Been experimenting with the strain screw.

[scooter123]

As a mechanical engineer part of the appeal to the S&W revolver is it's combination of simplicity and accessability for "tuning". Since I didn't want to make any changes that couldn't be reversed, I've been making up some shims that fit under the strain screw using standard stainless steel shim stock.

Because ammo for revolvers is a bit catch what you can, I also wanted to find the reliability limit for Speer ammunition due to the reputation for CCI primers being a bit hard. In addition, I like the 38 +P Speer Gold Dot as a defense load for the home, so I wanted to insure function with that particular ammunition.

Guns used were a 67-1, 610-3, 620 no dash, and 617 no dash. Mainsprings are the factory mainspring in each model and all 4 have a 14 lbs. rebound spring installed. I also took the time to do some light stoning on appropriate surfaces per the Kuhnhausen manual, however on the MIM guns much of this work has aready been incorporated into the guns. The firing pins in all 4 guns are currently the factory firing pins. I do have extended pins for the 610 and 620 but haven't decided yet if I want to try using them to achieve a lighter DA trigger on these guns. Right now it's nice to have every gun with nearly identical triggers.

Currently the SA break is just a hair over 3 lbs. and the DA pull is right about 8 lbs. per a cheap modified fish scale, so those weights can probably vary by 1/2 lbs.. Next time I place an order at Brownell's I intend to order the Lyman digital so I can get a good accurate result.

At this point, all 4 have a 0.010 thick shim installed under the strain screw and all 4 have proven to be 100% reliable for ignition. However, the 617 has only been tested with the Federal 36 gr. bulk ammo from Walmart simply because I've found it to be clean shooting in regards to cylinder fouling. Tomorrow I have to do a bit more sight tuning with the J Point I've installed on the 617 and will try it out with Winchester Xpert HV. Might also try it with some CCI Stingers if I have find some at the range.

Today I decided to lighten the DA trigger on the 610 a bit more and installed a 0.013 inch shim. That change of just 0.003 inch had a VERY noticable effect on ignition reliability with 40 caliber Blaser Brass. On each loading of the cylinder I had a misfire on 1 or two rounds. However, every misfire did ignite on the second strike. Function was 100% in single action.

Anyhow, at this point it seems that an 8 lbs. DA trigger is basically the reliability point in the K, L, and N frames for ammunition using CCI primers. One thing that was a bit of a revelation is that also seems to apply to the rimfires, however more testing of a selection of ammunition is required before I would make that a blanket statement.

BTW, the measurements for the shim are an inside diameter of 0.147 inch and an outside diameter of 0.215 inch. Unfortunately a 0.250 diameter round won't fit into the recess in the frame, so I had to resort to grinding the OD to size using a diamond burr in a Dremel, a rather fiddly exercize.

For those who want to take a more direct route and file down the strain screw, I would suggest using either a micrometer or digital calipers to insure you don't over shoot. It would also be a good idea to have a spare strain screw on hand just in case you do take it too far.

I'll also note that the 45ACP moon clip revolvers have a bit of a reputation for misfire issues with the factory setup. On the model 25 and 625 the use of an extended firing pin is probably essential and a cautious approach should be taken.

 

[500 Magnum Nut]

Reduce the rebound spring now, and the DA will be lighter. Reduce the drag on the center pin (or anywhere for that matter) and the DA is still lighter. Stone all high spots on the insides, add a few drops of good oil and the DA is still lighter.

Harder to get on N frames as the cylinders are so much heavier, but the same applies. Rimfires are tough to do because of the rimfire case.

Enjoy experimenting!

 

[scooter123]

I've experimented with various rebound springs, didn't like the effect it had on the SA break. Using a 12 lbs. rebound spring took the SA break a bit below 2 lbs. a trigger light enough that I'm not completely comfortable with it and I wouldn't ever permit a new or inexperienced shooter to use a trigger that light. I also didn't see enough gain in reducing the DA pull to justify the lighter SA trigger that comes with a lighter rebound spring. The trigger bar actually acts as a toggle link so it has a large leverage advantage on the rebound spring.

