fit to throat confusion

[dahermit]

Rifle shooter wisdom gleaned from The Fouling Shot et. al. says: "...size the cast bullet to .0005 less than the diameter of the throat..." and, "...the bullet must fit the throat..."

After studying the diagrams of typical rifle chambers and reading many articles on the fitting of bullet to a particular rifle, I have observed the following.

All common (not custom) cast bullets have cylindrical driving bands. The "throat" of standard SAMII chambers are tapered.

Therefore, given a tapered throat (a 30-06 Ruger #1 in this instance), the actual measurement derived will be dependant upon where on the throat taper the measurement is taken. Up next to the chamber is .3135, further down towards the leade it is much less.

Just how then is a standard Saeco bullet with cylindrical driving bands fitted to a standard tapered throat by measuring? My practice up till now is to seat the bullet nose into the lands until it stops...I reason that the first driving band is then contacting the taper of the throat at some point along the taper. I have been sizing the bullets at .309, one thousanth over the .308 groove dia., been getting 100 yd. groups of slightly over an inch for 5 shots...most frequently 4 shots in under 3/4 inch, one shot usuall opending the group to 1 and 1/8 to 1 and 1/4 inch groups.

Regards,

dahermit

[MGySgt]

dahermit -

Your groups are outstanding but to answer your question, I need to ask a few questions.

What are the actual measurements of your rifle? - Chamber/Throat/Lead and bore diameter.

If you are looking at diagrams of chambers your are looking at the SAAMI standards and we all know that each rifle is a little bit different, chamber reamer new and sharp, or old and resharpened? Some chambers are a little bigger and some a little smaller.

For target work - yes the accepted is .0005 under throat/lead. If that is a hunting rifle, I would suggest you go .001 under throat/lead. (A little grit in the chamber can cause a bullet not to chamber or extract - been there, done that and didn't get the shot!)

If the throat/lead is tapered go .001 under the smallest diameter. The ones I have measured may have been tapered, but the difference may have been .0001 or .0002, not enough to worry about.

Drew

[Bass Ackward]

Rifle shooter wisdom gleaned from The Fouling Shot et. al. says: "...size the cast bullet to .0005 less than the diameter of the throat..." and, "...the bullet must fit the throat..."

After studying the diagrams of typical rifle chambers and reading many articles on the fitting of bullet to a particular rifle, I have observed the following.

All common (not custom) cast bullets have cylindrical driving bands. The "throat" of standard SAMII chambers are tapered.
dahermit

da,

Not sure what you are asking. The above quote from the Fouling shot is refering to a throat that has a ball seat type of throat. That is a machined space cut by the reamer with parallel sides that ends with a slight taper into the leade of the rifling. So it has an end taper will generally match the angle on the end of the rifling until it wears away. But the ball seat portion does not. Thus the above description.

So not all SAAMI throats are tapered. In fact, while case dimensions and headspace are standardized for safety, throat specs can differ wildly. This is why there is such a vast difference in what you will find from manufacturer to manufacturer.

Since the throat area of a reamer is the vanguard for the reamer, it cuts the farthest and takes the most wear. In order to make as many rifles as possible from a single reamer, these dimensions are made generous to accomodate wear and resharpening.

To molds, Lyman Loverin designed or style bullets are tapered. This is usually from a diameter that is slightly above bore (land height) to some larger dimension that is usually well above the bore (groove). For 30 caliber, my 311466 is .302 to .312.

Matching a taper is the hardest shape to fit, but often unnecessary as you have found out with your success. Loverins usually provide enough centering that they function well in standard mold offerings. If you can find one available. They seem to be favorites in the group buy section of this site. Bore rides are also supposed to provide centering and guidance to support enough of the bullet until the bands enter the bore and seal.

Did this answer your question?

