Bore riding question.

[Blammer]

Ok, I don't quite get it.

When a bullet is 'bore riding' what does that mean?

I'm a little slow on the land and groove bit and this bore riding normanclateur is throwing me for a loop.

Can someone explain how the bullet design and the land and grooves work together with this bore riding stuff?

[Phil]

Hi Blammer,

There are two components to a rifle (or pistol) barrel. The bore, which is the hole drilled and reamed through the barrel as the first operation, and the grooves, which are then cut, swaged, broached, or hammer forged, to a spiral pattern. This spiral pattern (or twist) is what imparts the spin to the bullet. With jacketed bullets you are only concerned with the groove size.

With cast bullets one more factor enters into the equation. That is the throat size. The throat, in a rifle barrel, is that portion leading from the case mouth to the groove diameter of the barrel. Most times this throat is somewhat larger than the groove diameter. For instance, a 308 Winchester chamber may have a throat diameter of .310" or .311" leading to the .308" grooves. So we have to typically size our cast bullet to fit this throat regardless of true groove size.

The nose of the cast bullet must ride snugly on the bore of the barrel, the small diameter that was drilled and reamed first. On a typical 30 caliber barrel the bore size is supposed to be around .300" or so. To get a cast bullet to ride the bore of the barrel it needs to be about .001" or .002" larger than bore size. If the nose of the bullet is smaller than the bore of the barrel it will not center the back (throat diameter remember) of the bullet in the barrel. The bullet will cock, or slump, somewhat, which lets the bullet yaw inside the barrel. This results in the bullet exiting the barrel crooked which is fatal to good accuracy.

Thats the short answer anyhow. If you have any more questions please just ask. I'm sure others will chime in with perhaps a clearer explanation. I'm home sick today so maybe I'm not as alert as I should be.


Cheers,

Phil

[Blammer]

That explains it PERFECTLY!

thank you very much!

[Tom Myers]

A picture is worth a thousand words. Maybe this will help to visualize what Phil described so well. It is a cross section of my 40-65 Ron Long Chamber drawn to a scal of 1 pixel to 0.001" and showing the cartridge and bullet fit in the chamber.

Tom Myers
Precision Ballistics and Records (http://www.uslink.net/~tom1/)

http://www.tmtpages.com/LinkSkyImages/forum_images/BoreRide-1.gif

[Blammer]

that is an awesome pic!

[uscra112]

Nice post, Tom.

Let's also mention that "bore-rider" boolit designs have a goodly part of the length at or just barely above bore diameter, and that this works great if the lands are wide enough to give it some support. The old 2-groove Springfields had two lands which covered about half the circumference, and bore-rider noses were great. However, Marlin's early Microgroove barrels wouldn't shoot better than an open choke shotgun with them, because there is so little land area that the nose slumps to one side and the boolit exits the bore crooked. So look at your particular barrel before you decide what boolit design to use.

[montana_charlie]

Tom,
That is a great drawing. It clearly shows several important relationships between a loaded round and it's chamber/barrel working area.

However (don't I usually have a however to complain about?)...

Blammer clearly states that he doesn't have a real tight grip on bore and groove terminology. So, it might be safe to assume there are some other terms found in a chamber, which he might also be holding rather loosely.

Then Phil steps into the line and provides Blammer with a quick course in barrel building. He describes the bore, adds the groove, and brings in that third factor...the one that is ignored by shooters of jacketed bullets...the throat.

Phil does a decent job of describing a throat, and finishes with an explanation of how it holds the bullet's rear end while the nose gets centered up in front by having a close-fitting bore riding section.

Overall...a fine job, done in a few words.
Enter your drawing...incorporating a 'thousand words'.

Everything about it is graphicly clear, technically accurate, and leaves no doubt about what fits where. It even illustrates a properly trimmed case mouth.

But, the 'new term' that Phil introduced to Blammer...the 'throat'...is illustrated in your drawing in a way that might just cause Blammer to decide Phil is talking through his hat.

