S & W N-frame Project

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mtngun
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Re: S & W N-frame Project

Postby mtngun » Sat Jan 13, 2018 6:23 pm

SHOOTOUT between a smooth bullet and a grooved bullet.

I went into this shootout very prejudiced because I had previously experimented with smooth coated bullets in a rifle and the smooth bullets would not even stay on the target. In general, grooves were very important in rifle bullets especially at higher velocities. Nonetheless I was aware that many people have been shooting smooth bullets satisfactorily at lower velocities. Different guns and different velocity regimes may play by different rules. I try to keep an open mind and try different things and see what works.

The smooth bullet for this test had exactly the same form as the 180 gr. BB control bullet. Same alloy, same coating, same sized diameter, yada yada yada. Neither bullet was crimped so the lack of a crimp groove was not an issue for this test. Due to the lack of grooves, the smooth bullet weighed more than the grooved bullet.
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To my surprise, the smooth bullet shot decently and left the barrel squeaky clean.

Stats (compared to the control load tested in the January 5 post)
Avg 6-shot group: 3.52" grooved vs. 3.76" smooth (not significant)
Avg 6-shot ES: 43 grooved vs. 51 smooth (not significant)
Avg 24 shot % std dev: 1.3% grooved vs. 1.6% smooth (not significant)
Avg 1393 fps grooved vs. 1350 fps smooth, but difference prolly due to weight.

While the smooth bullet was slightly inferior in every category, the differences were not significant.
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This test is going to force me to rethink what I thought I knew about revolver bullet design. It all goes back to the eternal question that Dr. Mann attempted to answer "what causes cast bullets to fail?" Perhaps deformation does cause cast bullets to fail in some applications, and perhaps grooves do play a critical role in reducing that deformation, but in this particular apllication grooves don't seem to do much. Perhaps there is some velocity or pressure threshold where smooth bullets begin to fail? At this point all I am sure of is name, rank, and serial number. :lol: :roll: :lol: :roll: :lol:

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Re: S & W N-frame Project

Postby mtngun » Sat Jan 13, 2018 7:06 pm

Testing the bevel base bore riders ....
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The 180 gr. teased me by shooting my first sub 2 inch group, but after that it went crazy, with too much velocity variation and too many vertical fliers. I'm not sure what caused it to fail but this design was definitely a step in the wrong direction.
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It's hard to see in this photograph unless you zoom in, but every bullet hole was oval with the 220 grainer, and one hole was keyholed. Apparently the 220 was on the ragged edge of stability.

The heavy bullet did help the powder burn better, turning in outstanding velocity variation.

The Miller stability formula claims this load has a stability factor of 2.27. However, Mr. Miller's formula was designed around high velocity rifle bullets, not transonic revolver bullets.

The classic Greenhill formula claims this bullet needs a twist of 21.7 inches -- the actual twist is 20 inches. However, the classic Greenhill formula does not correct for velocity, altitude, bullet material, etc..

The Bowman-Howell formula does correct for velocity and bullet density, and predicts that this bullet requires an 18" twist. That seems closest to the truth in this transonic application.

Transonic velocities are known to destablize bullets, especially blunt bullets. I am not aware of any simple formula to predict transonic stability but from now on I will seriously consider the Bowman-Howell formula for transonic applications.
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What's next?
-- test the 180 gr. BB control bullet without lube.
-- recut the 180 gr. BBBR with a "DD" band on the tip of the bore rider, so it can grab the rifling sooner.
-- it looks like 200 - 205 gr. is going to be the max for this 20" twist barrel, and I've already tried a 200. Perhaps I will try a 205.
-- in general, rethink my bullet design philosophy for wheelguns. Grooves do not seem to be that important, so I need to figure out what is important.

