How does the bearing length of a bullet effect accuracy?

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  • Last Post 18 March 2021
Wm Cook posted this 15 March 2021

Could anyone explain the pros and cons for the amount of bearing surface a bullet carries? 

Below are two molds from Accurate.  Unless I am reading this wrong )always the chance, I'm not the sharpest crayon in the box) the 31-150S has a bearing surface of about 55% of the bullets entire length.  The 31-155W has a bearing surface of about 28%.  The bearing surface includes the required gas check on the shank, lube band and the non tapered mose of the bullet.  These numbers skip over the changes made by the sizing process.

So my question is how does the bearing surface effect bullet flight?

I'll add that the 155W has a 1.6 degree taper from .345 to .545" of its over all  length where as the 15S has a straight body from .230 to .440.  In addition the 150S has a bullet tip radius of 1.2 degrees and the 155W has a tip radius of 1.6 with a second approaching metplat radius of .05 degrees.  Is the taper and radius differences fashion design or does it have a purpose. 

Ihave a lot to learn about cast bullet accuracy.  Thanks, Bill.

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RicinYakima posted this 15 March 2021

Here is my hypothesis; Increased bearing length gives you a much better chance of aligning the bullet with the center of the bore when chambering. At whatever angle it is to the centerline of the bore, it will be the same on firing, down the bore and on exit. That is the bullet never "centers" or moves from the it original relationship. Longer bearing length gives you more options for seating depth and thus fitting the throat snugly decreasing blow-by of young gases.  

I have been told by scientists, that I trust, that the nose of the bullet effects flight characteristics. Smoothness of air flow and shape of compression waves are very important for long distance supersonic accuracy. Also the distance from the nose to the first driving band can effect air flow for the length of the bullet.

If you are looking for accuracy, check "The Fouling Shot" for what bullets the benchrest winners are using and study their designs.

HTH, Ric

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Larry Gibson posted this 15 March 2021

As rick mentions the longer the bearing surface the easier it is to get proper alignment of the bullet in the throat of the rifle.  Also during acceleration the longer the bearing surface the less the bullet can cant in the bore.  Note that bore rider noses, even those that fit tight, with bullets having minimal bearing surface can cant very easily in the bore. 

The longer bearing surface will keep the center of rotation closer in line with the center of rotation and the center of form.  The closer all three are together on launch the less wobble or yaw the bullet will have and thus will be more accurate.  

LMG

 

Concealment is not cover.........

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Bud Hyett posted this 15 March 2021

Ric has the interesting point, the bullet in the leade is how it will fit in the barrel. Then the shape will influence the flight after exiting the barrel. Shooting cast bullets is systems engineering, there are many factors to accommodate.

The harder bullet will center better, the softer bullet will move forward and take a set in the rifling as it is presented. Slightly off-center in the leade is slightly off-center in the barrel and slightly off-center in flight. The initial push from primer ignition and powder pressure pushes the bullet forward as it starts to spin where softer bullets will take a set. The harder and longer bearing surface bullet will resist and straighten out to self-center as it engages the rifling in the leade. 

I've long felt the Remington 700 style bolt face and setting the bullet into the leade is like putting a piece on a lathe between centers for ultimate precision. I am not a fan of the claw extractor bolt face since the case can slide around for alignment, Having said that, there have been some very fine score and groups shot with the claw extractor bolt face. 

With almost as many mold as rifles, I come back to the RCBS 30-180-SP and SAECO #315 with each new rifle or barrel. These are known bullets that shoot well and work well on the target. The SAECO is tapered and you can set it into the rifling to center on ignition. This bullet is also flat-nosed and drifts more in the wind with the resultant velocity loss, it shoot better at 100 yards.. When the load is good with these, then I experiment with heavier bullets for better wind resistance.

Ric is right on the researching the results in the Fouling Shot. You'll find a lot of concurrence with the choices. The heavier bullets with longer bearing surfaces predominate. 

Farm boy from Illinois, living in the magical Pacific Northwest

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John Alexander posted this 15 March 2021

I think Ric has nailed it.

If you need any proof look at what competitors shoot.  In 30 and 32 caliber a very high percentage of competitors shoot bullets of 200 grains or over.  An even higher percentage of the winnere do. 180 grains is about the lightest bullet you see at the top. Of course the bullets with higher sectional density also produce the highest ballistic coefficient for a given shape -- a side benefit.