When you look at drawings of chamber reamers supplied by commercial firms, you see 'throat' and 'freebore' used rather interchangeably.
If there ever was a difference, I suppose a freebore was a straight cylinder, and a throat had some taper. Don't know if that was ever true, but today the terms seem to be 'the user's choice' when labeling a drawing.

But your 'throat' label appears in the area of the chamber which might cause some confusion.
That's not a real problem for anybody who has been looking at chambers for a while, but Blammer already said he is a little fuzzy on terminology.
Add to that the fact that Phill described the throat as usually being one or two thousandths larger than groove diameter, and Blammer (or any newcomer) might find himself wondering 'which end' of your (tapering) 'throat' he should be measuring to find that oversized dimension. The label has the effect of steering him away from Phil's 'throat'...which is your 'freebore'...and presents him with the difficult task of measuring the tiny differences found in (what most call) the 'leade'.

It's your drawing, Tom, and a fine one. And, how it's labeled is your business.
But, if it caused Blammer to have a question that he didn't feel like asking...maybe my nit-picking answered it.
CM

[Tom Myers]

CM
Your are quite right in your assessment of the labeling procedure on the software Image. When I first designed the Chamber Drawing software it was somewhat primitive and the labeling process was not given enough thought to eventual changes. The original label of "Throat" simply encompassed the area from the chamber mouth to the end of the throating reamer's cut.
I have not changed it yet as I am still getting opinions on exactly how the individual areas should be labeled.
So far, my inclination is to keep the "Throat" as original planned and then, if the design so indicated, use " Free bore" to describe a cylindrical or slightly tapered space between the end of the chamber and the start of the angled taper or "Leade" that extends down to bore size. Thus "Throat" may then be divided into "Free bore" and "Leade".

I am open to suggestions as I would like the software module to clearly show the relationship between chamber, cartridge and bullet with a minimum of confusion.

Tom Myers
Precision Ballistics and Records (http://www.uslink.net/~tom1/)

[45 2.1]

Nice illustration, but I have yet to see an actual throat slug that was that uniform. The area from the case mouth forward about 0.4" is quite irregular in all the samples i've measured. Does your program allow imput from actual measurements?

[montana_charlie]

So far, my inclination is to keep the "Throat" as original planned and then, if the design so indicated, use " Free bore" to describe a cylindrical or slightly tapered space between the end of the chamber and the start of the angled taper or "Leade" that extends down to bore size. Thus "Throat" may then be divided into "Free bore" and "Leade".

I am open to suggestions...
I certainly can't claim to be an authority on the subject, so this is more like a 'feeling that it's right' based on how I usually see the terms used...and my understanding of what they mean because of how they are used.

Throat seems to be the most widely accepted label for the area between the actual 'chamber' and the beginning of the leade. By 'chamber', I mean the space occupied by the brass case.

Certain kinds of cartridges (I'm thinking about paper-patched bullets) sometimes call for 'extra room', to provide space for bullets seated much further out than more 'normal' loadings. I have read of PP bullets which are only seated a tenth of an inch into the case mouth.

Obviously, a configuration like that would require that the steel becomes an extension of the brass...essentially creating a two-part cartridge case to envelope the bullet. Why it would not make more sense to simply use a longer case, and seat the bullet deeper into it, is a mystery to me, as the guys who do it appear to be seeking greater powder capacity more than anything else.

It is in this context, I think, when the term 'freebore' comes into play.
I think it applies to that area ahead of the actual chamber which acts like a cylindrical extension of the case...and is usually several tenths of an inch in length.
Also (if I understand correctly), the long freebore eliminates the need for any 'throat', although it is sometimes referred to as a paper-patch throated rifle.

You can go any way that you choose, but if your drawing contains nothing more than chamber, throat, and leade...I think it will be perfectly understandable.
Perhaps you can set your software to throw in the freebore label when it detects the use of the 'paper-patch' term.

