambering in lever guns. So there is a fine line of just right snug of the bore riding nose section and the need for ease of chambering. In most instances with say a lever gun- if the nose section is just barely snug and doesn't impede chambering, then accuracy will be good. If it is too snug then the lever will be balky or may not chamber at all. If too loose then accuracy will likely suffer.

[DrCaveman]

Ok I guess I am still missing a piece of terminology, I need to study drawings of rifle barrels more carefully.

I understand the nose engraving into the lands, I have done that with some jacketed bullets for the 30-06. Also with lead in my handi rifles, though they are 45-70 and 357 mag. But if the nose is engraving, then what makes it be a 'bore rider' vs any old normal boolit?

I am still not seeing the distinction, maybe I am being more blunt than my rnfp's

[fouronesix]

Ok, I'm not doing to draw a diagram, take a photo, resize it, transfer it and post it here.

Look at a regular RNFP bullet. It should have one to a few grease grooves in between the drive bands. The top groove may or may not be a crimping groove. The top drive band is usually the same diameter as the diameter of the rest of the shank of the bullet. With a non-bore riding bullet the nose from that point alone its ogive to the tip of the bullet, regardless of ogive shape, is not large enough to touch the lands. That portion of the nose therefore is unsupported in the bore.

The bore begins where the throat ends.

With a bore riding bullet, the nose from the front drive band for a distance forward until it starts its ogive to the tip is parallel in form and is approximately bore diameter (diameter from land to opposing land). That parallel portion of the nose RIDES ON THE LANDS IN THE BORE. Therefore is called a bore rider.

[DrCaveman]

Fouronesix, I think I gotcha. Thanks.

So now, it sounds like friction would be a little greater for bore riders than non. And, it seems that the lee 312-160 mold I mentioned is possibly the only bore rider I have.

Are any of the lee 30 cal boolits 150 gr and up bore riders? The parallel portion of the nose is certainly smaller than the front driving band, but not sure if it is actually big enough to be supported by the lands.

Prob me being blunt again, thanks for the patience.

[RoyEllis]

Quickest way to see if the forward "bore riding portion" of your boolit fits the rifle bore is to turn one around & stick the nose into the muzzle...if it's a tight slip fit or does it have some wobble room. You really want the fit to be snug or a little better to help keep nose slump at bay as your loads get faster.

[DrCaveman]

So then, why have a section of boolit nose merely snug to leads when you could have full groove diameter all the way and get even better stability?

Is it just a dance of boolit weight vs ballistic coefficient, or some other friction/pressure based reason?

[milkman]

Reread post 2. It is difficult to force a groove diameter boolit very far into the rifling since it is being engraved by the lands. A bore riding nose will slip into the bore (without being engraved), enabling you to use heavier boolits without seating them deeply past the neck of the case.
Milkman

[Tom Myers]

As I am gaining momentum in my 30-30 loading of cast boolits (it has gone from total garbage to passable) I realize my knowledge base is very lacking regarding "bore riding" boolit designs. Looking for wisdom from you guys.

********************

And I am sorry if I have flip-flopped bore and groove in the above description. If I have done so, then it means I am WAY more confused than I thought regarding bore-riders.

Thanks

A Picture is worth a thousand words.

A bore riding bullet designed to fit a SAAMI Spec, 6mm Rem chamber.

Hope this helps.

http://www.tmtpages.com/LinkSkyImages/forum_images/6mm_Remington/6mm_Rem.~_SAAMI~TMT_C244-237-100-RN.Jpg

[youngda9]

The .35 diameter section between .54 and .64 in length is the bore riding section of this bullet. Shoots well in my 358 Winchester.

http://www.accuratemolds.com/img/bullets/detail/36-230D-D.png

[DrCaveman]

Thanks, diagrams helped a lot.

Next question: at what point (length of boolit outside of case, or prob % of weight being more important) is a bore rider needed? Seems that if 50% or so of total boolit length is groove diameter (maybe like the lee soup can?) then stability would be ok without any need for bore riding.

Is there a rule of thumb, or does everybody do it a little different?

[runfiverun]

stability isn't the issue look at the rcbs silhouette boolts they are about 70% nose.
if you push them too hard you lose accuracy.
think about the reasons why that would happen.

[DrCaveman]

I found a similar thread started around June of last year, it helped me understand a bit. Also reading more of fryxell's book is helpful... But very time consuming. Think I need to print that sucker out and read it cover to cover, break time at work is a great opportunity to crank out a chapter or two. Reading from phones is no fun.

