S & W N-frame Project

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mtngun
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Re: S & W N-frame Project

Postby mtngun » Tue Dec 19, 2017 2:24 pm

Water tank results using 15.6 gr. WC297 for about 1400 fps when the bullet hits the water. Coated and quenched reclaimed shot at ~25 BHN. It so happened that all of today's bullets bounced off the side of the water tank and suffered some damage, but I tried to position the damaged side out of the photo.

Here's the 180 gr. BB from the No-Name barrel with Taylor Junior forcing cone.
-- to my eye the engraving is deeper at point #1 than at point #2, though the difference is slight.
-- 100% of lube retained, points #3 and #4.
Image

Here's a 180 gr. GC with a bore-riding nose. I have not shot this one on paper yet, but I chose it for the water tank because the bore riding nose does a good job of highlighting any uneven engraving.
-- the bore riding nose is definitely engraved at point #1 but only faint engraving at point #2.
-- I think the line of missing coating at points #3 and #4 is due to the crimp. I don't plan to crimp this bullet when I load for paper, but I was in a hurry when I assembled today's rounds and didn't want to fiddle with the crimp die setting.
-- 100% of lube retained, points #5 and #6.
-- the gas check came off in the water tank. The check was a very tight fit but it seems common for the scouring action of the water to pull checks off, so I don't think it is anything to worry about.
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Switching to the GM barrel with a very snug 5 degree forcing cone. Here's the 180 gr. GC with bore-riding nose.
-- minor skidding at point #1. Not surprisingly, bullets skid a little more in the 6 groove GM barrel than in the 10 groove No-Name barrel.
-- to my eye point #2 did not engrave as much as point #1, but the difference is slight.
-- the GC had been pulled part way off even though it was a tight fit.
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Here's the 180 BB from the GM barrel.
-- to my eye the engraving is deeper at point #1 than at point #2, but the difference is tiny.
-- both bullets fired in the GM barrel today suffered a large patch of missing coating. The missing coating could be due to bouncing off the side of the water tank, or it could be due to scraping action in the barrel's forcing cone.
Image

Thoughts:

-- I was pleased that all of today's bullets showed only minor misalignment.
-- the Taylor Junior forcing cone may have mitigated the misalignment of the No-Name barrel.
-- I believe the GM barrel has very good alignment, nonetheless some coating may be getting scraped off in the forcing cone. Perhaps the forcing cone is too rough or its opening is too tight?
-- I don't understand the physics of a Taylor throat, other than to speculate that it gives the bullet a little more room to make the turn into a mis-aligned barrel. Perhaps a long 1 degree taper would accomplish the same thing, but that will be another experiment for another day.

What's Next?:
-- I might as well try a Taylor Senior in the No-Name barrel, but based on the minimal misalignment of today's bullets, I'm guessing it will be hard to improve on the Taylor Junior. We'll see.
-- since the Taylor Junior benefited accuracy in the No-Name barrel, I want re-cut the GM barrel for a Taylor Junior.

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Re: S & W N-frame Project

Postby mtngun » Tue Dec 19, 2017 6:23 pm

Here's a sketch of the "Taylor Junior" forcing cone, in blue.

The red line represents a straight line connecting the beginning and the end of the Taylor Junior cone -- the straight line is approximately 1.5 degree per side, about the same angle as used in many rifle leades. Would a simple 1.5 degree cone work as well as the Taylor Junior? I suspect it would.
Image

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Re: S & W N-frame Project

Postby mtngun » Wed Dec 20, 2017 3:39 pm

Recutting the forcing cones in both barrels today.

Usually, except for benchrest rifles, I indicate the barrel on a snug fitting gage pin. In theory a snug fitting gage pin's error should be less than 0.001".

On expensive benchrest rifle barrels I use a gage pin to do an initial dial-in, then follow up by indicating directly on the grooves with a test indicator.

Since these were cheap barrels and a revolver is not a benchrest rifle, I had previously used a gage pin to dial in the barrels. Today I started out using a gage pin in the No-Name barrel, but my forcing cone turned out visibly off-center even though I had dialed in the gage pin carefully, so then I switched to a Mitutoyo indicator with a 1.5" long tip reading directly on the grooves, as shown in the photo.