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Re: S & W N-frame Project

Postby mtngun » Sat Jan 20, 2018 6:23 pm

Since the 185 gr. Keith bullet was a dud, I recut that cavity with a band near the tip of the nose, turning it into an "almost wadcutter." The thinking was that the bullet would not have to travel as far to contact the rifling. On the other hand, the aerodynamics would suffer, and that might hurt accuracy, or it might not even stabilize at 100 yards.
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Today the 200 gr. Keith-Wadcutter averaged averaged of 8.25", compared to 7.33" for the 185 gr. Keith when tested on January 5. Due to the lousy accuracy I gave up after only 2 groups.

The bullet holes were round and there was no hint of instablity.
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Re: S & W N-frame Project

Postby mtngun » Sat Jan 20, 2018 6:31 pm

A 180 gr. bore-riding bevel base did not do so hot when tested on January 13. One theory was that the bore rider was not making contact with the rifling in the forcing cone, so I recut that cavity to have a 0.357" band on the tip of its nose. That increased the weight to 183 gr. but everything else was left the same.
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The re-cut 183 averaged 4.28" vs. 4.93" for the original 180. That's probably not a significant difference, and in any case it is nothing to write home about.

The band on the tip of the nose did not improve accuracy on either the bore rider on on the Keith. Maybe the jump to the rifling is not that important, or maybe the tip-band hurt the aerodynamics enough to affect accuracy?
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Re: S & W N-frame Project

Postby mtngun » Sat Jan 20, 2018 7:17 pm

Since the "smooth" version of the control load had shot surprisingly well on January 13, I decided to try a couple more versions of the control load.
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The uncoated version of the 180 gr. BB did "OK." If you disregard the one wild flier, the average group drops to 3.53", practically the same as the control load's 3.52".

However, velocity variation was significantly worse than the control load -- 67 ES uncoated vs. 43 ES coated, and 1.8% SD uncoated vs. 1.3% coated. Bear in mind that these loads were not crimped. The no-crimp loading worked satisfactority with the coated control bullet, but it's possible that the uncoated bullet is more slippery and would benefit from a crimp?

FYI there was a 5 - 10 mph wind quartering across the range when these groups were shot, and that may have contributed to the horizontal dispersion in two of the groups.

A borescope inspection revealed no leading with the uncoated bullet. At the beginning of this project there was light leading with uncoated plain base bullets, but that was in a different barrel with a different forcing cone and a different bullet. My guess is that this barrel's improved alignment and improved forcing cone are responsible for the reduction in leading.
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The unlubed but coated version of the control load did not do well. Perhaps it was the shooter's fault or perhaps it was the 5 - 10 mph wind quartering across the range, but the unlubed bullet did not even try to shoot nice groups. I have no explanation because the smooth bullet was also unlubed yet it shot OK.
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Summary of results:
-- the uncoated bullet had worse velocity variation, while the other 3 were about the same.
-- the unlubed bullet may have had worse accuracy while the other 3 were about the same.
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Recent lessons learned:
-- reducing the jump to the rifling by adding a 0.357" band at the tip of the nose did not help accuracy.
-- this particular barrel / cone combination does not lead with plain base bullets, at least not at 26 ksi.
-- it did not like the Keith designs, but I can't explain why.
-- accuracy was mediocre with the the bevel base bore riding design, but I can't explain why.

What's Next?
-- do an alloy shootout with the 180 gr. BB control load, using both coated and uncoated bullets.
-- try a GC version of the 180 gr. bullet? I have tried GC's previously but so many things have changed since then.
-- try a smaller meplat?
-- try 300MP powder? It may be too slow to hit the desired velocity, but the slower burn rate might be gentler on the cast bullets.

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Re: S & W N-frame Project

Postby mtngun » Mon Jan 22, 2018 7:21 pm

I have been thinking about why the bullets with the .357" band added to the tip of the ogive did not shoot well. :geek:

There are two challenges that any bullet has to overcome. First it must fly straight while passing through the gun. Second, it must fly straight while passing through the air.

