S&W 646 conversion to 10mm about to begin


Black Snowman
October 22, 2004, 11:25 AM
The gun will be here next week and I've already contacted S&W about a replacement cylinder. Their recommendation was to start with a 686 Cylinder and rechamber it since I wanted to keep my original titanium cylinder unmodified.

They said a 686 cylinder from them is about $175.00. I plan on ordering an unfluted 686 cylinder, cut it for moon clips, then rechamber it for 10mm.

I'm working with Simmon's Gunsmithing (http://www.simmonsguns.com/) for the transfer and modifications to the cylinder. I'll be sure to post more info as the project progresses.

When I'm done I should have a gun with all the power of a 610 but in a much handier package. When I get CCW there's a good chance this will be one of the guns I carry.

For those of you not familiar with the 646 it's a 6 shot .40 S&W in a medium frame with a titanium cylinder. Here's a pic.

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October 22, 2004, 10:20 PM
I thought they went to the Ti cylinder because of the diameter of the .40 S&W didn't leave enough thickness to be safe with steel ? Is it going to be a six or seven shooter ?

While you're at it why not have your smith trim that lug off ? That would be sweet if you could get it down close to the weight of a model 19 with 10mm power. :D

Black Snowman
October 22, 2004, 11:27 PM
I heard a lot about the Ti cylinder and such but S&W says as long as I don't hold them liable they think it should be fine ;)

I think the Ti cylinder was more of a marketing gimick. I'm not metalurgist but I think as long as I don't make any over-pressure loads good ol' steel should be up to the task.

Black Snowman
October 22, 2004, 11:57 PM
7 shot is not going to be an option. Reaming out for the larger caliber is cutting into the margin of safety as it is.

I don't have a S&W 686 or 646 to check on at the moment so I'm using my Taurus 669 as a basis for comparison.

If I reamed out my 357 Taurus enough to chamber a 10mm I would be reducing the thinest wall in the cylinder from 0.80" to 0.77" or about 4%.

That 4% reduction in thickness would mean more like a 16% reduction in strength (a guestimation assuming I'm not ruining any kind of heat-treating in the process).

The 10mm and .357 Mag operate at about the same pressures so I'd be cutting signifigantly into the safety margin of the cylinder.

I did accidently proof test my 669 with about a max and a half charge of Universal in one of my reloads without it suffering any ill effects. Looking at the 7 shot S&W I'd say they sacrificed more than 0.03" of metal between cylinders when they added another round.

In fact some quick math tells us that if they had the 0.10" between cylinders that my Taurus has they would have had to go to 0.04" walls. I know they didn't get that small because there has to be 0.06" or the rims will over-lap. So that means the S&W cylinder should be larger than my Taurus and give a decent amount of extra cushion making the thinnest the S&W goes to about the same as what the Taurus starts with.

Still, I'll look into heat or cryo treating the cylinder to maximize strength.

Anyone have a 686 cylinder they can put a micrometer on and give me the thinnest point in the wall? I was replying on THR to a post about the strength of the cylinder and I wanted to see how much I'm really losing.

October 23, 2004, 07:02 AM
Why not find out out what type of cutter is used cut the Ti alloy? You may can just buy another 646 cylinder and have the chambers reamed down to 10mm if the cutter isn't cost prohibitive. Less machine work and keep the advantages of using Ti.

Brian Williams
October 23, 2004, 01:39 PM
Check with Tamara, here, or g33 over on glock talk, g33 had a 646 reamed to 10mm at the shop that Tam worked at.

Black Snowman
October 24, 2004, 11:11 AM
Thanks Brian.

Majic, that was my 1st instinct and when I emailed them that was specificly what I was looking for, but their suggestion was still the 686 cylinder implying that there aren't any 646 cylinders available. Even if they have a few, I imagine they're reserved for warranty work.

Nathan Detroit
October 24, 2004, 10:36 PM
Here are some numbers taken from a 686. The chamber wall thickness at the midpoint is 0.089". The thickness at the bolt cut is only 0.042". The nominal case diameter for the .357 is 0.379 and the case diameter for the 10mm is 0.425. That means that you have to increase the diameter of the chamber 0.425-0.379=0.046". That in turn translates into a radial increasse in the chamber of 0.023". That 0.023" results in a chamber wall at the midpoint of 0.089" - 0.023 = 0.066". The thickness at the bolt cut will be 0.042 - 0.023 = 0.019". That is getting mighty thin. Now it might be that the bolt cut is so close to the case head that the amount of load the the chamber wall at the bolt cut has to carry is substantially below that closer to the case mouth. Good luck.


