S & W N-frame Project

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mtngun
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S & W N-frame Project

Postby mtngun » Sat Oct 14, 2017 1:04 pm

I retired my Dirty Harry era M29-2 twenty or so years ago. It was a disappointment from the get go, with mediocre cast accuracy, leading in the forcing cone, a habit of unlocking the cylinder with full snort loads, a broken side plate screw, and an uncomfortable square butt grip frame. The cylinder throats were sloppy and the forcing cone would swallow an entire 45 acp bullet. :o

Then I brought it out of retirement briefly for an experiment with 0.434" plain base bullets, which I posted about elsewhere. The 0.434" bullets cured the leading problem, but required turning the case necks to chamber. I experimented with reaming the chambers to accept the fatter cartridges, but got in a hurry and botched the job. That was the end of the 44 magnum experiment. No tears were shed for the M29 because it had already been retired, anyway, and replaced with a Ruger Speed Six 357 that did everything I needed a packing pistol to do. The little Ruger is surprisingly accurate and has been 100% reliable despite being rode hard and put away wet.

But .... the Speed Six snubbie is no target gun, and my aging eyes can no longer shoot iron sights satisfactorily, so the Speed Six is poorly suited for accuracy testing of revolver bullets. A Freedom Arms is not in my budget, and for that matter, neither are most new Rugers. :lol: So I thought "I've got that old piece of chit M29 laying around, why don't I do something with it?" :roll:

One problem with the M29-2, besides it's sloppy tolerances, is that the 44 mag is just too much cartridge for it. Smith and Wesson fans will disagree with me, but I came to that conclusion after putting thousands of rounds through it. It doesn't hold up to the recoil and it's not comfortable to shoot.

I believe the N frame is better suited for a lesser cartridge, and I considered re-configuring my M29 for 41 magnum. However, the 41 magnum with heavy cast bullets is nearly as powerful as the 44 magnum, so I was afraid that it might also be a little too much cartridge for the M29.

A rimmed 10mm magnum cartridge might be ideally suited for the M29, but that would be a 100% custom proposition, making it more expensive. I may yet build a 10mm cylinder & barrel for the M29, but I'm going to save that project for another day.

That leaves the humble 357 magnum. Used 357 cylinders are readily available for not a lot of money, and I had a chunk of 357 barrel left over from the Contender Carbine project.

A used cylinder was ordered from ebay and should arrive in a few days. I suspect that I will be dissatisfied with the accuracy of the factory cylinder, and may eventually make a homemade cylinder with tighter tolerances, but that'll take some time and perhaps more tooling, so the used factory cylinder gets the nod for now.

The barrel is ten groove, 20" twist, and 0.3475" bore. I think I only paid $15 - $20 for it at the time, though the ebay seller has since raised the price to $50. Apparently I never did measure its groove diameter -- they can vary quite a bit on 35/9mm caliber barrels, anywhere from 0.355" to 0.358". I'm tentatively aiming for an 8" barrel, because anything longer strikes me as more of a short rifle than a handgun. The barrel will be shrouded and fastened with a nut at the muzzle similar to a Dan Wesson. For today I merely turned the blank down to 0.680" and started making a barrel nut (actually 2 nuts, one being a spare). The nuts are threaded .670" - 36, same as the N-frame's barrel tenon.
Image

I hope to get this project running in about 2 weeks. The only hard part should be making the shroud.

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Re: S & W N-frame Project

Postby mtngun » Sat Oct 14, 2017 3:44 pm

Castle nuts complete.

They were made from a burnt-out 416 stainless rifle barrel, which is sort of "half-hard." Damaged nuts were a common problem on the Dan Wesson revolvers, so it might be better if the nuts were 3/4 hard, but then again, that's why I made a spare nut. :lol:
Image

FYI I'm using indexable carbide tooling to cut the 0.670" - 36 threads, but the inserts were not pointy enough for these very fine threads, so I sharpened one point on an insert on a diamond wheel, using a guide to maintain the proper angle. It worked well if I do say so.
Image

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Re: S & W N-frame Project

Postby mtngun » Thu Oct 19, 2017 9:14 am

The used 357 cylinder showed up. It's throats were spot on at 0.358" (why couldn't S&W make my 44 throats spot on at 0.430"? :lol: )

I still need to set up the end shake on the 357 cylinder, more on that later. At the moment the end shake is too tight with the 357 yoke -- it rubs enough to drag significantly -- and a little loose with the 44 yoke.

I wanted to measure how much the cylinders are misaligned with the barrel. I need to know because the entrance to forcing cone has to be at least big enough to "swallow" the misalignment. The more misalignment, the bigger the forcing cone needs to be.