It seems to me that neither of the designs above will have much bearing surface in a factory chamber (+ - .300" bore). Most of the bullet will be stuffed back in the, usually not-aligned, case neck (in factory chambers) so they are not good options for maximum accuracy but are obviously hunting designs where targets are big and close.

John

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Wm Cook posted this 15 March 2021

That helped quite a bit.  The bhn and seating squarely on the lands will start the bullet traveling in the right direction and the longer bearing surface will reduce the yaw. 

Bud's comment about nose profile has been in my mind throughout this thought process.  I believe its described as two different processes with one being called a Cherry cut capable of RN versus a lathe cut which is dedicated to only a FN?  I wonder what the smallest metplat is on a lathe?   And I would guess that the BC has less of an impact at 100 then 200 yards with a FN.

Back to the bearing surface once more.  Where do you draw a balance between the bearing surface, where it makes jam depth in the lands and where the gas check winds up in the neck/body of the case? 

I'm a new member of the CBA in fact my membership card just came last week.   But spent decades looking at result sheets to see who was shooting what in short range benchrest competition.  And I've spent decades loading at jam depth to .010 - .015 off of jam with jacketed bullets. For those that can remember it, is there such a thing as a Benchrest Primer for cast shooters like Brennan put out for jacketed benchrest shooters way back when?

Being new the CBA I haven't seen my first copy of the fouling shot yet.  Always good to know what the winning components are.  Thanks again, Bill.

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Bud Hyett posted this 15 March 2021

Go to the "Matches" header and drop down for Match results, both Military and Traditional. These lists the molds, lubricant, alloy, and size used for a particular rifle. These are good reference for a starting point, but your rifle is unique.

 

Farm boy from Illinois, living in the magical Pacific Northwest

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Wm Cook posted this 15 March 2021

Thanks Bud.  I found the match results and that looks like it would be a lot of fun.  And I figured out an answer to the question about the balance between bearing surface vs olgive vs gas check location.  It is what it is. Find the lands and see where the gas check ends.  On some molds it will be a no go because of the check being in the case and on others all you can do is adjust seating depth with whatever tolerances the bullet design and the reamer allows.  Again, thanks.  Bill.

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John Alexander posted this 16 March 2021

Larry says: " Note that bore rider noses, even those that fit tight, with bullets having minimal bearing surface can cant very easily in the bore."

I have read this several times and would like to respectfully disagree. It doesn't seem to me that such a chambered bullet could "cant very easily."  A long nose bore rider like  the 299 design with the nose engraving *to near the front provides a lot of length of guidance while chambering even if the nose only bears on the lands. It also allows only a short section of bullet in the case mouth for easier rotation to align as the bullet is chambered insead of having a lot of bullet in the case neck in an off center factory chamber misaligning the bullet. The proof is in the pudding and a large share of CBA matches requiring factory chambers are won with long nose bore riding bullets. I suspect that good bore alignment is part of the reason for their dominance. Performance tops theory based on assumptions.

With a custom tight neck chamber and maybe with bumped bullets the answer is different.

John

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Larry Gibson posted this 16 March 2021

John

 

I respectfully have to disagree with you.

Have you ever recovered (?) any bullets, especially bore riders and noticed land engravements only of one or two lands on one side of the nose?  Many of us have.  That fact, not theory, answers the question without hypothesis that the bullets tipped or canted in the bore during acceleration. Note I said "very easily tip".  I didn't say always. Because many bore riders are very successfully used in CBA matches does not prove such bullets can't tip or cant in the bore.  It only proves with the load/velocity used the bore riders used didn't cant enough to adversely affect accuracy....but since no bullets are recovered during CBA matches we/you don't really know....do we/you?

When it occurs has to do with the velocity level, acceleration rate and hardness level of the alloy.  Might surprise you to know some do shoot cast bullets at other than the usual velocities as in CBA matches.  Also, sorry to burst your bubble, but the CBA match results are not the "end all" definitive proof of everything cast bullet.  Many cast bullets are shot for a lot other types of shooting than CBA matches, probably a lot more than are shot in CBA cast bullet matches.  Might be a heretic for mentioning that here but it is a fact.

LMG

Concealment is not cover.........

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Ross Smith posted this 16 March 2021

Larry: If the bullet is out of round, won't that affect engraving by the lands?