Black Snowman
October 25, 2004, 12:35 AM
Thanks for the measurements Nathan.

10mm brass is very strong in the last 0.3" or so of the cartridge and doesn't expand at all from what I can measure on my spent brass. The thickness tapers off as it goes up but the brass under the bolt cuts should still be quite strong.

On the Ti cylinder the 40 brass is thinner at that point and is running at almost the same pressures. The stresses with 10mm shouldn't be any higher.

I'm banking that the thicker brass will be enough to eliminate any problems there. The bolt cut only comes to a short line where it is at it's full depth so risk of failure is minimized.

From G33s report on his conversion at Glock Talk. (http://www.glocktalk.com/showthread.php?s=&threadid=216693&perpage=25)

"S&W buddy would not go on record, but noted that as .40 ammo developed up to 35,000 psi, the cylinder could handle 37,500 psi."

Now since I'm planning on working with a stainless cylinder that data probably doesn't apply. Odds are it's either stronger or weaker by some measurable degree than the Ti.

Since S&W recommended it, I'm hoping it's going to be on the stronger side but don't want to take any chances. I'd rather know these things before I shell out the money for the cylinder and work rather than after I ruin a brand new handgun and possibly my hand with it.

Lloyd Smale
October 25, 2004, 05:42 AM
very interested in finding a 646 for a simualar project. I looked and i dont see them on there web sight is this model discontinued?

Brian Williams
October 25, 2004, 09:36 AM
Hey Lloyd check Gunsamerica they had 2 yesterday at $560 and $599

Lloyd Smale
October 26, 2004, 04:29 AM
thanks Brain keep me posted on the progress.

Jim Watson
October 26, 2004, 09:44 AM
Very interesting, I have never gone along with that "titanium is necessary to hold .40 pressure in a "L" cylinder" business. There are a lot of steel alloys and heat treatments available if their usual stuff wasn't strong enough. I think titanium is a dead end in firearms and you are going the right way.

October 26, 2004, 08:34 PM
"While you're at it why not have your smith trim that lug off ? That would be sweet if you could get it down close to the weight of a model 19 with 10mm power."

A 66 is 37 oz, my 686+ Mountian Gun is 37.5 oz with full sized houges on it. My 646 non PC & thus full lug is 35 oz.

Long load it to 10mm specs & Voila-

What we need is a 5"&/or4" 610+ Montain gun. (Odd number chambers = stronger cylinder. If they can do a 8 shot .357in an N frame why not a 7 shot .401?


October 26, 2004, 09:20 PM
A 66 is 37 oz, my 686+ Mountian Gun is 37.5 oz with full sized houges on it. My 646 non PC & thus full lug is 35 oz.

So lets say we could get a 646, keep the Ti cylinder but rechamber it to 10mm and shave off the lug. Down to about 31-32 oz. SWEET ! :D

Black Snowman
October 29, 2004, 02:34 AM
At 10mmMan's suggestion I'm going to put off spending extra money on modifying the gun until I get my results from these. (http://10mmtalk.com/forums/index.php?showtopic=472)

Combat Controller
March 22, 2007, 02:11 AM
Well, I started thinking of 10mm revolvers that were easy to pack a few weeks ago, and my 610 is just not quite small enough. I was going to go custom but then I ran across a 646 non PC. While searching the interweb I came across this thread from the S&W forums... How did your conversion go? I know G33 from glocktalk is pleased with silvertips...I ordered a spare .40 cyl from S&W today, so if it gets screwed up, I can still shoot .40... I was thinking of using the smith you posted, did you have a good experience?

March 23, 2007, 11:53 PM
I too am very interested. The ultimate caliber in the ultimate handgun, how did it go?

Combat Controller
March 24, 2007, 12:09 AM
Found the gunsmith who did the original conversion for "G33" on glocktalk, and he is happy to do it, I have to keep the pressure down though, he said keep it as a Winchester Silvertip gun. I have no problem with that. The more I dryfire it, the more I love this revolver.

March 24, 2007, 01:18 AM
I recently got a 646 and I love it! I have heard of a couple conversions to 10mm using the original Ti cylinder, but I like your idea of using a 686 cylinder better.