Now, I admit to being afflicted with a certain amount of tunnel vision which sees many things in a BPCR aspect. I do remember seeing military rounds (I think) which have a round nosed bullet poking way out from the case mouth. So, it's entirely possible that I'm 'missing something' which is well known by others.
CM

[Tom Myers]

That drawing is actually a composite of three drawings, each drawn by a software module that draws from a database of dimensions that are entered and stored until needed to draw an image.

The Chamber module will draw Pistol, Bottleneck, Rimmed, Rimless, Belted, Straight or Tapered Chamber.
The Cartridge module will likewise draw all the different Cartridge configurations that the chambers will accept.
The Bullet module is the Precision Cast Bullet design software that has been altered to draw the bullet to the 1 pixel to 0.001 inch scale.
Once the three drawings are made, the chamber drawing is then loaded into the Windows Paint program and then the bullet drawing is loaded, superimposed and positioned in the proper relationship to the chamber and finally the Cartridge drawing is loaded and superimposed at the proper relationship to the chamber and bullet. The resulting composite is then saved as a bitmap file which can be either printed out or saved for future reference.

Once all the measurements are made and entered into the databases, it is only a matter of a few minutes to produce the three drawings and merge them into a final image.

Here are some examples of the full sized images and some that have been reduced in size by about 50% that have been stored on my website. They were all made from actual measurements of chambers and cartridges. Some of the bullets are proposed group buy bullet dimensions.

Tom Myers
Precision Ballistics and Records (http://www.uslink.net/~tom1/)

http://www.tmtpages.com/LinkSkyImages/ChamberFit/ChamberFit.htm

http://www.tmtpages.com/LinkSkyImages/forum_images/25-20/25-20%20WCF%20-%20Clymer_W-W_Cart_Cust_25_80_GC_Reduced.gif

http://www.tmtpages.com/LinkSkyImages/forum_images/25-20/2520Data/25-20_Bullet_Fit.htm

http://www.tmtpages.com/LinkSkyImages/forum_images/23-35%20Win/25-35%20Win%20-%20Clymer_Sammi_Cart_Cust_25_80_GC_Reduced.gif

[303Guy]

If I may express my understanding of the terms free-bore and throat; Throat is that part of the chamber or bore that seats the boolit/bullet ready to be fired. Free-bore is the distance the projectile must travel before engaging the leade. Free-bore should be cylindrical and of groove diameter so as to guide the projectile into the leade and bore. So, a chamber with free-bore would still have a tapered throat and a leade but with a straight, groove diameter section between the two. Free-bore should not be confused with long-throated. Just my take on it.

[stubshaft]

Fantastic diagrams and pics Tom.

[mpmarty]

To see "freebore" take a look at a chamber drawing for a Weatherby cartridge. Ignore the double radius shoulders or "venturi" part of the Weatherby venturi freebore concept.

[steg]

Now that was an education, thanks guys GREAT post!

uscra112 I had that problem with an 1895 marlin, it was very accurate with factory boolits(love that spelling) but with a cast 300 flat nose I was lucky to hit paper at 100 yds, ended up selling the gun to a guy that just wanted it for factory ammo................steg

[felix]

Throat = (freebore + leade) or....
Leade = (freebore + throat).

I prefer the first one. ... felix

[blackthorn]

303 Guy said:
“If I may express my understanding of the terms free-bore and throat; Throat is that part of the chamber or bore that seats the bullet ready to be fired. Free-bore is the distance the projectile must travel before engaging the leade. Free-bore should be cylindrical and of groove diameter so as to guide the projectile into the leade and bore. So, a chamber with free-bore would still have a tapered throat and a leade but with a straight, groove diameter section between the two. Free-bore should not be confused with long-throated. Just my take on it.”