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RicinYakima posted this 16 March 2021

Larry, IMHO tipping of captured bullets can be caused by several things:

if the center line of the bullet is not in the centerline of the bore, the nose is pushed at an angle

if neck wall thickness is not equal, the nose is pushed at an angle

if the case is full length resized, the plunger ejector in the bolt face with push the case crooked in the chamber

upon acceleration and as the rear of the bullet slumps from pressure, the nose can tip in any direction

gas checks not seated square will push a nose if the body becomes mushy from pressure

The nose just sitting on top of the lands doesn't make it supported. When I was working with the 311299 vs the 311284, the '299 didn't always mark the bullets from the same lands. The '284 was easier to work with for me and I have used it 25 years.

Have not shot a match since August of 2019 because of the virus, but have used up over 500 primers in my 30/06 plinking. Especially now that I have a chance to shoot steel targets out to 500 yards.

I don't know of any other international organization shooting cast bullets for precision (benchrest) than the CBA.

Respectfully, Ric

 

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Larry Gibson posted this 16 March 2021

Ross Smith posted this 12 hours ago

 

Larry: If the bullet is out of round, won't that affect engraving by the lands?

 

If larger than the bore they probably would be slightly engraved.  However, not to the depth of of the grooves as when canted during acceleration.  There is a difference between slight engraving of the bore riding nose on chambering [with a concentric bullet that slight engraving should be even from all the lands] than with land groove engraving on one side of the bore riding nose caused by tipping/canting during acceleration.

LMG

Concealment is not cover.........

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Larry Gibson posted this 16 March 2021

Ric

 

I'm not criticizing the CBA matches at all.  I also shoot some CBA matches [at Ben Avery range in Phoenix] or at least did before COVD....  My objection is the use of the match results as an "end all" to prove some point when the point really is not germane to the match results.  That bullets with insufficient bearing surface can and do tip/cant in the bore under acceleration is one of those "points".  As I mentioned to John, there are many more cast bullets shot for other reasons and at much different rates of acceleration than at CBA matches.  Besides, we really don't know if any of the bore riders at CBA matches tip or not under acceleration because no one I know of recovers any bullets used in CBA matches.  

As you mentioned in your first post and reiterated in the second bullets do align better (as in more concentric to the bore) if they fit the throat and have the bearing surface to mitigate the issues you mention.  My own  and several others HV cast bullet shooting has proven cast bullets with 60+ % bearing surface give the best accuracy.  It is why the Loverin style bullets always had the reputation for better accuracy at higher velocities.  

BTW; "bearing surface" as i refer to it is that length of the bullet than will be 100% the diameter of the groove diameter.  Bore riding noses, even if they are "bearing" on the top of the lands are still basically an unsupported and can tip/cant into the groove area under acceleration.  I believe you mentioned that.

LMG

Concealment is not cover.........

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RicinYakima posted this 16 March 2021

Larry, Know you were not dissing the CBA matches, but in my experience, high velocity shooters seldom catch bullets that are "readable" because of deformation. And I agree with your %age of bullet support, same as I use. Ric

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Bud Hyett posted this 16 March 2021

Four decades ago, Ed Doonan and I built a bullet recovery box to test many things including bullet tilt. The box was eight feet long with oiled sawdust as the medium. This was successful, but dirty as we used the oil from oil changes. A baffle every foot with a wax paper square inserted allowed us to track a bullet and recover it easily. This would capture cast and some jacketed bullets, the varmint bullets blew up.

We found light loads will tip, a prime example is the .45 rifle caliber Lyman 335 grain Gould Express in Marlin Micro-groove using 9.0 to 12.0 grains Unique seated into the lands where it engraved. You could see the bullet was going down the bore sideways. We next tried seating just off the lands, neither made any difference. The same bullet shot with 50 grains of Reloder #7 went straight, no sign of tipping, and almost out the end of the box. These bullets were hard alloy that dropped at .459+ and were sized in a .460 die simply to lubricate.

All the above loads shot groups under two inches for ten shots. Interestingly, you could feel the rifle torque against your cheek when shooting the 9.0 and 10.0 grain load.

Shooting Plain Base today with a choice of bullets, I'd like to test this question with several hundred shots each of several bore-riding and tapered into a target with a bullet recovery box. Unfortunately, I do not have such a bullet recovery box.

I realize the .45 caliber Gould Express is not a bore-riding bullet. However, the test is interesting.