What was interesting is that shimming the strain screw on the 617 so that the trigger matched the centerfires didn't result in misfires. Granted, this was with the Federal ammo, however it does point out that the DA trigger on the rimfires can be lightened somewhat from the factory setting. Later today I hope to put more range time in with the 617 and try a variety of ammos avalable.

Currently the 610 has a double action pull that is just superb. It is hitch free and dead smooth from start to break. Part of this I attribute to the trigger stop pin in the rebound spring on the N frames. I noticed when checking the smoothness of the rebound slide with the hammer out of the lockwork on the K and L frames that there is an increase in friction towards the end of the trigger stroke. Since I've fully polished the bores of the rebound slide on all 4 guns using diamond lapping compound and all 4 are lubricated with RIG gun grease, I attribute this increase in friction to the rebound spring buckling within the bore. At some point I plan on adding that stop pin from the N frames to each of my guns to see if that pin is acting to reduce the spring buckling and is the reason why the 610 is so perfectly smooth.

Fact is the 610 is now so smooth that some shooters wouldn't like it. It's nearly impossible to stage the trigger by feel, which for me is a good thing because it reduces the temptation to stage the trigger to improve accuracy.

I've also noticed a very slight hitch, perhaps 1/8-1/4 lbs., in the trigger stroke of the K and L frames right at the beginning of the stroke. Watching the lockwork on these guns shows that this hitch is cause by the DA sear on the hammer flipping forward by perhaps 1/64 inch at the start of the trigger stroke. On the 610 the DA sear does not do this. One thought for correcting this would be to reshape either the DA sear on the hammer or reshape the DA sear on the trigger to eliminate this. By my estimate only about 0.005 inch of material would have to be removed, however without knowing the depth of the hardening on these parts I hesitate to do this. To be honest, that hitch is so slight it's meaninless. However the 610 is now so perfectly smooth that it's a bit hard to resist trying to get that same level of smoothness on the K and L frames.

One other thought is a matter of Theory. What is the effect of changing the hammer weight on Ignition. Using a lighter hammer would provide a faster hammer stroke and in theory that would increase the kenetic energy delivered to the primer. The problem with this theory is that you end up with a loss in Inertia and I've always found it much easier to drive a heavy nail with a heavy hammer as opposed to a light hammer. I wonder if anyone has ever experimented with the effect of a bobbed hammer on ignition versus a hammer with the spur in place. If so, I would be very interesting is hearing the results.

As for lubrication, I've been using RIG gun grease on the rebound slide and hammer pivot, light oil everywhere else. I don't do much shooting outdoors, the nearest outdoor range is about an hour and fifteen minutes away. Fortunately, there is an excellent 50 yard indoor range 15 minutes from home. So, I don't see any issues with using a light grease in some areas. However, somewhere I have some high tech teflon bearing greases stashed from my days of tinkering with RC models, may have to dig them out and see if they are any better.

 

[bountyhunter]

I never saw the attraction of filing strain screws because the metal is gone. If you get misfires you need a new screw. I will slightly bend the mainspring and also back the strain screw out a bit (not more than about 1/2 turn) and loctite it if I want to leave it there.

As for where to set spring force: measuring DA pull is an indirect measurement because it also includes the rebound spring. I use a trigger pull gauge with dental floss tied directly to the hammer pulling straight back a fixed distance to measure actual hammer force from the mainspring. Cock it, pull the trigger and hold it pulled, then measure the hammer spring force.

If you adjust it until you start to get misfires, measure the actual hammer force and then crank it up about 10%. Always fire DA to get the misfire threshold as that is worst case.

 

[bountyhunter]

One other thought is a matter of Theory. What is the effect of changing the hammer weight on Ignition. Using a lighter hammer would provide a faster hammer stroke and in theory that would increase the kenetic energy delivered to the primer.

There is an optimum mass of hammers (or strikers) and in every case I have ever seen, the mechanical requirements of the piece force the mass to be well above optimum. Ergo, lightened hammers and strikers give more striking energy than stock. I have seen a special SW DAO hammer that is very much reduced which allowed DA pulls in the 3# ball park. You can also find reduced mass strikers for Glocks and the like. They do work.

