Bearing Length

45idaho
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Bearing Length

Postby 45idaho » Fri Oct 24, 2008 6:56 am

I had a Blackhawk in 45 Colt for years that I couldn't get to shoot very accurately. I measured the throats and they were between .448 and .450 so I had them taken out to .452 which helped a lot, but it still didn't shoot as accurately as I thought it should. I noticed that heavier bullets (325 gr) shot better than lighter (255 gr) ones. One spring I was able to collect a bunch of perfect fired bullets left over from the melted snow and noticed the lands had stripped when engraving the bullets. On the lighter bullets they had acutally stripped some at the base and the heavier bullets had stripped at the nose and the stripping stopped by the time the base of the bullet engaged. These were water dropped wheel weights.

I did some reading on Taylot Throats and after years of discouraging accuracy decided to give it a try as a last resort. After Taylor throating the accuracy didn,t seem to change. After recovering more perfect bullets from melted snow I found that the stripping problem was a lot worse. Most probably from a higher velocity when engaging the lands and a longer land taper from no lands to full land height because of the super gentle taper. Like the old Mark V Weatherby's not shooting well because of the freebore. I'm sure the fast 16" Ruger twist had a lot to do with the stripping all along and bad allignment was my main problem.

My custom linebored revolvers have a slower twist (22" in the 454 and 18" in the 475), hopefully good allignment (I can't seem to find a way to accurately measure it), and very short/small 11degree forcing cones. I notice very little stripping of bullets, a hell of a lot better accuracy and virtually no leading. I wish I would have customized the 45 a long time ago to save me all the time and grief of trying to force a bad pistol to shoot! Heavier bullets shoot better in these revolvers also, so I am a believer in bearing length for accuracy.

I know there are a huge amount of variables, but how much bearing length do you feel is necessary for good accuracy? Do larger calibers need a longer bearing length to maintain the same bearing length / bullet diameter ratio and what do you think this ratio shoud be? I am sure most people are like me, wanting to shoot a light bullet for velocity and lighter recoil but just heavy enough to retain accuracy.

I know nose length also has an effect on this. A shorter nose makes more bearing length, but a shorter nose also causes more jump / higher velocity before engaging the lands which I am sure hurts accuracy. It seems to me that a wheeler built for light bullets should have a very slow twist and short cylinder to minimize bullet jump before engaging. It seems that most revolvers have cylinders that are long enough to take .4"+ bullet noses which is a bad thing for light short nose bullets. God I wish I had the time, Money, means, etc. to try all this stuff out!

45idaho
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Joined: Wed Apr 30, 2008 7:03 pm

Re: Bearing Length

Postby 45idaho » Fri Oct 24, 2008 9:07 am

I think your design program has a small glitch. I was working on a bullet for my 45-70 and noticed that the same weight bullet for the 458 bore had a longer bering length than for my .451 pistol bore. Then I noticed that the program must use a .451 bore diameter for figuring the bearing length for both .452 and .459 bullets and the tangential ogive gets shorter as bullet diameter grows. I'm sure if the program used a .458 bore for figuring bearing length for the .459 bullet, the calculated bearing length would be shorter than the same weight of bullet at .452 in a .451 bore.

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mtngun
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Re: Bearing Length

Postby mtngun » Fri Oct 24, 2008 9:17 pm

I appreciate your effort to create the perfect cast bullet wheelgun !!! There's never enough time or money, is there ?

Bearing length ? I don't have a formula for how long a bullet should be, but I can tell you that "decent" designs will have no more than 1 caliber of the length unsupported -- and by "unsupported," I mean the unsupported nose.

An example of a "decent" design would be Elmer's 44 bullet. I don't have one in front of me to measure, but I'm thinking the ogive was about 0.320" long, maybe a little longer, making the ogive length about 3/4 of the diameter. I personally think Elmer's ogive is longer than ideal. He may have done that on purpose because he was keen on long distance stability and because he needed more powder space with the 44 special case and the powders of that era.

Another example of a "decent" design is Veral's LFN. Its unsupported nose is what -- about 0.300" long ??? Like Elmer, Veral advocates using a long nose so you can stuff more powder in the case.

At the time Veral developed his designs, pressure testing equipment was not available to the common man. We now know that, when loaded to equal pressures and shot out of a "real" handgun that you can carry on your hip all day long, deep seated short nose bullets produce almost the same velocity as shallow seated long nose bullets (except in the case of very heavy bullets that cramp the powder space). So with a few exceptions, I don't buy the "powder space" excuse for long unsupported noses.

On "better" designs, the unsupported section will be much less than 1 caliber long, more like 1/2 - 2/3 of a caliber long. This is true even for cast spitzers that are used in 200 yard benchrest competition. You never, ever, see long unsupported noses winning cast benchrest matches. Never.

An example of a "better" design is the Eagan target bullet. Again, I don't have one in front of me to measure, but from memory, the unsupported ogive is only about 2/3 caliber in length. That is on a bullet that is over an inch long. Another way to look at it is that about 85% of the bullet is supported.

I don't worry about lube grooves when calculating bearing length. Lube grooves don't seem to create any support problems.

So to sum things up, I like to see 75% or more of the bullet supported, and the unsupported nose should be no more than 2/3 of a caliber long.

