Spring Rates and Trigger Pull Weights

Double-action revolvers are well known for their long, heavy double-action trigger pulls. Factory double-action trigger pull weights generally run from 12 to 15 pounds for most revolvers, with 4 to 6 pounds for a single-action pull. The trigger pull can be greatly improved just by changing springs, but it isn’t always obvious what a final trigger pull will feel like with which springs, so it’s not clear which pound-strength to buy.

I’m focusing on Smith & Wesson revolvers in this report, and two springs control the trigger pull in S&W revolvers. The mainspring (a.k.a. hammerspring) controls the hammer, and the rebound spring controls the trigger. The mainspring controls the double-action trigger pull weight but has little effect on the single-action trigger pull. The rebound spring affects both single-action and double-action trigger pull weight.

Wolff Gunsprings makes springs for J-, K-, L-, and N-Frame S&W revolvers as well as for many other brands. The K-, L-, and N-Frame guns use a flat mainspring, and the J-Frame guns use a coil-type mainspring. All four frames use the same round wire coil trigger rebound springs.

Wolff makes two Power Rib mainsprings for the K-, L-, and N-Frame guns: a standard power (Type 1) and a reduced power (Type 2). The standard-power mainspring is recommended for critical applications, such as law enforcement, while the reduced-power mainspring is recommended for target and non-critical applications, according to Wolff.

The Power Rib mainsprings have central ribs that run most of their lengths. The springs have a unique variable-power design that improves the smoothness and consistency of the trigger pull and essentially eliminates stack up, according to the manufacturer.

Results of Changing Springs

I used a Timney trigger pull gauge to measure the trigger pull weight with the S&W factory mainspring and both of Wolff’s mainsprings and all the rebound spring weights the company offers, which are from 11 to 16 pounds. I put the springs in a new .38 Special S&W Model 67 K-Frame revolver.

The trigger pull with the S&W factory springs was 4.5 pounds in single action and 13.0 pounds in double action. There is no “weight” rating for the mainspring, but the S&W factory rebound spring has a weight rating of 18 pounds, according to Wolff.

The single-action trigger pull was over 4 pounds with all of the mainsprings when combined with the factory 18-pound rebound spring. Trigger pull weights decreased fairly consistently as the rebound spring weight decreased, though the 12-pound rebound spring had a slightly higher average pull weight than would be expected. The single-action trigger pull was reduced (mostly) to around 3 pounds or less once the rebound spring weight reached 13 pounds or less. The lightest single-action trigger pull with the 11-pound rebound spring was 2.8 pounds with the factory mainspring and the Type 1 Wolff mainspring and 2.6 pounds with the Wolff Type 2 reduced-power mainspring.

Wolff says its standard Type 1 Power Rib mainspring is equivalent in strength to the S&W factory mainspring, but I found that it reduced the double-action trigger pull on my gun by 3 pounds compared to the factory mainspring, and it maintained a consistently reduced pull of 2 to 3 pounds when combined with all the rebound springs. The Wolff reduced Type 2 Power Rib mainspring reduced the double-action trigger pull by about another 2 pounds. With all three mainsprings, the double-action trigger pull did not change much after rebound springs of 13- and 14-pound weights were installed.

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But how reliably does the revolver operate with a lighter trigger pull? Reducing the strength of the mainspring reduces the force of the hammer strike. Lighter trigger pulls are wonderful, but the gun still has to “go bang” every time the trigger is squeezed.

I have determined that when it comes to reliable ignition with a lighter trigger pull the brand of ammunition you use matters. Federal has a reputation for having the most sensitive pistol primers, and they are very popular among competitors who tune their guns to have the lightest trigger pull possible and still be reliable. But this means that even though a gun reliably fires Federal primers, it might not reliably fire some of the other brands.

Before conducting this test, I couldn’t say just how sensitive the other primer brands were, so I included several brands. Magnum primers, by reputation, have thicker cup material to better handle magnum pressure. Here again, I couldn’t say how this might affect sensitivity, but I included some magnum primers in my test ammo. I used handloads with a variety of primer brands.

All ammo was test-fired in double-action mode. Many believe a double-action trigger pull produces less hammer force because it does not travel to the rear as far as when the hammer is cocked in single-action mode. Thus, the less distance the hammer moves during the firing cycle, the less force it generates to hit the primer. I don’t have a way to measure this but took the idea to heart and fired all loads double action with the thought that if it fires with a “weaker” hammer fall from double action, that should be the more rigorous test of the mainspring’s reliability.

