9mm J-Frame conversion

Well, after trying to find a 940 for a long time, I've decided to see if I can make my own from a 640/340 by adding a 940 cylinder. My first question is what models of 640 have the right frame window dimensions for the 940 cylinder? The second question is whether there is a bore difference between a 940 (at about .355) and a 640 (at about .357)? I don't know if I want to try to find a barrel, although I know I can get cylinders. Any suggestions out there?

There has been more then one 940 in the classifieds in the last couple weeks.

I don't know anything about the frame window, but, the bore will be fine. 9mm is actually .356 and its quite common for them to be .357.

9mm J Frame Convertion

The early J frames with the 38spl sized window are what your looking for to accept a 940 or 940-1 9mm cylinder..
The difference between the 940 & 940-1 cylinders is the newer ejector ratchet or older ones with two pins..
There was a 940-2 made with a magnum sized frame window but I have never seen one but do know they exist..
A 640 No Dash 38spl would have the correct frame window to accept a 940/940-1 cylinder, a 640-1 is the 357 mag frame window..
I was able to aquire a 940-1 cylinder with extractor ratchet new & also aquired a center pin & ejector rod which were magnum cylinder lenth that I had to cut down for the convertion of my 642 No Dash to 9mm para..
After first fitting the cylinder to the yolk I had to add two .002" endshake shims to the 940-1 cylinder, then after checking barrel/cylinder gap & headspace I then went to work filing the ratchet to fit the 642s existing hand which was a very time consuming project but allowed me to convert the 642 to 9mm & back with just removeing the endshake screw, swaping cylinder & retightening the screw.
I haven't range tested this convertion yet but after reading other threads about convetions & the very miniscule difference between .355" .356" & .357" don't think I'll notice any accuracy loss between the 38spl & 9mm para..
Plan on shooting same paperplate targets at exact distance with similar ammos to see which is most accurate, I know which cost the least the 9mm para..
I also have a 940 & can tell you they can be had & sometimes for very "Decent" prices..
My 940 I traded straight up for a $175 Ruger SP101 38spl.
It did have a homemade endshake screw & broken firing pin & S&W sent me all the parts to repair it FREE of charge..
It may also be easier to find an extra 38spl cylinder & pay to have it reamed to 9mm para & cut for the moon clips..
I was very lucky as I posted a WTB 940 cylinder in the classifieds & found a fellow who had bought the parts to convert a later J frame to 9mm but his frame window was too large & they couldn't be used, I actualy traded an old Alessi Bodygaurd shoulder holster for the whole setup..
Here's a pic of my "942" 9mm Airweight shown with the 9mm cylinder attached & the 38spl next to it before I put my Spegel grips back on.
It's a very neat convertion & will give a good range report after I wring it out..
2nd pic is the 940 & "942".. My Current Avatar!!
Gary/Hk
642/942 Convertable Airweight

940 & 942

940/640 9mm conversion

Thanks for the information and the reply, Gary. You have an impressive set-up there. I intend to step up my search for either a 940 or a 640 and 940 cylinder to put into it. If you run across either, please let me know. I'm anxious to hear about your 942 range report. Thanks again.

Dave

I think you can order a 940 cylinder from S&W -- at least, it's on their parts list. http://www.smith-wesson.com/wcsstore/SmWesson/upload/other/RevPartsList_Retail.pdf

What model 60 frames would work for the same conversion? Would it be possible to use any frame that is not one of the extended magnum versions?

Any older J frame 38spl with the smaller window size could be converted to take the 9mm cylinder..
I measured the window size from the forceing cone to the recoil shield of both my J frames & the 642/942 is 1.6010" & the 940 is 1.6135"...
I'm not sure if S&W will sell a cylinder to an individual as it's a factory fitted part but it is listed on thier webpage above for around $123.00, Midway USA also lists a 940-1 cylinder for the same money but list it as a special order 60+ days, Also note the Rusted Cylinder in the pic..
MidwayUSA - Smith & Wesson Cylinder Assembly S&W J-Frame Model 940
Pred here on the forum converted a M60 to 9mm & was planning on doing a M649 Bodygaurd as well..
It will be a while before I get to the range, I don't get to go every week as I once did & only go a few times a year now, this realy helps with ammo costs but I'm not getting the fun from my guns I once did..
I won't carry the 642 in 9mm till I know what it can do & if the chambers are sticky, which is a common problem with the 940s.. My 642 is my only Airweight & gets carried more than all my other guns combined, None Better IMO..
I have a new set of cylinder hones that can cure these sticking case problems BTW Thanks to the great folks here on the forum who replaced them for me, My old set fixed several 940s thoughout the USA.. They are available for use by any forum member, They realy helped smooth out the chambers of my newly aquired M686 No dash BTW..NICE!!
I have three dozen moon clips that came from TK Customs that are .040" thick but now they only list .035", They were made for Ruger SP101 9mm, I've heard that they all come from Ranch Products who has a booth at the local gunshows & will probobly pick up a few more if thier available..
Smith & Wesson also has moon clips & thier part # is 191410000 for a package, they also list them seperatly on thier website.. Also plan on getting some of the Ruger SP101 9mm Moon clips for the 940 the next time I order parts from Ruger..
I've always liked 9mm revolvers & now have a 940 the 642/942 convertion & a 4" M547 & am always interested in aquireing a 3" M547 at a "Decent" Price..
Gary/Hk