Run, all I can think of is this word 'slump' that keeps getting thrown around. Since my assumption is that once driving bands engage rifling, the boolit will square up, I guess I need to do a lot more reading. Apparently it is not as simple as that, though my 358429 would seem to disagree.

Makes no sense to me that a boolit would give a hoot whether it was pushed at 1500 fps with 20k pressure or 1800 fps with 24k pressure, based on boolit shape. I thought fit, hardness, and lube were determining factors, all else held equal.

Am I right then to think that bore rider designs complicate the 'fit' issue in an exponential manner? One design, one alloy, one sizer die, will get you TWO sets of numbers which are not necessarily related nor adjustable independent of one another? Seems like either a crapshoot, or something warranting a custom mold design.

FWIW my lee 309-150 fp nose fits 'snugly' in the muzzle of the marlin 336 MG. So it should work, I conclude, at reasonable velocities. Maybe I need to upgrade molds to get past ~1800 or so.

[btroj]

Dr caveman, I asked similar questions last year.
Think of it this way. The bullet, case, chamber, and bore are basically in line. Or are they? Bullet is in throat, centered pretty well. Then the pressure rises. If the bullet has any place to slump, or flow, into then it will. Key is, will this be symmetrical?
The long bore rider nose can be good, if it fits snug in the bore. If even a bit lose the slump will NOT be symmetrical. Even if the nose is snug in the bore the rest of the bullet can change shape enough under pressure to give less desirable results. The chance of the bore rise being snug enough are also somewhat slim.
Like anything, this is all relative.

Pressure, and rate of pressure rise, is important. Pressure curve matters because the amount of slump that can occur in the lease, between case neck and bore, is defendant upon pressure when bullet is in that area. Fast pressure rise can cause much slumping into this potentially unsupported region, slower pressure rise can let bullet get into bore before slump can occur into unsupported regions.

Hope that helps.

[John Boy]

all I can think of is this word 'slump' that keeps getting thrown around.Slump is a condition respective to alloy hardness and has nothing to do with being or not being a bore riding bullet or whether the bullet is jumping the leade or in the leade for better axial position

[runfiverun]

smiling.
you also have to remember that you have to spin the thing, 1/4" of bearing surface isn't gonna do that, band-aid on the back or not.
ballance and limits.

[DrCaveman]

So, are we talking centrifugal forces sending the nose 'outward' at the instant of rifling engagement of bearing surfaces? If so, it makes sense that this could be a problem when pressures are at a certain point, where a portion of the (bore riding) nose is also engaged by rifling but the rest of the nose is not.

Leading to non-uniform concentricity and a resultant wobble?

Am I getting it?

John boy, are you saying that slump is similar to obturation in that it is a function of boolit hardness/toughness and pressure? So a harder boolit may solve this accuracy problem though leading wasn't the issue?

Thanks for working through this with me guys.

[btroj]

8-)Hold on a second Run, are you saying it has to do with RPM?

I had never thought about the bearing surface and it's ability to grip the rifling without stripping.

more I learn the less I know.

[swheeler]

8-)Hold on a second Run, are you saying it has to do with RPM?

I had never thought about the bearing surface and it's ability to grip the rifling without stripping.

more I learn the less I know.

Boy howdy, me too! But yes you have to get the bullet rotating without stripping or it isn't going to shoot for beans, so the simplest way around this problem is to use the slowest twist that will stabilize at desired velocity. Some call it a RPM theory, I call it common sense.

[swheeler]

Of course some of the RPM effects can be overcome by bullet design/bearing length, hardness, and where and when peak pressure is applied and how long sustained. Obturation and deformation are not the same thing, anybody can have a bullet obturate the bore/seal it, but if that same obturated bullet deforms far enough and pushes nose to one side(which does happen no matter) then the major contributing factor to inaccuracy is rpm'S. There's a fellow here who has been all over this like white on rice for many many years. Facts are facts no matter how nyou try to twist(pun intended) it.

[runfiverun]

glad you come back with the second post.
that one [and a couple ahead of it] explains why the rpm threshhold can be overcome.
and why a bore riding nose won't go much past it.

you have to think in terms of up and down from the bottom of the chamber if you use a fatter boolit how does it really help?
if you use thicker brass?
if you change the shape of the boolit from paralell to a sloping shape in the throat area and size to scuff the area ahead of the chamber.

look at tom's picture.
now look at all the white surrounding the boolit and in front of the case.
now make the area around the neck bigger,and add some more white in front of the chamber.
that's more representative of what you are really dealing with.
see how the shape of the boolit would change?
it would end up looking more like youngda9's boolit.

now the whole round is stuck hanging off the nose of the boolit with all that white showing around everything.
worse yet it's holding the case on an angle too, all the way back to the bolt face,so that when the round is fired it's shooting it's gas charge [and the base of the boolit] at the roof of the throat.
need i go on?

oh.. this thread is gonna get linked to my 'something that may help' sticky too.
this will make the 4th or 5th thread dealing with throats and alignment tacked on the back of it.