[joeb33050]

You got your cartridge case neck. Further out you got your rifled barrel.
I've got plenty of cold swaged lead impressions of this area.
You got your throat, ball seat, leade, free bore, your knurlman, and I don't know what they are. Definitions vary, a lot.
I can draw what's in my guns, but don't know the names of these areas, and I don't think it matters. What does matter is the dimensions, and I thinkwe get screwed up on the names.
I've got a 30/30 bench gun that Mark Penrod built. The chamber:
At 2.080" from the base the chamber neck ends, and measures .326" dia.
There's a .3095"/.310" cylindrical section from 2.080" to 2.225" or .145" long.
Then the rifling starts, the lands may be tapered but I can't see it if they are.
I can successfully shoot .312" diameter 311/314299s in this gun, with the bands almost all in the case.
I can also successfully shoot .309" diameter 311/314299s in this gun, with the bands almost all the .145" out of the case neck.
I've won matches and shot groups in the high 2s with this gun and .312" diameter bullets, It averages a little under 1" for 5 five shot 100 yard groups.
I think that it shoots a bit better with the .309" bullets, but the difference is not great.
I think we would know a lot more if we got people to make impact castings of the "throat", got somebody to draw the picture, and then watched how different bullets/diameters shoot. If we had a library of these maybe we could learn something about this "throat"business.
Remember Harrison's warning about nose riding bullets and small lands, Maybe we should stop our love of the bore rider nose bullets and get some experience with groove diameter bullets.
I wish I could do the CAD drawings to keep records with, I think this would be worthwhile.
My gun/s shoot pretty good when the bullet does NOT fit the throat, whatever the throat is.

joe brennan

[Char-Gar]

I am anything but an expert and numbers and drawings give me a headache. In my simple mind, it is all around straightness. The bullet must enter the barrel straight to give it best accuracy.

The rifle chamber must be straight with the bore and the bolt head straight with the rear end of the chamber. The case needs to fit the chamber and not flop around. The fit of the bullet to the throat (or whatever) is just one more element in keeping things straight.

Throat (or whatever) designs and dimensions vary as do bullets and how they fit in the "whatever". When it is all boiled down, the bullet that fits the best to provide a smooth straight entry into the bore will be accuracy. Of course it depends on alloy, powder charge, etc. etc. etc.

[porkchop bob]

I agree with Charger in his reply #5 and with Joe B in his reply #4. The boolit has to be supported by the throat and guided concentrically into the rifling of the barrel. A cone shaped throat does neither.
Bob

[Phil]

One time I tried an experiment just for fun. I chambered a barrel with a reamer that had no throat section at all. Then I throated it with a throating reamer to just a bit shy of where I wanted to shoot the cast bullet I wanted to use in the rifle. Next I took a piece of barrel steel that had been bored and reamed but not rifled. I reamed it with the same throating reamer that I used to put the throat in the rifle barrel. Finally I threaded the exterior of the piece to 7/8X14 so I could screw it into my loading press, and made a set of punches to get the bullets into and out of the die. Thus, I had a bullet custom fit to the throat of the barrel it was being shot in. Short version is that it worked very well indeed. So well in fact that I made another die to work with an M1A for a friend and it worked very well also. The rifle shot exceptionally well with 311291's so treated and loaded with my old standard charge of IMR4198. That charge. by the way, works well in any full size caliber thirty to 8mm round with cast bullets. IE all the WWII era military rifle cartridges.

Merry Christmas all,

Phil

[Bass Ackward]

Maybe we should stop our love of the bore rider nose bullets and get some experience with groove diameter bullets.

My gun/s shoot pretty good when the bullet does NOT fit the throat, whatever the throat is.

joe brennan

Joeb,

I know you already understand a lot of this, but I will write in detail so others can follow.

I don't see any mystery to throats. The barrel, the bullet design, and the load still are responsible for achieving the accuracy. Because breach seating eliminates the transition from the throat to bore under pressure and remains the most accurate method for shooting cast. The job of a throat is to maintain the prefect conditions you strive to create from molding to the loaded cartridge.

And bore ride bullets serve a purpose and fulfill a need for guns with less than "ideal" throat conditions. But with bore rides comes the disadvantages that can and do alter perceptions of false limits on cast velocities that aren't really valid. Cast gets the reputation for velocity ranges and limits that they do because of all the different bullet designs and throat shapes and sizes. And the misunderstanding of how to minimize those effects from shooters who decide to use cast.

Bullet obturation from pressure is a fact. Obturation is an uncontrolled deformation of your precisely crafted bullet. Deformation occurs until a stronger metal stops the process. Exceede the ability of the hardness to resist obturation and you better have a throat design to support or better yet, minimize it. Thus some rifles can run soft cast at significantly higher velocities than others.