If it were only a matter of flying straight while passing through the gun, then we'd all be shooting full wadcutters. The bullets with the band added to the tip of the ogive were close to being full wadcutters.

But full wadcutters are not known for being accurate at distance. I am not knowledgeable enough to explain the physics, but I know from experience that changing the shape of the ogive or the balance of the bullet can affect accuracy owing to the way the bullet flies through the air.

Here's an interesting experiment that illustrates how changing the balance of a bullet can radically affect accuracy. That experiment continues in this second thread and then in a third thread. I want to attempt to duplicate that experiment with my 180 gr. BB control bullet.

Up to this point I have standardized on a 75% meplat and a short secant ogive, because it has been my experience that a 75% meplat is streamlined enough to avoid the stability problems that sometime plague bigger meplats. And indeed, my 75% meplats make round holes in paper. However, I'm wondering if they are getting buffeted around in the air enough to affect accuracy? So I will try some smaller meplats and longer ogives. I'll try to find the optimal tradeoff between the design that is best at flying straight through the gun and the design that is best at flying straight through the air.

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Re: S & W N-frame Project

Postby mtngun » Mon Jan 22, 2018 7:47 pm

While re-reading David Bradshaw's experiments with the "volcano'd" WFN's, I stumbled across this tidbit about the Taylor throat in one of David's pet Ruger 45LC's. Its 0.800" long Taylor freebore strikes me as longer than ideal, however it may have been cut that long to clean up the barrel constriction that is present at the tenon of many mass-produced revolvers.

Note that his Ruger has 0.005" mis-alignment yet is still a good shooter when combined with the Taylor throat. Its cylinder throats are 0.451".
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Re: S & W N-frame Project

Postby mtngun » Tue Jan 23, 2018 6:32 pm

Some trial and error revealed that air-cooled reclaimed shot bullets would be dinged by a 6-jaw chuck, but water quenched bullets would not be dinged (a 5C collet might work better but I did not have the appropriate collet on hand). So I heat treated the bullets prior to cutting the hollow point.

I used a broken center drill that had been reground to a sharp point.
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The shallow hollow point shaved 6 grains off the weight.
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Re: S & W N-frame Project

Postby mtngun » Thu Jan 25, 2018 11:51 am

I should mention that the cavity for the 180 gr. BB control bullet was originally cut to drop out right at 0.358" with reclaimed shot, with the expectation that PC coating would increase the diameter. Now that I am doing some uncoated experiments with other alloys I needed the bullets to drop out a little fatter, so I recut the cavity a bit fatter and that increased the weight to 183 gr. with reclaimed shot. I doubt if that small change in weight affects performance but I'm just noting it for the record.

The wind finally stopped blowing so the alloy shootout and the hollow point shootout have begun. Because the uncoated bullets have more velocity variation than the coated bullets, I decided to switch from no crimp to the Lee collet crimp, with the hope that the crimp would improve velocity variation. It did not, more on that later, but nonetheless I'll stick with the Lee collet crimp for the alloy shootout.

First up, the solid nose uncoated bullet, cast of reclaimed shot, but heat treated at 468F for 38 BHN as measured with a 10mm / 150 kg test. I had tested uncoated reclaimed shot before but that was quenched at only 400F for ~25 BHN.

The 38 BHN uncoated bullets averaged 4.30" vs 4.21" for the 25 BHN uncoated bullets, certainly not a significant difference, and no difference in velocity variation, either.
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Next, the hollow pointed uncoated bullets, also 38 BHN, so this is an apples-to-apples comparison to the solid nose load. It takes more than 4 groups to "prove" a significant difference but for now let's say that the hollow point's 3.17" average is the best this gun has done with any load.
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Summary of results to date:
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Lee Martin
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Re: S & W N-frame Project

Postby Lee Martin » Mon Jan 29, 2018 5:59 am

Dan - outstanding thread. Would you mind if I shared the link on my forum? I know our members could learn a lot from your project.

Thanks,

-Lee
www.singleactions.com


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