S & W N-frame Project

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mtngun
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Re: S & W N-frame Project

Postby mtngun » Mon Jan 29, 2018 6:21 am

Go right ahead, Lee.

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Re: S & W N-frame Project

Postby mtngun » Mon Jan 29, 2018 11:05 am

Here's a goofy bullet design. The nose is similar to the various designs made for IHMSA silhouette, but the base has an 8 degree boat tail. While the boat tail might improve the BC a tiny bit, that's not why I did it. I did it because an 8 degree boat tail is reputed to improve stability in the transonic zone.

(FYI boat tails made for supersonic velocities are typically 13 degrees, but they can cause instabilitly at transonic velocities. A gentler boat tail angle of 7 to 9 degrees is recommended for the transonic zone.)

But .... it's easy to imagine how any boat tail could cause problems for a cast bullet -- gas seal? reduced bearing area? :cry: I was skeptical that this goofy design would even hit the target, but nonetheless I wanted to try it just to say that I tried it.
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There was a 5 - 10 mph breeze, and I don't like to attempt group shooting with this wheelgun if the wind is more than 5 mph, but I wanted to see if these goofy boat tail bullets would even hit the target. They did, but accuracy was poor.

I am going to assume that the boat tail is probably to blame for the poor accuracy. If I had to do it over I might try a rebated boat tail, but for now I will recut the cavity to a conventional bevel base and try again.
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Re: S & W N-frame Project

Postby mtngun » Mon Jan 29, 2018 11:24 am

The Aerodynamics of Revolver Bullets

Ross Seyfreid once wrote that the BC of revolver bullets is zero. That's only a slight exaggeration. Even the more streamlined "silhouette" type revolver bullets still have a sucky BC. Combine that with low velocity and you get a lot of wind drift.

Here's the drift table for a 5 mph wind, assuming a 180 gr. bullet with 0.18 BC launched at 1360 fps at my local conditions.

2.3 inches of drift at 100 yards. That's a lot when you are trying to shoot 2 inch groups! :lol: :roll:

Note that the bullet crosses the speed of sound at 130 yards, so 100 yards is not far enough to test long range stability. When the weather permits I'll have to do some 200 yard testing.
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Besides the horrendous wind drift, revolver bullets operate in the transonic zone:
When the velocity of a rifle bullet fired at supersonic muzzle velocity approaches the speed of sound it enters the transonic region. In the transonic region, an important thing that happens to most bullets, is that the centre of pressure (CP) shifts forward as the bullet decelerates. That CP shift affects the (dynamic) stability of the bullet. If the bullet is not well stabilized(a perfect spiral football throw), it can not remain pointing forward through the transonic region (the bullets starts to exhibit an unwanted coneing motion(wobbly football throw) that, if not dampened out, can eventually end in uncontrollable tumbling along the length axis). However, even if the bullet has sufficient stability (static and dynamic) to be able to fly through the transonic region and remain pointing forward, it is still affected. The erratic and sudden CP shift and (temporary) decrease of dynamic stability can cause significant dispersion (and hence significant accuracy decay), even if the bullet’s flight becomes well behaved again when it enters the subsonic region. This makes accurately predicting the ballistic behaviour of bullets in the transonic region very hard.

Well, the fact is that magnum revolvers do operate in the transonic zone and we have to deal with that. There is no simple fix, no black and white rules.

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Re: S & W N-frame Project

Postby mtngun » Wed Jan 31, 2018 9:24 am

I recut the boat tail cavity to a bevel base.
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The bevel base version of the "silhouette" bullet was not as inaccurate as the boat tail, but it was still inaccurate. A switchy 5 - 10 mph wind did not help, nonetheless there was no hint that the bullet wanted to shoot small groups. :cry:

This "silhouette" design was a failure, but I am not sure exactly which feature is to blame. Besides the smaller meplat, it had a longer ogive and a different band layout compared to the "control" bullet. I may make another attempt at creating a small meplat bullet, but with all other features exactly the same as the control bullet.
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Re: S & W N-frame Project

Postby Joeb4065 » Wed Jan 31, 2018 2:32 pm

This thread is the most interesting thing I have read in many moons. Thank you for all the research and writing it up.

Joe

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Re: S & W N-frame Project

Postby mtngun » Wed Jan 31, 2018 4:34 pm

I'm glad you enjoy reading about my adventures, Joe4065. I've been casting for 36 years and I'm still learning new stuff. That's what makes it interesting.