Where you see the difference in throats is when you apply pressure in excess of the hardness. The ideal situation, is for a fully supported bullet at the same diameter as a bore. Breech seating does this. The ideal throat would attempt to replicate this support until the slug can enter and be supported totally by the bore. But the bullet must still chamber. And .... this is a temporary condition at best.

Most cast benchrest quality throats are cut so that they are .0005 over groove. The problem is that anytime you cut, you leave tool marks. These marks will wear away quickly depending on the pressure a bullet is subjected too and the rate heat is applied to this area. These marks can easily take .0005 on a side to clean up. Thus, the .0005 throat that was cut opens to .0015 over bore or more depending on how sharp the reamer was, the type of steel, how fast it was pushed, etc. You could very easily lose .001 on a side and end up with a throat .0025 over bore. You can't cut without roughing to some degree.

So what happens is that a guy get's a new gun and slugs. He gets his measurement and records them, accepting these as fact and believes his job to be over. He makes his choices for bullet fit of a one diameter slug at that time. And depending on his chosen pressure levels, his throat cleans up over time and the leade angles change. He loses the fit he attempted to create in the first place and loses some level of accuracy. Odds are, he never slugs again to follow his throat out until it smooths and eventually stabilizes.

So a "perfect" thoat for cast is impossible to create or cut and breech seating will always remain king. Throats are a fluid and ever changing condition. So as a shooter you accept cast limitations and the velocity levels that come with those, OR you must learn how to fit .... or choke to achieve success at higher pressure levels or simply be happy in the cast velocity zone.

[fecmech]

Bass --could you please explain "breech seating" if you would. I have heard the term many times but am not sure what it really entails. Thanks Nick

[wills]

Breech Seating:
http://www.assra.com/getting_started.htm

[charger 1]

Would having a mold made that was identical to throat of that gun, filling said throat,and touching rifling ,used only in that gun not for the most part accomplish the same thing?

[Bret4207]

IMHO the throat area of the chamber, that area between the end of the cartridge neck and the start of the rifling, should be a guide for the boolit into the rifling. It should support and guide the boolit straight into the rifling without being so large as to allow the boolit to obturate beyond a reasonable limit. Whether this is a hangun or rifle it should act the same. If the throat is oversize it would allow the boolit to obturate more than it should and might allow the boolit to wander to one side or the other of the throat and become misaligned. Too small a throat either gives you "Ruger-itis" where the boolit is too small for the barrel or in a rifle won'
t allow you to seat the nose into the rifling lead.

Im my perfect world the throat of a rifle would be large enough to allow the perfectly fitted nose to engage the lead while the first drive band would be just contacting the tapered throat. In my real world I seat to engae the rifling if possible and run my boolits fat enough to fill the grooves or maybe up to .002+ over if I can or have to. I don't load real hot for rifles. In revolvers I've lucked out so far as all my throats are larger than my grooves so I go to throat size.

Is this about as everyone else follows things? FWIW- I am a fan of the Loverin style rifle boolits and Keith style or maybe I should say SWC style revolter boolits. Support the body and keep the nose no longer than needed if possible. That being said, I love the look of the bore riders but they don't always work for me.

[charger 1]

Thats my take on it to. "ruger-itis" I like that..Ya its a definate ailment, but can be cured with a 460-461 throating reamer

[Bass Ackward]

Would having a mold made that was identical to throat of that gun, filling said throat,and touching rifling ,used only in that gun not for the most part accomplish the same thing?

Charger,

Read Phil's response above. The answer is yes. Until the throat changed enough that fit was no longer perfect.

Most guys that shoot cast at cast pressures / velocities, never need that kind of fit we are addressing a "perfect" world. Nor would most notice if they lost it. Only people pushing higher pressure applications will ever notice the accuracy loss as they lose fit. Especially if they are shooting soft lead, like for hunting. Then you notice it right away.

When it first started on my 35, I was using a 210 grain bullet at 2400 fps. Sub MOA groups just started to open up quickly to 2". I seated out farther and accuracy came back to just above MOA, but not as consistent. Fliers became more common and were wilder. I always wanted to stay light to keep my velocity up and thus had to make a 220 grainer. It required I change the olgive taper a little. Accuracy came right back to sub MOA .... for about 200 shots. So I made up a 250 that I am still able to use as my leade angle has stabilized at this time. Took about 800 rounds to get to this point.