There are several ways to measure misalignment, here's mine. I made a "tenon stub" with threads that fit snugly in the frame. The stub was bored 0.358", in the same setup as cutting the threads, so the stub's ID is concentric with the stub's OD.

A 0.357"+ pin gage (my set happens to be PLUS sizes) will slide into the throats, or slide into the tenon stub. If the throats are perfectly aligned with the tenon stub, then the .357"+ gage should be able to slide through the stub and into the throats.

Of course the throats are not perfectly aligned on mass produced revolvers, so a .357"+ gage will not go. So then I try a .356" gage, and a 0.355", and so on, until I find a gage that slides through. Repeat for each hole. Today's test was performed with the 44 yoke and the OEM bolt -- and that may make a difference. Here are today's results:

Hole #1 -- 0.354" pin
#2 -- .353" pin
#3 -- .353" pin
#4 -- .353" pin
#5 -- .353" pin
#6 -- .354" pin

What does that mean? It means the throats are out of alignment with the barrel by 0.003" - 0.004". That's about what I expected, and it could have been worse. The bullet will shave lead unless the forcing cone entrance is big enough to allow for the misalignment, so the entrance to the forcing cone will need to be at least 2 times 0.004" oversize, or 0.366". :cry: If I had a custom cylinder that was line-bored or line-indexed, then the forcing cone could be tighter. Hopefully one of these days I will be able to make that custom cylinder, then I'll set the barrel back enough to tighten up the forcing cone.
Image

I'll repeat this test after I get the end shake sorted out and the oversize bolt fitted.

Also, note that I ground off the square corners of the grip frame. I've wanted to do that from day one but couldn't bring myself to do it because it would lower the resale value. Well, I've had this gun for 30 years and still have no intention of selling it, so I might as well make it comfortable to shoot. :lol:

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Re: S & W N-frame Project

Postby mtngun » Thu Oct 19, 2017 9:34 am

Even though the 357 cylinder's exits are spot on at 0.358", there is still a problem with the throats. You see, in between the 0.380" - 0.381" chamber body and the 0.358" throat exit is a funnel. The funnel starts at 0.381" and tapers down to 0.358" over about 0.100" of length (approximately 6 degrees per side). That means the bullet will be unsupported while it passes across that 0.100" long funnel.

If the bullet obturates at that spot, then it gets squeezed down as it enters the 0.358" throat. Every time a bullet obturates and then gets squeezed down, that distorts the bullet and may harm accuracy.

If the bullet does not obturate at that spot, then it will not be supported, and it may be exposed to hot gases. So with or without obturation, the oversize funnel is bad news. :evil:

The oversize funnel throat appears to be a SAAMI spec, as highlighted in red. What were they thinking? If I ever make a custom cylinder then I'll employ a proper rifle-style throat instead of the stupid oversize funnel.
Image

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Re: S & W N-frame Project

Postby Brad » Fri Oct 20, 2017 2:47 am

It is worse than you think. Are your chambers actually just minimum length? Is your brass max length? Make an impact cast of the cylinder end and throats area and measure for yourself.
I bet you have .025+ free space before even hitting that taper.
Make an impact cast and do some measuring. What is the longest possible case you can have and still allow the brass to expand and release the bullet?
Yes, the case mouth may be in the taper a little.

Now get some 357 max brass and trim them to that length. You can easily eliminate .025 to .050 of that open space.

Might help, might not. It certainly won't hurt.

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Re: S & W N-frame Project

Postby mtngun » Fri Oct 20, 2017 9:01 am

A 0.379" pin goes in 1.295" deep, so the chambers are pretty darned close to SAAMI spec. If I trim cases to 1.285", that's 0.010" gap, pretty reasonable.

Eventually I'll design a bullet just for this cylinder. Target bullet considerations are a bit different than hunting & defense. My Speed Six bullet is 160 gr., but I'll probably end up at 180 gr. or even more in this cylinder. My Speed Six bullet is designed for reliable chambering, but a target bullet should make light contact with the throat when chambered.

The mis-alignment is consistently on one side. I'm trying to figure out a practical way to correct it? Moving the bolt window over 0.003" by filing would fix it, but then it would need an oversize bolt to fit the oversize window, and I don't think the oversize bolt that I ordered is that oversize, but we'll see when it shows up. It's frustrating because you know it hurts accuracy when a cast bullet slams into the forcing cone 0.003" off center. Well, I guess that's why people pay big bucks for custom cylinders that are line bored or line indexed. :lol:

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Re: S & W N-frame Project

Postby mtngun » Fri Oct 20, 2017 4:12 pm

The Power Custom oversize bolt showed up, so here are some numbers:

Oversize bolt = 0.106" - 0.107" thick
OEM bolt = 0.1014" thick
bolt window = 0.105" gage pin goes, 0.106" pin starts but won't go all the way through, so let's call the bolt window 0.1055".
357 cylinder bolt cuts = 0.104" gage pin goes easily, 0.105" gage pin is an extremely snug fit, 0.106" pin won't go.

