Forcing Cone Problem

[teamgeek]

I apologize for my delayed return to this thread. Family illness and a pistol tournament down in Phoenix got in the way. I was finally able to test the 686-6 for accuracy in my Ransom Rest. Following warmup shots from all cylinders, I fired five consecutive five shot groups at 25 yards on the same multi bullseye target. I moved the target between each group so as not to disturb the Rest. I used 357 cases with a slightly increased powder charge, selected using QuickLOAD, in an effort to duplicate the performance of my standard 38 special bullseye ammunition. The average group size was 1.90 inches. The average velocity was 734 fps, standard deviation was 25 fps.

For reference, I can report that my model 67-5 shoots 1.26 inch average groups with my standard 38 special bullseye ammunition under the same test conditions. I have recorded average velocity at 750 fps, standard deviation at 24 fps.

About 50 rounds had been fired in the 686 by the end of this testing. Limited leading was present in the barrel beyond the forcing cone. For comparison, my model 67 showed less leading after firing about 180 rounds in the centerfire stages of Phoenix match.

I also slugged the barrel and each cylinder throat on the 686. I used round Hornady lead balls driven into clean lubricated throats and barrel. To be sure I get accurate readings, I bump up the resulting lead slugs after they are in place. The cylinders were very consistent at .3576. The barrel minimum groove dimension was .3552. The barrel was tight at this minimum at the muzzle and at a point just forward of the forcing cone. Between these two locations the barrel was loose on the slug. I did not push the barrel slug over the burrs at the forcing cone.

My plan now will be to fire full power jacketed 357 ammunition to see if the burrs can be worn away. If this works, I will repeat the accuracy testing with bullseye ammunition.

 

[teamgeek]

I am grateful for all the input. Following suggestions from the Forum, I fired fifty 357 rounds loaded with 110 grain jacketed bullets using the maximum charge of WW 296 listed in the Hodgdon Reloading Data Center. Before and after pictures are included with the “after” picture being the darker of the two.

To remind everyone, the pictures are taken using a lens that views the rifling at 90 degrees from the direction of bullet travel. The forcing cone reamer cuts each rifling land in a way that produces a ramp. It is at each of these ramps that a burr has been created and pulled over into the adjacent groove effectively making the land wider but just at the forcing cone. The ramps on the other lands have similar burrs.

The two pictures are of the same location in the barrel. It is apparent that fifty rounds of full power jacketed ammunition had little or no effect on the burrs.

I will be considering other actions that have been suggested. I would appreciate any additional comments.

 

[Nostrom]

I just picked up a new 632 UC i ordered online, and the forcing cone has to be seen to be believed, looks like it was done with a Sawzall. I've dozens of Smiths over the years and I've never seen anything like this, it's Rock Island-tier bad.

Pics to come later.

 

[Yendor357]

Wow, a Ransom Rest and a chrono? Very cool.

I also think your 686 ought to group better than 1.9 at 25 yards.
Different ammo maybe?

 

[teamgeek]

Hello to all those following this thread. As a next step, I ordered a 9mm throating reamer from Brownells. It arrived and was marked “35 CAL Pistol”. My measurements indicate that this reamer is .3595 inches in diameter on the cutting surfaces.

I removed the barrel from the 686. Please see the attached image of it mounted in my lathe. I made successive shallow cuts with the reamer followed by observations with my Hawkeye Borescope as I tried to remove the burrs. It seemed to me that the shallow angle of the cut, combined with either the poor machining properties of the metal or course cutting surfaces on the reamer, tended to maintain the burrs. I eventually advanced the cutter about a tenth of an inch to achieve removal of most but not all the defects. An image inside the barrel following reaming is included showing the most prominent remaining burr. I am not impressed with the finish left by the reamer.

I also removed the constriction I previously reported just forward of the forcing cone using a cast lead lap and 400 grit abrasive. The barrel groove diameter now measures .3562 at the breech end tapering to .3552 at the muzzle. Lapping of this barrel seemed to proceed quickly compared to other barrels I have lapped indicating that the metal might be softer.

I reinstalled the barrel and will repeat some accuracy testing.

 

[teamgeek]

I fired 12 fowling shots and then six consecutive 5 shot test groups through the 686 at 25 yards using my Ransom Rest. The ammo consisted of carefully prepared lead target loads in 357 cases similar to my best target 38 Special handloads. The average group size was 1.60 inches. This is an improvement but still not up to the accuracy of my M-67.

A summary of my actions:

• I returned the 686 to S&W twice for repair. The barrel was replaced but the second barrel included similar burrs. S&W declined to take additional action on the second return.
• I tested with loads in 357 cases that were patterned after my best 38 special loads.
• I was largely able to remove the burrs found at the forcing cone. To achieve this, it was necessary to move the origin of the rifling forward in the barrel increasing the jump of bullets as they leave the cylinders. An image is included showing bullets in the back of the barrel for this 686 and in my Model 67 for comparison.
• I used an inexpensive throating reamer with a larger than optimal diameter (.3595). I declined to secure a custom reamer in a diameter closer to the nominal value of .357. Also, the barrel steel may be of a type selected for ECM rifling processes but not particularly suitable for forcing cone reaming.

I am inclined to conclude that barrels with burrs in the forcing cone cannot be sufficiently improved to justify the cost and effort. Please let me know if you have questions. I would also appreciate any additional comments or suggestions.

 

[Protocall_Design]

You might want to recheck for barrel constriction with the barrel installed. When you take the barrel off, the diameter mostly springs back to original size. When you put it back on, it will be compressed again unless you have turned a couple thou. off the back where it butts up against the frame so it's not twisted on as tight.

 

[-BTM-]

Im unclear, why did you go oversized with the throating reamer and did you use it in the barrel?

 

[rwsmith]

T
What don't pass?? They don't have a quality inspection.

'Passed' means it went out the factory door. The customer trying to shoot his brand new revolver is the QC. And when you send it back they either say it's 'in spec' or replace it with an equally poor part. That's why I stick with the old ones.

 

[teamgeek]

Thanks to Protocall_Design, BTM and rwsmith for their great comments and questions.

I can report that the barrel seemed very tight when I removed it. Based on the position of the barrel at a more reasonable torque, I calculated an amount I then removed from the shoulder of the barrel before reinstallation. I will, however, go back and make some measurements with lead slugs on the installed barrel and report what I find.

In the case of the throating reamer, it came from Brownells at the oversize dimension of 0.3595. I had hoped it would come out slightly over .355 since it was listed as a 9mm reamer but I suppose they have only one they supply for both 9mm and various “35 caliber” chambers. I have not tried personally to regrind any of my chambering or throating reamers primarily because I do not have the right equipment. I chose not to wait the time or spend the money for custom rework of the Brownells (Clymer) reamer, or for a custom reamer.

Yes, I did use the 0.3595 throating reamer in the barrel at the forcing cone in an effort to clean out the burrs.