So this is why I would always recommend someone start smaller and shorter than your desired dimensions if you are building. It helps if you use a seperate throater to cut a leade angle that is cast friendly right from the start. Something less than 3 degrees for a straight case and 1 degree or less for a bottle neck. Then it will wear faster and stabilize sooner. Phil had the process right above.

Otherwise, you have to work with or chase what you have.

[charger 1]

In my particular situation of running hard cast ie 21bn+, the throat entirely full with a .5 degree taper, and ball powders noted for far less erosion I think I'll get sick of shooting it before the accuracy is in the toilet

[Bret4207]

BA- How long did it take to see a change and do you believe the change is due to wear from powder errosion or actual mechanical wear from the boolit?

[Bass Ackward]

BA- How long did it take to see a change and do you believe the change is due to wear from powder errosion or actual mechanical wear from the boolit?

Bret,

My barrel took 180 rounds of jacketed to get things smoothed up enough to shoot HV with soft lead. Then I got maybe 200 rounds with the 210 grain before things moved. I consider that fairly soon. Then maybe 200 rounds with with the 220. I was getting nervous. Now I have at least 800 rounds, but I just fire lapped this gun again using the Tubb's System to decrease the tapper even more and correct any small dimensional abnormalities that remained. I find this system preferable in that it is faster and cheaper to shooting several hundred jacketed now.

The purpose was to see if it improved any for the use of soft lead and then if so, how or why. Because this barrel glistened before I started! You know, another danged experiment.

As to mechanical wear, I can tell you this. I was using a plastic wire nut and 600 grit to clean up bad tooling marks in a forcing cone on a Redhawk that was causing or destroying plain based bullet bases and causing leading. When I was finished, you could visibly see the clean, 600 grit, spiral swirls, while very slight, looked rough compared to the gleam of the lead polished bore. I thought to myself, this was going to take a couple of hundred shots to polish out to get to the same bore like condition. Do you know how many shots it actually took?

Six! One cylinder full of soft, 14 BHN bullets at 20,000 psi and CSI couldn't tell the forcing cone was ever roughed up. So if lead can polish 600 grit marks that fast, imagine that very slight errosion from powder is polished away with each lead shot. The polishing action will be highest, where pressure peaks and bullet obturation is most severe.

So my answer is that all wear is powder errosion, and the lead just polishes the damaged surface. Chicken or the egg?

[Bret4207]

Bass- You tickled a memory- I recall a gun that I handled/saw/owned/worked on (who knows?-CRS) that had an unusual chamber. A couple of the lands appeared to stick out further to the rear of the throat than the others. I suppose they could have been a bit higher or lower than the the other lands. When it was chambered the reamer didn't hit them the same as the others, or maybe the lands weren't concentric with the grooves. All I recall is the 2 lands extending way beyond the other. I wonder if this would cause the same type of change you talk of? The wear would be accerlated on those lands I would think. Interesting question, for me at least.

I also like the fire lapping idea having spent what seemed like years on the end of a lapping rod in my youth. I have the gear to do it and a couple rifles that would likely benefit. Time, I need time...

[felix]

Indexing rounds will exhibit what you see, Bret. The guy who owned the gun beforehand prolly had to do this to get any accuracy at all because of some misalignment. ... felix

[joeb33050]

Joeb,

Because breach seating eliminates the transition from the throat to bore under pressure and remains the most accurate method for shooting cast.

(I don't know this. I'm pretty well convinced that breech seating was invented because the straight tapered cartridges of the time had no necks and couldn't be reloaded with accuracy. 32/40, 38/55, have no neck. Thus the only way to get accuracy is to breech seat.
The ASSRA guys and the CBA guys shoot under different conditions. I wonder how the ASSRA guys would do if they had a certain 15 minutes to shoot 10 record shots.
I'm not saying it ain't true, just that I know of no proof.)

And bore ride bullets serve a purpose and fulfill a need for guns with less than "ideal" throat conditions.

(I'm starting to think that Harrison was right, and that we like bore ride bullets because they "should be" more accurate. I'm having a lot of luck with straight .309 bullets without the nose. Maybe I'll never know, bore riders have worked for me for many years-but I'm losing faith.)
joe b.

I also don't know how to reply to points in a post. I have to add something here.
joe b.