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Re: S & W N-frame Project

Postby mtngun » Thu Feb 01, 2018 1:09 pm

A new cavity, cut exactly the same as the 183 gr. BB control bullet except for a smaller meplat. This way, if it shoots different we'll know it's due to the meplat and not due to something else.
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Aaaaand .... the 50% meplat was less accurate, 6.38" vs ~3.5" for the control bullet. So yes, Virginia, a smaller meplat does hurt accuracy in this particular case.

It's got to be due to the way the smaller meplat flies through the air, because the parts of the bullet that contact the gun are exactly the same. Presumably the smaller meplat makes the bullet less stable at transonic velocities, but I'm not knowledgeable enough to explain it better than that. :?

I've never run into this phenomena before. I'm going to recut this cavity to 62% meplat, try that, then recut it to 85% meplat. I may try hollow pointing the various meplats, too.
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Re: S & W N-frame Project

Postby mtngun » Thu Feb 01, 2018 1:39 pm

Getting back to the hollow point shootout ......

The HP'd uncoated bullet shot very well so I had high hopes for the coated version. This was quenched after coating so the BHN was about 25.
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Accuracy was not so great, mainly due to a single flier. If you disregard the flier then the average was 3.4", about the same as the solid nose control load, but I don't like to disregard fliers without a darned good reason.

Velocity was 1408 fps today vs. 1369 fps when I shot the uncoated HP. Past experience with this bullet tells me that accuracy deteriorates above 1400 fps and can cause fliers. What caused the velocity to increase?
-- I had been using a 12+ year old batch of CCI 550 primers, but ran out of those and switched to a fresh batch about a week ago.
-- also about a week ago, something fell off a shelf, landed on my RCBS 10-10 powder scale, and messed up its calibration, so I gave it a tune-up and recalibrated it and compared it to test weights and to two other scales. I trust it today, but can't verify that it was calibrated correctly prior to the recent tune-up. :lol:
-- the temperature of the shooting shack is controlled, and I'm still using the same jug of powder, so those things are constant.

I'm guessing either the fresh lot of CCI 550's is hotter than the old lot, or else the 10-10's calibration had drifted prior to its recent tune-up. Either way, the 15.6 gr. load is now faster than it used to be. I decided that henceforth I will tweak the powder charge to maintain about 1370 fps, which seems to be the sweet spot.

In the meantime I can't draw any conclusions about the HP with this particular bullet. The uncoated HP was slightly more accurate while today's coated HP was slightly less accurate, but the differences were small and it takes a lot of shooting to "prove" anything.
Image

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Re: S & W N-frame Project

Postby mtngun » Thu Feb 01, 2018 2:00 pm

Getting back to the alloy shootout ....

Coated air-cooled reclaimed shot. As discussed in the previous post I decided to tweak the powder charge to hit 1370 fps, so these loads used 15.4 gr. WC297 instead of the usual 15.6 gr.. FYI Quickload believes that 25,000 psi is required to hit 1370 fps, so this is a mild load.

If I air-cool this alloy as it drops from the mold onto a towel, I typically get 12 - 14 BHN, with some variation due to how quickly they air cool. But after baking the coating on these bullets, I set the cast iron pan holding the bullets on top of the stove to air-cool. Due to the inertia of the cast iron pan, that may cause the bullets to cool slower than dropping them on a towel. At any rate, today's bullets measured 11.3 BHN with a 10mm ball & 150 kg load. I left the coating on when testing BHN.

These soft coated bullets turned in the best average I have shot with this gun to date. 8-) Their velocity variation was excellent, too.
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To make the comparison with uncoated bullets apples-to-apples, I gave them the same 400 degree cure for 25 minutes followed by the same set-the-cast-iron-pan-on-top-of-the-stove air-cooling.

The uncoated bullets turned out 11.0 BHN, and shot much better than I expected them to. They left the barrel squeaky clean, too.
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Previous tests with air-cooled bullets in this gun gave inferior accuracy (and leading, in the case of uncoated bullets) but that was a different bullet with a different load in a different barrel with a different forcing cone. Lots of things have changed since then.

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Re: S & W N-frame Project

Postby mtngun » Tue Feb 06, 2018 3:45 pm

THE meplat shootout continues ..... today I tried a 62% meplat with the usual control load.
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Results so far:
50% meplat -- 6.38" average at 100 yards
62% meplat -- 4.48" average
75% meplat -- 2.76" average

Next time I will try an 85% meplat.
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