I tested the Wolff standard-power Type 1 mainspring in the Model 67 with five different primers. It reliably fired 100 out of 100 of four standard non-magnum primers: CCI 500, Federal 100M, Remington 1½, and Winchester WSP. However, it was not completely reliable with CCI’s 550 Magnum Pistol primers, setting off 94 out of 100 rounds. This could be considered acceptable since this gun is chambered in .38 Special and will likely not see magnum primers in any factory ammo in this caliber.

With the Wolff reduced-power Type 2 mainspring in the Model 67, I experienced many misfires. It only fired four out of six Federal rounds and three out of six Remington rounds. I stopped after only one cylinder of each of these brands because this mainspring was clearly too lightly powered.

However, not all guns are the same, so I tried the reduced-power mainspring in an S&W Model 65 (also a K-Frame revolver but chambered in .357 Magnum). My Model 65 has had a fair amount of use, so the movement of its parts probably produces less friction than the newer Model 67, which could rob energy from the hammer speed and strike. Ignition performance with the reduced-power mainspring was much better in the Model 65 than it was in the Model 67. The Model 65 reliably fired Federal primers 60 of 60 rounds. However, it misfired one round out of 36 rounds of Remington 1½ primers. It really didn’t like CCI 550 primers, failing to fire three out of 12 rounds.

Fix It or Forget It?

So what if your gun’s mainspring is not 100 percent reliable with your primer of choice? What’s the next move? Do you give up and put the original spring in and write it off to experience? That’s one option. The other is to fix it by increasing tension with the strain screw.

Tension can be increased in two ways. One is to add a shim between the end of the strain screw and the mainspring, so when the screw is tightened all the way, it puts more tension on the mainspring and the hammer will hit harder.

Some shooters use a spent primer cup as the shim. Primer cup thickness varies from roughly 0.017 to 0.025 inch. Caution: Only spent primers should be used as shims. Do not tinker with live primers because the primer mixture is very sensitive, and anything that can go wrong might go wrong! Remove the anvil from the spent primer cup and give it a quick cleaning to remove any leftover residue.

Not everyone will have easy access to spent primers, so they can make shim material from spent cases that can be cut with wire cutters. Case wall thickness of .38 Special cases at the mouth runs from about 0.009 to 0.013 inch. They can be folded or combined to increase thickness if need be.

The other method is to use a longer strain screw. S&W has two frame styles (square butt and round butt), and the strain screws are different lengths. The square-butt strain screws are longer than the round-butt screws. My two guns have round-butt frames, so they are candidates for the longer square-butt screw. However, those screws’ lengths are too long for a round-butt frame, so they have to be shortened to work correctly.

The S&W strain screw has an 8x32 thread, so you can also use a setscrew. For reference, one full turn of an 8x32 screw is 0.031 inch. I measured trigger pull while adjusting the strain screw and found that a quarter-turn of the screw changed trigger pull weight by an average of a half-pound. This information might be useful if you use a setscrew for your strain screw. If you do use a setscrew, you’ll probably need to use a thread locker to hold it in place because unlike a strain screw, a setscrew can’t be tightened until it’s “tight,” and a thread locker will help prevent it from unscrewing on its own. But use only a medium-strength thread locker that can be removed with normal tools, such as Loctite 242 Blue. Don’t use a permanent thread locker because they require high heat to undo them, and it’s best to not expose your gun’s frame to that much heat.

I used the primer-as-a-spacer method with the reduced-power mainspring in the Model 65. The spent primer shim added 1.5 pounds to the double-action trigger pull. The shim helped, but it still wasn’t at 100 percent reliability with all primers. It set off 98 out of 100 CCI 500 primers, 99 out of 100 Remington 1½ primers, 99 out of 100 CCI 550 magnum primers, and 100 of 100 Winchester magnum primers.

According to many enthusiasts, .22 LR and .22 Magnum rimfire revolvers need a strong hammer strike to reliably ignite rimfire ammo. Rimfire rounds can be less reliable to fire than centerfire rounds, so reducing hammer-strike power might not be in your best interest. I suggest you consult a gunsmith before reducing the trigger pull of such guns.

The rebound spring, in addition to applying resistance to the trigger pull, also determines how quickly the trigger returns forward into battery. World-famous revolver speed shooter Jerry Miculek has perfected his skill by ensuring that the trigger returns to battery as fast as possible. If your goal is to shoot your S&W revolver very fast, make sure that whatever rebound spring you install will get the trigger back to battery before you stroke it for the next shot. That’s absolutely necessary if you want to shoot as fast as Jerry.