S&W's reason for using the Ti cylinder had nothing to do with strength. The 646 was first made as an IDPA competition gun, the lighter Ti cylinder rotates faster which allows a quicker lock time and lighter trigger pull. The first 646s were made by the Performance Center (PC) and had a lighter, slab sided barrel, later S&W did a run of 900 "non-PC" guns with the regular, full-lug, style barrel.

Here's a pic of my "non-PC" 646:


Combat Controller
March 24, 2007, 01:59 AM
That's the gun I got! I really like the trigger pull, and I ordered a spare Ti cyl for $150. The best part is I can still shoot .40 in it.

March 24, 2007, 05:56 PM
If you want to experience some thing close to 10 mm power in your 646, you can load .40 S&W cases with Hodgdon's Longshot. I have fired these from my 610, and they are very potent. Of course, 10mm will always do a little better.

I like the idea of a smaller 10mm revolver.

March 24, 2007, 08:51 PM
I wonder if the chambers on a five shot 696 cylinder could be sleeved to 10mm? That should be plenty strong....probably pretty expensive though.

Seven For Sure
March 24, 2007, 09:53 PM
That won't work. 696 has .429 chambers. 646 has .401. I have two 646's and an unfluted 686 cylinder ready to go. Don't know if I want to do it yet, the 646 is sweet as is and I love the 10mm. Not if I'm stuck to silvertips. Basically, those suck in power for 10mm and bullet technology these days. If you reload, you can long load the .40 to get very close.

Seven For Sure
March 24, 2007, 09:54 PM
Plus, they only come in 20 rd. boxes, I can shoot DT way cheaper!

March 24, 2007, 10:07 PM
You do know that there are other runs in the works for more S&W 610 10mm revolvers, available later this year?

March 24, 2007, 10:11 PM
I am looking for a "donor" GP100 or a 686 to modify to 10mm/40 S&W. I am going to have Clements Custom do the conversion.

March 24, 2007, 10:14 PM
That won't work. 696 has .429 chambers. 646 has .401.

Which is why they would need to be sleeved.

March 24, 2007, 10:30 PM
You do know that there are other runs in the works for more S&W 610 10mm revolvers, available later this year?

(Down on my knees; arms reaching up to the heavans)

Combat Controller
March 24, 2007, 11:06 PM
Well, I don't want to mess with hot .40 loads, as I don't really care for the round. I have other 10mm guns, and light loads, or winchester are fine for carry. It is a 10mm after all. This way I can swap out from carbine, to 610, to Delta Elite to 646. I talked to the smith today who did the conversion of the other 646, and he told me to send the gun. It is $15 per chamber including polishing and whatnot, he asked for the whole gun to check headspace. I think I will have him put in a tritium front sight while he is at it.

Seven For Sure
March 25, 2007, 10:24 AM
That sounds very reasonable. Good luck with it and please let us know how it turns out.

Combat Controller
March 25, 2007, 09:42 PM
I will do so!

Combat Controller
July 14, 2007, 03:21 AM
It is done! I just got back my bored out 646 .40 S&W revolver (http://i7.tinypic.com/4uixfki.jpg). It now accepts 10mm cartridges. What little I could garner from the internets told me at least one person has done this, possibly two. I used the same gunsmith and I am going to follow the advice I was given about pressure. The 10mm can spike up into the 40k psi range, and the gun is only rated for a 36k psi .40 cartridge. The Winchester Silver Tip 175 gr is supposed to shoot well in this gun and at 33k psi, should be safe. I am hesitant to develop a handload for it, and just go with the more costly, but lower pressure commercial load.

Here it is compared to my other two 10mm revolvers (http://i16.tinypic.com/5x6jsrp.jpg), a 5" and a 3" lew horton 610. Notice it is much smaller than the N frame 3" and should be easier to pack. As it is the same size as my pythons (and other S&W's), I can use the holsters with no problems. It is not a snub nosed gun, but I may tackle that one at some point, but it will require a Ruger and a lot of heavy custom work. So, for the time being, I am going to try and convince my gunsmith to make me a custom 1911 CCO in 10mm. He is hesitant, but heck, it's my money!

Seven For Sure
July 20, 2007, 12:03 PM
Awsome. I have two 646's. One NIB with a non fluted SS 686 cylinder waiting for conversion. A 10mm, L frame S&W revolver without the lock. What more could a guy ask for? You should try talking to these guys and ask them if they have any low pressure loads. Good shooting!

Combat Controller
July 22, 2007, 08:48 PM
I plan on it, thanks!

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