In theory you should get the same results with either method, but today that was not the case. Well, several things can go wrong using the gage pin method. The gage pin must have some clearance inside the barrel, so that clearance can cause a few ten thousandths error. Next, the gage pin rides on the lands while the test indicator method reads the grooves, and in a cheap barrel there may be slight variations in the groove depth from one groove to the next. Finally, if the bore is curved -- not an unusual thing even in expensive barrels -- that will cock the gage pin and throw the readings off.

Indicating directly on the grooves is not perfect, either. If the groove depth varies from one groove to the next, that will throw the readings off. If the bore is curved then the best you can hope for is zero runout where the indicator tip is positioned, but there will still be runout at any other position.

So dialing in a barrel can be complicated, and gunsmiths have endless campfire debates about which method is best. My personal take is that as long as you are using a reamer with a snug fitting pilot, then the reamer will try to follow the bore even if the dial-in is off a thousandth or two.

But I'm using a boring bar to cut these experimental forcing cones, and there's no pilot to keep the forcing cone concentric with the bore. If my dial-in is off, then my forcing cone will be off the same amount.

After today's unsatisfactory experience using the gage pin method, from now on I'll probably use the Mitutoyo to dial in all my barrels, even non-benchrest barrels.
Image

After re-dialing in the No-Name barrel with the Mitutoyo, I recut the forcing cone, and now it appears concentric -- though some of the diameters ended up bigger than I intended. The main freebore section was supposed to be 0.358", but it was actually big enough to swallow a 0.358"+ gage pin, so it's probably 0.3585" or thereabouts. That's not the end of the world, but I'm just noting it here for the record.

I made the freebore long enough to swallow the 180 gr. BB bullet, as shown in the photo.

The freebore is followed by a 1.5 degree leade into the rifling.

Like many of my projects, this "Whole Hog Taylor" forcing cone is just an experiment and I make no guarantees that it will work.
Image

Next, I recut the Green Mountain barrel's tight 5 degree forcing cone, turning it into a "Taylor Junior," though slightly different than the No-Name barrel's "Taylor Junior." If I have time I'll post a sketch later, for now let's note that a 0.358"+ pin will go in 0.200" deep, and that freebore section is followed by a 1.5 degree taper.

I had used the Mitutoyo to dial in the Green Mountain barrel, so the new forcing cone should be perfectly concentric, right? Well, it's close to concentric but not perfect -- it looks to be off about 0.0005" relative to the bore. How is that possible? Because I positioned the tip of the Mitutoyo a good inch inside the barrel rather than at the end of the barrel where the forcing cone is. That wouldn't be an issue if the bore is straight, but if the bore is curved then all bets are off. :roll: :twisted: :cry: Oh well.

If I knew for sure which forcing cone design was best, I'd make a piloted reamer to cut that forcing cone and that would make it easier to cut the cone concentrically. In the meantime a boring bar is more cost effective for these experimental cones.

While I had the GM's bore dialed in with the Mitutoyo, I also double-checked the runout of the tenon. I had originally cut the GM's tenon for 0.0072" offset as indicated on a snug fitting gage pin. But when indicated with the Mitutoyo, I measured 0.008" - 0.0085" offset at the tenon! :o Which is correct? They both are correct relative to the way they were measured, but there is some uncertainty in the measurements for all the reasons I have laid out in this post.

I'm not losing sleep over the imperfections, just noting them for the record.
Image

The GM's "Taylor Junior" cone will swallow only the nose of my 180 gr. BB, as shown.
Image

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Re: S & W N-frame Project

Postby mtngun » Sat Dec 23, 2017 5:59 pm

Testing the two experimental forcing cones. About 20 degrees outside, but warm inside the shooting shack. A bit of mirage but it was barely noticeable with the Weaver scope set at 6X.
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This is the 180 gr. bore riding GC that I mentioned in a previous post. It is a bastard design because I made it by re-cutting the 140 gr. GC cavity. There was no way to incorporate a crimp groove, and I'm not keen on the long check shank, but nonetheless it should serve to test the merit of a bore riding nose. If it shows merit, I will cut a better bore riding design.
Image

First up, the no-name barrel with its "Whole Hog" Taylor cone that swallows the entire bullet. I wasn't even sure if the bullets would hit the target after making the long leap to the rifling, but they did better than I expected. The control load averaged 4.58" in the "Whole Hog" cone vs. 3.73" for the "Taylor Junior" cone. The difference is not significant mathematically but nonetheless my gut instinct says that the difference is real. Also, the "Whole Hog" cone averaged a lousy 74 ES while the "Taylor Junior" averaged 35 ES.