Ross Seyfreid once said something to the effect that "the ballistic coefficient of a cast bullet is zero." In other words, don't worry about aerodynamics. A hunting bullet has to have a minimum of aerodynamics or else it won't stabilize, but otherwise, don't worry about it. It's certainly desirable for a 200 yard benchrest bullet to have good aerodynamics, but even so, you don't see VLD designs winning CBA matches. They use spitzers, yes, but abbreviated spitzers, with a short ogive. Support is more important than aerodynamics.

Getting back to your revolver...... you seem to have given this a lot of thought and there is not much I can tell you that you don't already know. But I do challenge your statement that a short nose has to "jump" further. Not necessarily true. The thing that has to "jump" is the front band. The unsupported ogive doesn't "jump" because it never touches the rifling. It is the length of the front band that determines how far the bullet has to jump, not the length of the nose.

You can get around this by using a long undersize front band, or else a long bore riding section. Say you make an extra long front band or bore rider 0.448" - 0.450" diameter. That willl drop into the chamber easily, yet it is still fat enough to touch the rifling. I can buy into that as long as you are realistic about tolerances. I don't think 0.0005" clearance is realistic. More like 0.002". As you have noted, if clearances are too tight, you can run into intermittent chambering problems due to residual lube or grains of unburned powder or just normal bullet tolerances.

I'm totally with you on the weight thing. I tried to get 240-250 grainers to shoot in my 44 from hell, but was forced to go to a 280 to get decent accuracy. I tried to get a 180 to shoot in my Marlin 357, but was forced to go to a 190 to get decent accuracy. I tried to find a good 165 grain load in my 30-06, but found that 180's were much more accurate. And so forth. Longer bearing length and additional lube grooves seem to fix a lot of problems.

The only other thing I can suggest is to find and read Ross's article on the paradox revolver, or any of his other Paradox articles. I think that was in Guns and Ammo, many, many years ago. Ross had a revolver made to shoot both shotshells and cast bullets. He had the rifling cut out for all but the last inch of the barrel. I think it was a 7 1/2" barrel with rifling only for the last inch, or something like that.

You may be wondering, "what does that have to do with me? I don't give a rat's ass about shooting shotshells." Ahh, but all revolver bullets have to "jump" before they touch the rifling. The Taylor throat has to jump a little farther. What Ross did was make the world's longest Taylor throat. It made the "jump" problem much, much worse, and he had one heck of a time getting it to shoot cast, as you can imagine.

But Ross found a solution to his "jump" problem. Do you think his solution might apply to our "jump" problem, as well ? Maybe. :idea:

Ross's Paradox article never got much attention, because only two or three people in the world care about Paradox revolvers. But If there were a Nobel prize for cast bullet research, Ross should get it just for that one article.

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mtngun
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Re: Bearing Length

Postby mtngun » Fri Oct 24, 2008 9:20 pm

45idaho wrote:I think your design program has a small glitch. ........ the program must use a .451 bore diameter for figuring the bearing length for both .452 and .459 bullets


That was true in the past, but I fixed it a few months ago, or I thought I fixed it. I'll revisit it as time allows. I'm busy right now, so it may be a few days.

45idaho
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Joined: Wed Apr 30, 2008 7:03 pm

Re: Bearing Length

Postby 45idaho » Sat Oct 25, 2008 7:44 am

You are right about the bullet not engaging till the front band, or even the full size band behind the crimp groove engages. The mold that you made me throws bullets with a .4" nose and a 80% meplat. When recovering these bullets, it is amazing to see how little of the nose is untouched by the lands. I don't have one in front of me, but I think it is only .2" or so. They almost look like a wadcutter after marked by the bore. The bore riding effect can't hurt, but I am sure full engagement would be better. The long nose bullet seems to start the bore riding effect long before a short nose bullet does. The point I was trying to make is I think cylinders of most modern revolvers are longer than necessary in order to accept the .4" and longer nosed bullets that are so popular today. It seems to me that a short cylinder (short throat) that would minimise bullet jump would be better than todays long throats for accuracy. The problem is that a short cylinder wouldn't accept the majority of heavier bullets that are readialy availiable.

I have a custom 5 shot 45 colt that will shoot a .4 nose 325 gr 80% meplat bullet into 2" or better at 50 yards. It is the best shooting revolver I have. The cylinder is shorter than my 454 and if it was chambered to 454 it would take a max nose length of .35. I was thinking of possibly boring it to 454, but I hate to screw up a great shooting revolver, go to the cost of making more molds, etc..... On the other hand, it may shoot lighter bullets better ...........

I am heading out to do some shooting and will let you know what happens.

Thanks again for your time! I sure appreciate the great molds you make and the awesome design program and info service you offer!!!!!!!!!!

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mtngun
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Re: Bearing Length

Postby mtngun » Fri Oct 31, 2008 7:42 pm

45idaho wrote:I think your design program has a small glitch. I was working on a bullet for my 45-70 and noticed that the same weight bullet for the 458 bore had a longer bering length than for my .451 pistol bore. Then I noticed that the program must use a .451 bore diameter for figuring the bearing length for both .452 and .459 bullets and the tangential ogive gets shorter as bullet diameter grows. I'm sure if the program used a .458 bore for figuring bearing length for the .459 bullet, the calculated bearing length would be shorter than the same weight of bullet at .452 in a .451 bore.


OK, I found one bug and fixed it. Let's hope there aren't any more bugs ;)


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