[Larry Gibson]

Of course some of the RPM effects can be overcome by bullet design/bearing length, hardness, and where and when peak pressure is applied and how long sustained. Obturation and deformation are not the same thing, anybody can have a bullet obturate the bore/seal it, but if that same obturated bullet deforms far enough and pushes nose to one side(which does happen no matter) then the major contributing factor to inaccuracy is rpm'S. There's a fellow here who has been all over this like white on rice for many many years. Facts are facts no matter how nyou try to twist(pun intended) it.

Very well put.

Centrifugal force could indeed cause nose "slump" to one side as DrCaveman suggests. However it is generally conceded that the "slump is caused more by increased acceleration where there can be weak spots in the bullet (softer alloy in some locations than others or hard antimony spots from too much antimony and too little tin) or the driving bands collapse on one side. This would cause an unsupported bore riding noet to tip to one side. The former and latter can easily be seen in HV/high RPM and higher pressure loadings. The latter is easily seen with too soft alloys.

Many who push the velocity and RPM threshold up use a softer (more malleable actually) alloy that is hardened (WQ'd or HT'd) so the bullet will set back evenly during acceleration. However, as acceleration increases it overcomes the bullets elasticity and the bullet obdurates and setbacks (slumps) unevenly. This unbalances the bullet. Of course the unbalanced bullets accuracy is adversely affected and a new, higher RPM threshold has been found for that bullet/load combination. At some point, using a cast bullet of ternary alloy, if the practical velocity is not reached with the cast bullet then an RPM threshold will be found when accuracy will get poor very quickly.

It is indeed the centrifugal force acting on the imbalances of the bullet that cause the inaccuracy in flight......at the RPM threshold.

Larry Gibson

[MBTcustom]

All I know, is the best most accurate brass is the one that is still in the chamber of the rifle, that has just been fired. Every thing we do to the case (eject, tumble, size, trim, debur, reload, chamber, and fire) has the ability to detract from that perfection. The boolit comes into play also, in very similar order.
At the end of the day, I realize that the brass itself is a weakness in accuracy design, and the breech seating black powder guys actually have the best "case" scenario (pun intended).
However, I think we can get pretty close, but things need to be considered that are usually taken for granted.
Not that I am some great target shooter, but I know where the problems are.
The big question is how do you accelerate a soft boolit into a controlled condition without damaging it in the process?
Sometimes this feels like using a sledge hammer to drive a finish nail into a piece of Sheetrock and examining it with a microscope to observe any damage! LOL!

[geargnasher]

Very well put.

Centrifugal force could indeed cause nose "slump" to one side as DrCaveman suggests. Centrifugal force causes "slump"? I don't understand how that can be. However it is generally conceded that the "slump is caused more by increased acceleration where there can be weak spots in the bullet (softer alloy in some locations than others or hard antimony spots from too much antimony and too little tin) or the driving bands collapse on one side. This would cause an unsupported bore riding noet to tip to one side. The former and latter can easily be seen in HV/high RPM and higher pressure loadings. ...and a few other anomalies.....The latter is easily seen with too soft alloys. Other things, like improper powder burn rate selection and some of the things Runfiverun covered are more usual culprits unless there is a major issue with large voids from poor casting technique.

Many who push the velocity and RPM threshold up use a softer (more malleable actually) alloy that is hardened (WQ'd or HT'd) so the bullet will set back evenly during acceleration. NO, that isn't why that alloy is chosen. However, as acceleration increases it overcomes the bullets elasticity and the bullet obdurates and setbacks (slumps) unevenly. First, if you launch it properly, there is no setback. Second, what would guarantee that it will slump UNEVENLY, if you force it to do so? Talk to the black powder guys about how uneven their nose slump is based on 1,000-yard targets. Perhaps something else is at play to damage the boolit's noses? This unbalances the bullet. Lots of things can "unbalance" the boolit, not a good thing. Of course the unbalanced bullets accuracy is adversely affected and a new, higher RPM threshold has been found for that bullet/load combination. Higher threshold with more unbalanced boolits? :) At some point, using a cast bullet of ternary alloy, if the practical velocity is not reached with the cast bullet then an RPM threshold will be found when accuracy will get poor very quickly.