The "Whole Hog" was definitely not an improvement over the "Taylor Junior," and it may have been worse. I'm not surprised but I wanted to try it just to say I tried it.
Image

Now for the Green Mountain barrel with its "Taylor Junior" throat. The control load averaged 3.3" with the "Taylor Junior" vs. 5.1" with the snug 5 degree cone in this same barrel. ES averaged 48 for the "Taylor Junior" vs. 50 for the 5 degree cone.

It takes a lot of shooting to "prove" a mathematical difference but my gut says the "Taylor Junior" was an improvement and definitely did not hurt.
Image

You may recall that the GM's "Taylor Junior" appeared to be about 0.0005" off-center despite dialing in the barrel with an expensive indicator. Here is a bore cam attempting to show what I mean. I was impressed that it still shot 3.3" despite the flawed cone.
Image

Summing things up:
-- with both barrels, groups averaged about 5" with a conventional forcing cone
-- recutting to a "Taylor Junior" shrunk groups to 3.73" in the no-name and 3.30" in the GM.
-- recutting to a "Whole Hog" opened groups to 4.58" in the no-name.
-- so the "Taylor Junior" is my best cone to date.
-- the bore riding bullet shot roughly the same as the 180BB.
-- a 1.5 degree leade seems to work about as well as anything. Some people claim that an 11 degree leade is superior, but I'm not seeing it.
-- since the GM shot 3.3" with an off-center forcing cone, that makes me wonder how it would shoot if the forcing cone were properly centered?

What's Next:
-- water tank tests are on hold until the weather warms up, which may be a long time. :cry:
-- should I try an intermediate Taylor, halfway in between the "Junior" and the "Whole Hog"? I'm skeptical, but may try it anyway in the interest of science.
-- I have enough GM blank left to make one more barrel, should I use it to make one more attempt at getting the alignment right and the forcing cone right?
-- now that I am pretty sure that a "Taylor Junior" with 1.5 degree leade is close to optimal, I may make a piloted reamer to cut the cone so there will be a better chance of cutting the cone right.
-- I may experiment with more bullet designs in the GM "Taylor Junior." After all, this project was supposed to focus on testing bullet designs. :lol: I.e., I may try a Keith bullet.

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Re: S & W N-frame Project

Postby mtngun » Sat Dec 23, 2017 8:11 pm

Sketch of the "Taylor Junior" currently in the GM barrel.
Image

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Re: S & W N-frame Project

Postby mtngun » Sun Dec 24, 2017 7:14 am

1994 article by Ross Seyfried "In Search of The One MOA Revolver"

Since my M29-357 is not line-bored and since my used cheapo barrels are not hand lapped PacNors, I am willing to settle for 2 MOA. :lol: :lol: :lol:

Ross's Taylor throat was 0.625" long (or about 1.4 times the bullet diameter). Diameter of the Taylor was "slightly larger than the cylinder throat," whatever that means. No mention of the transition angles at the beginning and at the end of the Taylor freebore.