 

[scooter123]

Bountyhunter, that's a very interesting post. It also confirms a hunch that primer ignition is primarily a result of kinetic energy transfer. Seems that perhaps S&W should look into producing Titanium Hammers instead of cylinders. If your indication of reliable ignition with a very light hammer bears fruit it's an indication that it's possible to produce a DA/SA revolver with identical trigger pulls in both modes of operation. That would make for a very interesting revolver to shoot.

The only problem is the titanium isn't that hard, so something would have to be done to address wear on the sear surfaces. Might have to do a bit of imagineering. Wonder how well a composite steel and carbon fibre hammer would stand up.

 

[bountyhunter]

Bountyhunter, that's a very interesting post. It also confirms a hunch that primer ignition is primarily a result of kinetic energy transfer. Seems that perhaps S&W should look into producing Titanium Hammers instead of cylinders. If your indication of reliable ignition with a very light hammer bears fruit it's an indication that it's possible to produce a DA/SA revolver with identical trigger pulls in both modes of operation. That would make for a very interesting revolver to shoot.

Hard to do: the low mass hammers I referred to are DAO because they have no cocking spur for SA use, in fact as much of the top mass is removed as possible. Because the moment of inertia (I think, been 33 years?) is proportional to the distance from the pivot point squared, the mass you remove farthest from the pivot gives the biggest bang for the buck. I have read that the snubs that have their hammer spurs bobbed actually have a measurable increase in strike energy (which is to say, mainspring can be further lightened before misfires). I never tied cutting the top off a hammer to see.

There was a pro on the Brian Enos forum who developed the DAO competition hammer for SW revos that was minimum mass. I just recall less mass is almost always better for transferring hammer energy to the firing pin or directly to the primer in the older style hammers.

I believe you are correct that titanium or "skeletonized" hammers could significantly improve strike performance if mass was significantly reduced.

 

[bountyhunter]

The only problem is the titanium isn't that hard, so something would have to be done to address wear on the sear surfaces. Might have to do a bit of imagineering. Wonder how well a composite steel and carbon fibre hammer would stand up.

That is a big problem: the present steel hammers are surface hardened about .006" deep and are very reliable for sear wear resistance. The point of a surface hardened trigger or hammer is that about 98% of the piece is NOT hardened which means it's less prone to chip or shatter, but only the surface you need to be hard is hardened. I have my doubts there is an analogous process for titanium.

 

[tomcatt51]

Lightened hammers are a good thing if you want to lighten the DA pull of your CF gun. The MIM hammers regularly get cut down to 1/2 their original weight. The firing pins in the newer S&W's (MIM FMFP guns) are titanium. Don't lighten the hammer on your (MIM) 617, the rimfires don't like lighter hammers. When you're squishing the case rim for ignition, more mass seems beneficial.

You also run into a geometry issue just shortening the strain screw. As you shorten the strain screw, and preload the mainspring less, you increase the installed lenth of the mainspring. This alters the hammer/mainspring link (or stirrup) geometry. You can get to the point of binding the link (or stirrup) against the hammer. A good reason to to put (bend) more arch in the stock mainspring or use a spring with more arch.

 

[tdan]

Take a minute to read a gunsmith's opinion on lightening factory springs regarding effect on revolver accuracy.

HOME

Go to the news and announcements section, and then read his blog from 9-9-2010 about firing pin impact.

I routinely "tweak" my Smith revolvers. After reading that article, I'm thinking of testing mainspring pressure ranges and seeing how that affects accuracy in my guns.

 

[tomcatt51]

After reading that article, I'm thinking of testing mainspring pressure ranges and seeing how that affects accuracy in my guns.

You still have the human part in the equation if you're just shooting groups. If you're getting inconsistent/inadequate ignition you'll see it as an increase in your standard deviations and extreme spread chronographing your loads. Heavier hammer falls (more mainspring) are also necessary if you have reloading issues like inconsistently seated primers.

 

[tdan]

You still have the human part in the equation if you're just shooting groups. If you're getting inconsistent/inadequate ignition you'll see it as an increase in your standard deviations and extreme spread chronographing your loads. Heavier hammer falls (more mainspring) are also necessary if you have reloading issues like inconsistently seated primers.

You are absolutely right about plenty of variables involved with centerfire handloads. I'll see if I can verify this gunsmith's results with a rimfire 617 first. I have enough gray hair as it is!