It is indeed the centrifugal force acting on the imbalances of the bullet that cause the inaccuracy in flight......at the RPM threshold.

Larry Gibson

Wouldn't it be better not to get the imbalances in the first place than allow them to happen and try to mitigate the effect with slow twist?

Gear

[geargnasher]

The big question is how do you accelerate a soft boolit into a controlled condition without damaging it in the process?
Sometimes this feels like using a sledge hammer to drive a finish nail into a piece of Sheetrock and examining it with a microscope to observe any damage! LOL!

An egg in a potato cannon would be an appropriate analogy.

Gear

[Larry Gibson]

Wouldn't it be better not to get the imbalances in the first place than allow them to happen and try to mitigate the effect with slow twist?

Gear

That certainly is the trick as we have discussed many times before. However, even you find a velocity level where accuracy diminishes regardless of what you do to mitigate it. Guess what? That is the RPM threshold for that load. The higher a velocity (with a given twist) that we can mitigate any imbalances to the bullet the higher the RPM threshold.

However, at a certain point with any cartridge/cast bullet combination capable of 140,000+ RPM with a "standard" twist barrel it is much easier to use a barrel with a slower twist if practical. Such as using a 12 or 14" twist in a .223 or a 12, 13, 14 or even a 16" twist in a .308W or '06. What is "practical" simply depends on how much you really want to spend shooting cast bullets to HV with a semlence of consistent accuracy easily. Or we can instead strive to mitigate imbalances using a "standard" twist and maybe get results with a lot of frustration and other equipment expense using the "standard" twist such as a 7 or 9" for the .223 or a 10" twist for the .308W or '06 these days. Just depends on how you want to get there.........easy way or the hard way..........the easy way is with the slow twist...........

Larry Gibson

[swheeler]

Very well put.

Centrifugal force could indeed cause nose "slump" to one side as DrCaveman suggests. However it is generally conceded that the "slump is caused more by increased acceleration where there can be weak spots in the bullet (softer alloy in some locations than others or hard antimony spots from too much antimony and too little tin) or the driving bands collapse on one side. This would cause an unsupported bore riding noet to tip to one side. The former and latter can easily be seen in HV/high RPM and higher pressure loadings. The latter is easily seen with too soft alloys.

Many who push the velocity and RPM threshold up use a softer (more malleable actually) alloy that is hardened (WQ'd or HT'd) so the bullet will set back evenly during acceleration. However, as acceleration increases it overcomes the bullets elasticity and the bullet obdurates and setbacks (slumps) unevenly. This unbalances the bullet. Of course the unbalanced bullets accuracy is adversely affected and a new, higher RPM threshold has been found for that bullet/load combination. At some point, using a cast bullet of ternary alloy, if the practical velocity is not reached with the cast bullet then an RPM threshold will be found when accuracy will get poor very quickly.

It is indeed the centrifugal force acting on the imbalances of the bullet that cause the inaccuracy in flight......at the RPM threshold.

Larry Gibson

And put the least little void, imperfection, or soft spot in the bullet, your group just went TU. That's why most of these HV cast groups shown are SINGLE groups, lets see 5x5 or better yet 10x10 groups.

[DrCaveman]

Wow, good discussion guys. I have re-read the entire thread about 5 times and I catch new things each read.

The RPM threshold is interesting, and ~maybe~ I have found it with my marlin. Of course there have been many other variables explained here to which I may not be paying proper attention, and I should refine my practices a bit to help narrow things down.

At least the rpm issue involves simple math, since I know the barrel twist and have a chronograph. It seems that I may need to get a little more consistent in my alloy in order to take results from one shooting which may have shown me an 'rpm threshold' but all that will change if my next batch is a little heavier or lighter with tin or antimony %.

Hmm, lots to ponder. I think I will try the 'more appropriate powder' direction first. Rx7 worked great using GC boolits in the 30-30, I will see what's what with down loaded plain base boolits.

After I shoot up all those I will revisit lapping or beagling the mold and possibly water dropping. I suppose I could just go cast some and water drop now...hmmm...casting or loading...maybe I make a pot of coffee and do both tonite

[DrCaveman]

Ok, to heck with that thought about rx7. I want to get this thing working without gas checks, and my load notebook shows that 18.5 gr was terrible.

Guess I will try to max out a unique load. 8.5 seemed very wimpy, but maybe that's what I am stuck with. Maybe I push it a little further, though the speed of unique compared to 'fast rifle' powders seems a bit off-track. Still think 2400 may be my ticket, I shall also tinker with charges there.