I respectfully beg to differ. If, as you suggest “freebore” were to be cut to be cylindrical “and of groove diameter” its purpose, as I understand it would be lost. I believe the purpose of cutting “freebore into a rifle is to allow the use of more powder (read increased velocity) without producing excessive pressure. I base my belief on what I have read with respect to Roy Weatherby’s magnum line of rifles and the reaction of custom cut Weatherby chambers (pressure wise) when the “freebore” is not cut in. Essentially, in my opinion, “freebore” and “long-throated” could easily be used interchangeably. With respect to the use of cast bullets, I see no advantage to a rifle with freebore as long as there is sufficient leade to allow the chambering of a bullet that is slightly bigger than the bore.


And then there is the “ball seat”: Custom mould maker Veral Smith of Lead Bullet Technology (LBT) said:

I believe “ball seat” is a military term, a hangover from the days when bullets were all called balls. That is all guess on my part, but I will clarify exactly what a ball seat throat is.

It is a throating type with a straight cylindrical section larger than bullet diameter, but not necessarily as large as groove diameter. Some high-grade rifles, which give superb accuracy even though groove diameter is much larger than bullet diameter, have a close tolerance ball seat throat, which removes only part of the rifling, and aligns the bullet perfectly. Some ball seats are cut significantly larger than standard bullet diameter, which is detrimental to accuracy. I prefer rifles with ball seats for cast bullets, as fitting bullets requires simply sizing to whatever will fit the ball seat closely.

The other type of rifle throating is tapered, and the length, angle etc varies dramatically from gun to gun. Very heavy usage eventually results in a long tapered throat, due to powder erosion. -- To get best results with this type, cut the bullet nose as close as possible to groove diameter, or the point in the throat that it will be when chambered. The bullet body is sized to fill the case neck to a close fit to the chamber, with a bearing length to come close to the throat taper. If you find all this confusing, don't be concerned about ordering a mold. Just send a throat slug with your order and I do all the figuring and cut the mold to fit.

So---something else to consider!!

[felix]

Powders were not slow enough for the case size when Roy designed the latter. His first was a 30 caliber, right? It sold beyond expectations, and other calibers were added to the line. Folks would not buy another 30-30 neck length, so, yes, he added neck length as freebore.

It's amazing how fast freebore is automatically created with loads at or above 50K cup. The best approach for a gun to be shot hard and often would be making a 30-30 neck and keeping the freebore at ZERO. ... felix

[303Guy]

That's fair comment, blackthorn. I have a book (two volumes) by Howe. I shall have a look to see how he defines it. (Maybe the use of the term has changed since his day?) I do seem to share your idea on how the throat should be shaped and how the boolit should be sized to suite.

The idea of "ball throating" is one I have wondered about but did not know the term for. I have two well used rifles that have formed their own "ball throating". I like it as my tapered cast and patched boolits contact the "ball throating" just nicely with a little force but not enough to push the boolit back into the case nor get left in the bore on de-chambering a round.

http://i388.photobucket.com/albums/oo327/303Guy/MVC-537F.jpg

Here one can see the rifling contact from the front edge of the patch to just under the 'u' and that's a tapered nose section. This would not be a bore-ride boolit as the rifling actually engages the nose section.

[303Guy]

I found this on the 'net. Thought it might be of interest.


Long-throating was done to soften that initial high pressure surge so large powder charges could be burned safely and progressively as the bullet traveled through the barrel. Many custom gunsmiths and wildcat cartridge designers who, for many years, had been expanding and sharp-shouldering cases for greater capacity, had also been longer-throating or partially freeboring as an additional safety factor for years prior to the Weatherbys.

Many gunsmiths had their hey-days during the WWII follow-up years when new rifles and actions were scarce and countless Mauser M-98s, 1903 Springfields and 1917 Enfields were everywhere. My own first .300 Weatherby magnum was built on a 1917 military Enfield, chambered and barreled at Weatherby, with their regular 3/4-inch free-boring. Our shop sent out many more guns for customers, many were M-70 Winchesters .270s converted to .270 Weatherby magnums.