 

[tomcatt51]

I'll see if I can verify this gunsmith's results with a rimfire 617 first. I have enough gray hair as it is!

For years I've heard it said you'll see a group size increase with a light strike on rf .22's. I've always seen an unacceptable increase in mis-fires first. The types of competition I shoot don't allow alibis.

 

[scooter123]

Saturday I took my 620 to the range to sight it in after installing a J Point reflex. Once I had the sight zeroed at 25 yards I then ran the target back to 35 yards and did a bit of bench shooting in single action using a sandbag. BTW, it's a 50 yard indoor range but 35 yards is the longest distance at whick I can still see the 3/4 inch red bullseye on an 8 inch shoot-n-c sticker.

Fired 7 rounds using Speer 125 grain TMJ Lawman. The first shot hit the bull, low center. This got me a bit excited and I jerked the trigger on the next shot, throwing it 4 inches high. Then I settled down and put the next 4 shots in a straight horizontal line centered on that first shot. That line had a total length from center to center of 1.8 inches. Natrually, I again got a bit excited and threw the last shot 4 inches high, 3/4 inches to the right of the previous flyer. Darned flyers drive me nuts, I just have to learn to calm down until I'm finished. Might help if I forget to take the spotting scope with me.

Now about the results, when I didn't jerk the trigger the deviation was horizontal. Total vertical deviation was 3/8 inch for the 5 shot string I put in the 10 ring. As for the horizontal stringing, I suspect that it's all me, I need to work on my release. I honestly believe that with this particular ammunition my 620 is capable of grouping at 3/4 to 1 inch at 35 yards. I was actually a bit shocked at how small the vertical deviation was. I'm going to have to head back to the shop where I got that TMJ Lawman and see if they'll order in a case to me, it's pretty obvious the 620 really likes this load.

BTW, at the start of this experiment I shot a box of 158 gr. Blaser LRN and won't make that mistake again. As I found out the new style rifling used on the 620 fouls BADLY with lead bullets. It took me a full 3 hours of work using peroxide/white vinegar soaked patches to get the lead out of the barrel.

Currently all 4 revolvers have 14 lbs. rebound spring installed and a 0.010 inch thick shim under the strain screw and I am going to call it good. As I found out with my 610, using a shim just 0.003 inch thicker caused lots of misfires with Speer ammo. With the 0.010 shim all 4 of my revolvers have shown 100% reliability for ignition so IMO that's the limit for tweaking the strain screw setting. The DA pull is right about 8 lbs. and it's easy to shoot with. The only further tweak planned is to add a trigger stop rod in the rebound spring, that should reduce the rebound spring buckling and get rid of that slight "hitch" just before the DA breaks on the L and K frames.

Note, after reading that article on the effect of primer ignition there are 2 more tests I want to run. One is to shoot all 4 from a sandbag in double action and see if I observe any vertical stringing. If I do see that, I'll probably back the shim down by 0.002 inch and test some more. The other is to test the 620 with some 357 Magnum ammunition to insure it's reliable with Magnum primers and also check for stringing. I don't shoot a lot of Magnums with the 620 due to the expense, however when the mood hits it'll be good to have some assurance that the gun will work properly.

 

[John F.]

Poor Ignition/accuracy in rimfires

Interesting thread! I once had a custom 10/22 built with Shilen barrel, etc. that would group 0.3" average for 8, 5-shot groups at 50 yards. It would group in the high 0.3"s with a good lot of Eley Standard (low grade match/practice ammo) and in the low 0.3"s with a given lot of Eley Tenex (best grade ammo.) This was consistent.

However, with a second lot of Tenex, it would routinely shoot 4 shots into a tiny group and throw a flier high, opening the groups to about 0.6" - 0.8", as I recall. My gunsmith looked it over and saw that the hammer spring was weak. He replaced it with a new hammer spring, and also saw that the firing pin face was chipped and uneven. He put in a new firing pin and ensured that firing pin protrusion and travel were optimum. Re-testing with the second lot of Tenex showed that the fliers were gone, and it now grouped as well as the first lot.

I've always heard of the importance of good ignition, but it was really cool to actually see the problem and eliminate it through correcting poor ignition!

Hope this helps,
John