Rx7 shot like a CHAMP when gas checks were applied. Maybe this says something about my process. 25.5 gr, near max, prob about 2000 fps (didn't have chrono yet), led me to remark: "best load. Use for hunting."

Good thing I took notes, sometimes they make more sense 6 months later than they did when I took them.

[btroj]

It isn't just velocity. Pressure AND pressure curve matters. I like 2400 in a 30-30 for up to about 1750 fps. If I want more I go to a slower powder. For max velocities I like to go slow as possible. For hunting I use RE15. This eases the push on the bullet.

[runfiverun]

nobody said it was easy.
if you care to take the time to make things work, you will have no more and no fewer flyers than you will with storeboughts.

i believe i have shown multiple groups every time i have shown any groups.
i think the last target i showed was 100 shots on the same target from the same day [from my 8 mauser] 5x20 shot groups using J-lube a 180 gr boolit and 32 grs of 4895 with a dacron filler/ no it was dryer lint filler.
it was showing the individual groups pretty clearly and how they shifted slightly throughout the day [due to mirage in the scope] but the entire group[s] was within 1-1/2"s.
pretty much what i expect from this rifle on a consistent basis even with jaxketed.

i don't expect a rifle to magically start shooting 1/2" groups just because i am using cast [sometimes it happens]

[DrCaveman]

Forgot I had imr 4227. A minimum load with this should put me around 1600 fps, with a relatively slow pressure curve, if I understand.

Loaded up 10@ 16.4 gr, no gas check. Lithi bee lube, boolit nose fits snug into muzzle but requires nothing more than a push...no whack or hard hand pressure needed. No wobble.

Driving bands are averaging about .310, some show 311 and even 314 at widest. Did not size these ones at all. Wish they averaged out bigger but apparently my mold cavities aren't quite equal. A good note is that they are larger than before minor lapping.

I should probably take better note of which boolit fell from which cavity, but that requires some serious discipline after the mold is up to temp and everything is moving so nice

[shredder]

"The big question is how do you accelerate a soft boolit into a controlled condition without damaging it in the process?
Sometimes this feels like using a sledge hammer to drive a finish nail into a piece of Sheetrock and examining it with a microscope to observe any damage! LOL!"

Nice! Man this discussion was getting pretty technical until this came up. Now I can relate to that.

Just kidding, I have my first real bore rider design, a lyman 311332, and am here soaking up all the great info.

[Larry Gibson]

"The big question is how do you accelerate a soft boolit into a controlled condition without damaging it in the process?

That is the question alright. Easiest answer, with a given barrel, is to use a lighter weight cast bullet with as slow a burning powder at close to 100% load density that ignites and burns consistently at the psi involved. If one can change the barrel to a slower tiwst and longer length (26 - 30") then that will also recuse the psi and accelleration rate for a specific velocity along with reducing the RPM.

If one has the "given" barrel such as on a milsurp, custom or factory rifle of a "standard" or faster twist then you are adapting the load to that barrel. If you are able to rebarrel with a slower twist and longer barrel then you also are adapting the barrel to the load. With one you've one way to go but with the other you can work from both ways which affect the "accelleration" which was the question.

Larry Gibson

[runfiverun]

if i were using a non gas check boolit and was looking for accuracy i'd target a velocity of @ 1200 fps then slowly move forward.
in some instances 1400 is doable,in others 1600.
you just get to a point where the base allows too much deformation [gas escape]
and it's ability to hold the rifling.
marlins round bumps are not helping the situation in any way.
have reasonable expectations for your combination.
i went through this in my 375 marlin with a couple of plain base boolits,i got to where i had decent accuracy at around 1400+ but the groups were lower than my sights would compensate for at 50 yds.

[DrCaveman]

Runfiverun

Marlin's round bumps? Are you talking about the rifling grooves?

The more studying I do is leaning me more toward your stated 1200 fps goal for starters. Dunno why I thought I could pump em at 1600-1700 without following boolit design (adding gas check), guess arrogance...like you say, have reasonable expectations.

And, like you, my super light loads (8.5 unique) shoot so low that it is hard to compensate with the sights. There's a good chance that I already found a load that grouped well, but since I couldn't find paper after 5 shots nor hit a clay pigeon at 50 yds, so I gave up on that one.

Now trying to bring POI closer to factory rounds by increasing velocity. But trying to cheap out and use a small-ish powder charge and no GC.

Well, the cake is being baked, but I don't know if I deserve to eat it yet.

Better load up some more to meet this new realistic goal. Good thing there is 100% chance of rain today, and its 40 outside. Good time for casting and loading more. Shoot Sunday, that's my weekly spiritual revitalizer. All due respect.