Also, you need to be fully aware of your purpose for using this gun. Casual shooting? Range gun? Competition? Duty and defense? If your revolver is for duty or self-defense, I suggest you keep the original full-powered springs in the gun. A lighter hammer strike risks a misfire. A lighter single-action trigger pull risks an accidental discharge. Many law enforcement duty revolvers were converted to double action for a reason—to disable the single action altogether because having your finger on a cocked trigger in a high-stress condition can easily lead to an accidental discharge. A long double-action trigger pull is more likely to require a deliberate action. But if your gun is intended for any other purpose—from casual shooting to competition—go for a lighter trigger pull.

I need to emphasize that the trigger pull weights in the graphs are specific to the individual springs I had and to my specific guns. The exact spring resistance will not be the same from one spring to another. Thus, my data is meant to be a guide and not a guaranteed pull weight data base. For example, I had two new Wolff Type 2 reduced-power mainsprings, and one had one less pound of pull resistance than the other. This might not seem significant, but it made a huge difference in the gun’s ability to set off primers. When installed in the Model 67, the weaker mainspring failed to ignite a single round of Federal primers, whereas the slightly stronger spring fired 67 percent of Federal primers. I found a similar difference in the pull weight between two standard-power Wolff Type 1 mainsprings.

A better trigger pull can be achieved easily by changing springs. My final setup with the Model 67 comprised Wolff’s regular-power Power Rib mainspring and a 14-pound rebound spring. It produces a very nice 8.1-pound double-action trigger pull and a 3.2-pound single-action trigger pull, which is a big improvement over the original weights.

Primer Sensitivity

Which pistol primers are the most sensitive? Which pistol primers are the least sensitive? This subject often comes up when shooters are adjusting the hammerspring or firing pin spring strength for a lighter trigger pull. Competitors are all about light trigger pulls because they feel better and make it easier for faster, controlled shots.

Double-action revolver shooters are especially keen on lightening the trigger pull for faster and smoother stroking and often lighten them as low as they can and still get reliable ignition. Federal primers are said to be the most sensitive and for that reason are the favorite of many wheelgun shooters. Many folks claim CCI primers are the least sensitive, with Winchester primers somewhere in the middle. Remington primers also tend to be somewhere in the middle.

Along with evaluating the reliability of ignition when changing a revolver’s mainspring and rebound spring as detailed in the main article, I conducted a test of the sensitivity of the various brands of primers using the S&W Model 67.

My procedure was to try 100 rounds of ammo with each of the four brands. I used standard, non-magnum Small Pistol primers, which are CCI 500, Federal 100, Remington 1½, and Winchester WSP. I fired the gun in double-action mode.

The mainspring was 99 percent reliable with Federal GM100M primers. (The Federal GM100M primers are the “match” version of Federal’s standard 100 Small Pistol primers. They are equally sensitive, but they are supposed to differ from the non-match version simply by producing a more consistent shot-to-shot ignition.) One misfire out of 100 means it’s right on the edge of reliability with the Federal primers. The other brands didn’t fare as well.

The brand in second place was the Winchester WSP. It was 77 percent reliable (23 failures out of 100 rounds). Next was the Remington 1½ primer with 64 percent reliability. The CCI 500 produced 54 percent reliability.

I don’t want readers to overgeneralize that all CCI, Federal, Remington, and Winchester primers will always show these same differences in sensitivity. There will always be variation in manufacturing tolerances that could affect sensitivity. And the results apply only to my gun with the specific springs used and only for the lot number of primers I tested. In addition, these results might not apply to Large Pistol (or any rifle) primers. But the results might serve as a guide for readers who want to do their own testing to see what primers are reliable in their guns if they are trying to reduce trigger pull weight.

Keep in mind that the numbers would change a little if the test were repeated. I would not be surprised if the ranking of primers that displayed similar failure rates could change places. For example, the Remington and CCI primers exhibit only a 10 percent difference in their failure rates, and it would take only a few different results from each brand to reverse their rankings.

All that said, there was a pretty big difference between the Federal primers and the nearest competitor: Winchester. The chance that a Federal primer would fail was one in 100. The chance a Winchester would fail was close to one in four. Now you can see why revolver shooters go with Federal primers when they reduce their revolver’s trigger pull.