Even with the line-bored revolver, Ross had to use only certain cylinder holes. He didn't say it was easy. :lol:

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Re: S & W N-frame Project

Postby mtngun » Sun Dec 24, 2017 7:48 pm

Another take on Taylor Throat dimensions

the barrel throat is lengthened one and one half to two calibers, and enlarged to slightly over groove diameter. The throat serves as the throat in a rifle barrel, enabling the bullet to become perfectly aligned with the bore before engaging the rifling. The "choking" effect present from tightening the barrel into the frame is removed as well. The rifling leade is a very gentle 1 ½ degrees. On average, when tested before and after using a Ransom Rest, 50 yard groups have been reduced 40 to 50%.
,,,
I am convinced Taylor Throating produces the greatest accuracy improvement value available. Line-bore chambering will produce the most accurate revolvers, but the cost is prohibitive for many. When the barrel is accurately recrowned; the forcing cone recut concentric to the bore; Taylor Throating is almost as accurate as line-bore chambering,
,,,
Taylor Throating aligns the bullet with the bore so it starts spinning straight instead of hitting the rifling off-center.


My experience so far largely agrees with that, except for the length of the Taylor throat, which in the case of my "Taylor Juniors" is less than one caliber, Interesting that he also uses a 1.5 degree transition from the Taylor freebore to the rifling.

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Re: S & W N-frame Project

Postby mtngun » Sun Dec 24, 2017 8:56 pm

Origin of the Taylor Throat

Apparently this was written by Mr. Taylor's son -- Taylor Junior? :lol:
Image
Image
Image

I blew up Mr. Taylor's sketch so you can read it.
-- it's not actually a straight freebore, but rather a tapered freebore.
-- the freebore taper is only 0.11 degrees per side -- not a lot, but nonetheless it's there.
-- including the 11 degree entrance chamfer, the total throat length is approximately one caliber.
-- no mention of a transition angle at the end of the freebore, but surely there must have been one?
Image

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Re: S & W N-frame Project

Postby mtngun » Mon Dec 25, 2017 5:05 pm

It's been too snowy to shoot so I prepped some bullets.

I'm not normally a Keith bullet fan but it actually makes sense for this application. It's long relative to its weight -- which may be a good thing or a bad thing depending on what you are trying to accomplish. The long bullet will take up more powder space but in this case that's OK because I will still be able to reach 1400+ fps at SAAMI pressures with room to spare.

I gave it an oversize crimp groove, not for oversize crimping, but to act as a shock absorber when the bullet crashes into the rifling.

The bevel base may make Elmer may spin in his grave, but in my experience a bevel base has a slight accuracy edge.

Elmer might object to the reduced diameter front band, too, but it's needed for reliable chambering.

Rather than Elmer's puny meplat I gave it a 75% meplat so that it'll be an apples to apples comparison to the other bullets in this thread.
Image

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Re: S & W N-frame Project

Postby mtngun » Wed Dec 27, 2017 1:59 pm

I had a notion to recut the "Taylor Junior" cone on the GM barrel, since the original cone was about 0.0005" off-center.

This time I dialed it in by indicating directly on the grooves in the leade area. Each tic mark on the indicator represents 0.0005", and you can split the tic marks by eye to resolve 0.0002" or better.
Image

After dialing in the leade area dialed in dead nuts, then I checked the runout on a 0.347" gage pin -- there was 0.002" runout (0.001" offset). In theory, if the barrel was perfectly uniform, and if the bore was perfectly straight, and if the gage pin was a perfect fit in the bore, then the gage pin method would give the same results as indicating directly on the grooves. But that's a lot of "ifs" and in this case there was a difference.

I also checked the runout on the OD of the tenon while I had the barrel dialed in -- there was 0.016" runout (0.008" offset). It was intended to have 0.0072" offset, the error is probably due to using the gage pin method to dial in the bore when I originally cut the tenon. :cry:
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OK, so after dialing in the barrel with the Mitutoyo, I attempted to recut the Taylor Junior, taking several light skim cuts, removing just enough material to center the cone. It was going well until I typed in a decimal place wrong on the CNC control. Oops! By the time I noticed the typo, it had already cut too deep, as shown in the sketch. Oh well, I had earlier contemplated testing an "Intermediate" Taylor cone, so now I had an intermediate cone whether I wanted one or not. :lol: :roll: Besides being longer than the "Taylor Junior," the intermediate cone was fatter, too. I did not like that but as long as it's here I might as well test it, in the interest of science. :ugeek:

Image

Here's the intermediate Taylor cone. At least it was concentric with the bore this time.
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Here's how the 180 gr. BB bullet fits in the intermediate Taylor.
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