For moving parts, clearance is required. I want to end up with might be called a "close running" fit, The general recommendations for a "close running" fit are 0.0002" - 0.0016" clearance.

The limiting factor is the bolt needs to fit into the 0.105" cylinder cut. That means the bolt can be no larger than 0.105", and more likely 0.1046" or thereabouts for reliable clearance. A 0.1046" bolt would have 0.0009" clearance in the 0.1055" bolt window, and that would be reasonably snug.

So the tentative plan is to stone the oversize bolt to 0.1046" - 0.1048" thickness, stoning a little at a time until the bolt snaps into the cylinder cuts reliably.

Earlier I had wondered if it would be feasible to improve alignment by filing the bolt window 0.003" on one side, but on further reflection that won't work because the resulting bolt window would be 0.1085" wide, and a bolt that was a snug fit in a 0.1085" window would not fit in the 0.105" cylinder cut. :P

The only other idea I have for improving alignment is to verify that the crane is closing all the way. Perhaps there is a tiny burr or high spot that prevents it from closing fully? I see no evidence of that but I'll inspect the parts closely under a magnifying lamp to be sure.

Will the oversize bolt improve alignment? No, the factory cylinder will still have 0.003" - 0.004" mis-alignment, the only difference is it will be rigid mis-alignment rather than wiggly mis-alignment. :lol: However an oversize bolt is essential for use with a custom line-indexed cylinder -- it would make no sense to line-index the cylinder holes if there were 0.004" slop in the alignment due to the OEM bolt wobbling in the bolt window. The oversize bolt was purchased with the idea that eventually I'll fabricate a line-indexed cylinder.

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Re: S & W N-frame Project

Postby mtngun » Sat Oct 21, 2017 11:31 am

SETTING END SHAKE

The used 357 cylinder and crane had less than zero end shake, in other words it was actually binding slightly fore and aft, such that it would not spin freely. It took me a while to figure out that the end of the crane was bottoming out against the keyed ring inside the cylinder. After a few strokes with a file the cylinder now spins freely yet with no perceptible end shake. :)

By inspecting the crane carefully under magnification, I could see slight peening at some of the corners, apparently due to normal wear and tear. Likewise there was slight peening at some of the corners of the frame where the crane closes. I stoned those peened surfaces until they felt flat. I don't think the peening was enough to have caused any problems, but we fixed it just to be safe.

The center pin that locks the rear of the cylinder is 0.124" OD and fits into a 0.1275" ID hole in the frame. For now I'll live with it but I'm making a note of the issue so that when the time comes to make a custom cylinder, I'll also make a custom center pin with about 0.1265" OD for a better fit.
Image

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Re: S & W N-frame Project

Postby mtngun » Sat Oct 21, 2017 12:04 pm

After fixing the end shake, I retested the cylinder-to-barrel alignment. This is with the stoned 357 crane and the OEM bolt.

FYI if a 0.3550" pin would go if I wiggled it enough and a 0.3540" pin went without wiggling, then I split the difference and called it 0.3545". The sloppy OEM bolt fit allows the alignment to wiggle quite a bit.

#1 -- 0.3545" pin
#2 -- 0.3550" pin
#3 -- 0.3545" pin
#4 -- 0.3545" pin
#5 -- 0.3545" pin
#6 -- 0.3540" pin
average = 0.3545" pin, or about 0.003" mis-alignment.

You can see the misalignment with the naked eye as you look into the tenon stub. It's exaggerated in this photo due to the camera angle, but nonetheless it looks the same on all 6 holes. I'm guessing the mis-alignment is due to tolerances in the location of the frame's bolt window. If there were some magic way to move the bolt window over 0.003", then alignment would be near perfect.Image

I'll retest after fitting the oversize bolt.

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Re: S & W N-frame Project

Postby Bob » Sat Nov 04, 2017 11:36 am

I think that the loose bolt issue is deliberate by some gun manufacturers for production purposes. If there's a little wiggle in the cylinder at full lockup, the cylinder machining & fit are a bit less critical. The bullets passage from the cyl throats into the forcing cone (especially jacketed bullets) will serve to align the barrel & cylinder a bit.

When you get to making your own cylinder, consider making it a bit longer so it fits the cylinder window better. Smith really dropped the ball on that little detail, IMHO.


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