I submit the real results are on the target whether paper or game.

Note: The only two bullets that traversed the box to leave a dent on the end panel were .30-'06 National Match 173 grain full-patch and the lead .45-70 500 grain round-nose Government load. 

Farm boy from Illinois, living in the magical Pacific Northwest

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John Alexander posted this 17 March 2021

Larry says: "Also, sorry to burst your bubble, but the CBA match results are not the "end all" definitive proof of everything cast bullet.  Many cast bullets are shot for a lot other types of shooting than CBA matches, probably a lot more than are shot in CBA cast bullet matches.  Might be a heretic for mentioning that here but it is a fact."

Larry,

My bubble huh? I don't think this ugly and unfair accusation of the CBA should stand unchallenged.  I have belonged to the CBA almost from the beginning and in all that time I have never heard one member say that CBA match results are the "end all definitive proof of everything everything cast bullet" nor imply anything close to that.  I don't know where you got that idea, but it is a false accusation and insulting. 

Most of our CBAs shooters aren't competitive shooters but many are very knowledgeable cast bullet shooters.  Over the years many of the most insightful and sophisticated articles in the Fouling Shot have been written by writers like yourself who are interested in high velocity or other ways to enjoy shooting cast bullets. These contributions are a valuable part of what the CBA has to offer and often have nothing to do with match shooting.

As far as bore riding bullets tipping, I never said they might not tip under some conditions.  I still argue that they probably don't tip "very  easily", or if they do it isn't enough to keep them from winning regularly in the only organized benchrest matches for cast bullets in factory rifles.  Performance tops arcane theories about what may or not happen to a soft bullet when the primer pops.

John

 

 

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Larry Gibson posted this 17 March 2021

John

Just a discussion, not insulting anyone in particular or CBA in general......just a discussion.  

You stated; " The proof is in the pudding and a large share of CBA matches requiring factory chambers are won with long nose bore riding bullets."

My point was/is in disagreeing with you was the CBA match results do not always provide "proof" of everything cast bullet related.  Specifically, the CBA match results provide no "proof" one way or the other that cast bullets can or do not tilt/cant in the bore during acceleration because of insufficient bearing surface.  I was simply pointing that out and meant no insult to you or CBA.  I've been a member for a few years and have religiously read The Fouling Shot every issue.  I also go through the match results both comparing the equipment listed to the scores.  There is indeed much valuable information to be gleaned from there. 

However,  all cast bullet knowledge as to what occurs during the firing of a cast bullet is not there.  That, simply, was my point and is not an "accusation" meant or implied.  It is simply a disagreement with what you implied in the quote.  

Now, I've not got a wedgie over this so please don't get one either.  It's just a discussion after all.  I think Ric's points and mine are that the longer the bearing surface is [as in % of bullet length that is/will be full groove diameter] is in a bullet that fits correctly the case neck, throat and lead has the better accuracy potential under many (that's not "all") uses.  The tilting or canting of bullets that are bore riders with minimal bearing surface can, under some shooting circumstances, be detrimental to accuracy.

LMG

Concealment is not cover.........

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lotech posted this 18 March 2021

Many of you are way beyond my level of expertise and your standards are more stringent, but I'm always open to learning. I've had good accuracy results with single diameter bullets, bore riders, and tapered bullets designs, assuming the latter are in their own category. I speak specifically here of the SAECO #315 and the Eagan MX3-30ARD. From my experience with several .30 caliber cartridges and a variety of rifles, the #315 almost always shoots very well. I don't particularly care for .30 caliber bullets under 180 grains. I much prefer those of around 200, like the Eagan design.  

I can't get small groups every time, but I've seen enough groups of around 3/4" at 100 yards using a heavy barrel Remington .308 that I'd call the Eagan bullet very accurate by my loose standards. I've also had good results with the bore rider  #314299 that has been hollowpointed, though it still weighs about 200 grains in an alloy approximating wheelweight or just a little harder, up to about 14 BHN. It won't quite compete with the Eagan as for accuracy, but, depending on the rifle, one inch groups aren't too difficult much of the time. 

The SAECO #301 is another that I've had very good luck with. Most of my work with it has been in the .308 cartridge, though I've done limited experimenting in 7.62x39, .30-40 Krag, and .30-06. 

Keep the comments and worthwhile advice coming.  

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