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Do-It-Yourself Ruger SP101 Trigger Job

A smoother, lighter trigger pull is always welcome, and tackling the job yourself is a good way to learn more about your revolver.

Do-It-Yourself Ruger SP101 Trigger Job

(Photo courtesy of Brad Miller PhD)

Millions of shooters love their double-action revolvers, but they are more difficult to master because of the long trigger pull. If that trigger pull is heavy or rough, the task is even harder.

Trigger pulls on revolvers straight out of the box tend to be heavy and some aren’t very smooth. This usually improves over time as you put more rounds through the revolver, but there are limits. You can have a gunsmith lighten and smooth the trigger pull, or you can do it yourself. Here’s how you can use a D-I-Y kit to make a Ruger SP101’s trigger smoother and lighter.

My Ruger SP101 is in .327 Federal Magnum, and the action is typical of a production gun; it’s heavy, right at 12 pounds. It is fairly smooth, but it has room for improvement.

M*CARBO DIY Trigger Parts Kit
The M*CARBO do-it-yourself trigger parts kit includes springs, shims, and grease. (Photo courtesy of Brad Miller PhD)

M*CARBO makes the do-it-yourself action job kit that I used, and it has reduced-power hammer (9.5 pounds) and trigger (7.5 pounds) springs. The Ruger factory springs have a rating of 14 pounds for the hammer and 10 pounds for the trigger (according to Wolff Gunsprings). The M*CARBO kit includes four 0.005-inch hammer shims and four 0.001-inch trigger shims. This kit works for all SP101/GP100 revolvers in .357 Magnum, .38 Special, .327 Federal, and .22 LR, according to their website. The company has video instructions for Ruger novices (I’m one), and the company also has a polishing kit that includes polishing compound and polishing tips for a Dremel and fine 1000 grit sandpaper. I ordered the parts kit and the polishing kit. I already had fine sandpaper.

A trigger job comes down to two things. Trigger pull weight is reduced by using weaker springs. Action smoothness is achieved by polishing surfaces of moving parts.

M*CARBO Polish Kit and Dremel Tips
M*CARBO’s polish kit includes polish and Dremel tips. (Photo courtesy of Brad Miller PhD)

Some of these parts are under spring pressure, and unless you are very careful, moving something in the wrong way will let them to go flying. I did some disassembly and reassembly with the parts in a one-gallon plastic freezer bag. If some small part did get away from me, it remained in the bag.

All critical parts are polished with a fine polishing compound. You’ll need a 1/4-inch drill bit and fine sandpaper. Other tools you’ll need are needle-nose pliers, a flathead screwdriver, 1/16-inch punch, 3/32-inch punch, feeler gauge to 0.001 inch, and safety glasses. Polishing will produce fine dust, too, so wear a dust mask.

The video instruction on the website is a must-watch. You see everything performed from start to finish. The narrator gives a detailed and clear description of everything he does.

Trigger Assembly Parts to Polish
Trigger assembly parts to polish: 1. Trigger Return Spring Tunnel; 2. Transfer Bar; 3. Pawl; 4. Cylinder Latch; 5. Trigger (* = sear); and 6. Trigger Guard (Photo courtesy of Brad Miller PhD)

First Things First

Again, make sure to wear safety glasses at all times. Be sure the gun is not loaded. Follow the disassembly instructions in the user’s manual to capture the hammerspring and remove the hammer and trigger assembly.

Once the trigger assembly is removed, be very careful to not squeeze the trigger. This will move the pawl (which rotates the cylinder), and a spring and detent under the pawl will be exposed and go ballistic into never-ever land (never-ever going to find that part). You’ll need to press the trigger slightly to remove the transfer bar, but only slightly. The pawl will be removed, so be sure to capture the detent and spring.

There are too many parts to describe their individual disassembly here, which is why the video is especially useful. If you don’t have access to the internet, go very slowly in your disassembly of the trigger assembly because of parts that might not be visible and under spring pressure. Make notes, take pictures, so you’ll have a guide for reassembly.

Hammer Assembly Parts to Polish
Hammer assembly parts to polish: 7. Hammer (* = sear); 8. Hammer Dog; 9. Hammer Pin; and 10. Hammerstrut. (Photo courtesy of Brad Miller PhD)

Go Easy

Keep in mind this is a polishing job. You don’t want to remove too much metal. You just want to polish where these parts rub against each other to reduce friction.


There are 10 parts to be polished. This is done with the felt bullet-shaped Dremel bits and polishing compound. You can polish these parts by hand, too, but the Dremel speeds it up considerably.

The parts to polish are: 1.) Inside of trigger return spring tunnel. 2.) Transfer bar, the whole thing. 3.) Pawl, just the bottom flat. 4.) Top surface of cylinder latch. The cylinder latch can be removed, but it isn’t necessary to do so. The M*CARBO guide does not remove it for polishing. That’s fine, but don’t allow it to slide sideways very far—or else. It’s under spring pressure, so secure it down with a rubber band or tape. If it moves too far sideways, the spring and plunger will be off into never-ever land. 5.) Trigger. Polish the sides and lightly polish the sear. 6.) Trigger guard, the contact location where it rubs against the underside of the pawl, inside where the trigger pin/shims go. 7.) Hammer; sides, underside, and top of sear. Light polish on sear “teeth.” 8.) Hammer dog. 9.) Hammer pivot pin. 10.) Hammerstrut, the whole thing.

Transfer Bar Before and After Polishing
The transfer bar before (top) and after (bottom) polishing. (Photo courtesy of Brad Miller PhD)

The trigger return spring tunnel is cleaned up with a 1/4-inch drill bit. Do not put the drill bit in a drill. The bit should be handheld only. And be very careful to not push the drill bit too far into the tunnel. You don’t want to damage the lip at the end of the tunnel because it captures the plunger. If you damage it, you’ll have to buy a new trigger guard! Follow this with fine (1000 grit) sandpaper, which you can wrap around a 15/64-inch drill handle. Then, follow this with polish on a Q-tip.

I approached the hammerstrut differently than the video instructions. The hammerspring will only rub against the corner edges of the strut. Imagine a square peg in a round hole. Only the outer corners of the square peg will contact the round hole. Lightly round the edges with a file, then polish them. Polish the flats of the lower-most portion because the sides of the strut can rub on the bracket.

Surfaces to Polish Marked in Blue
Surfaces to polish on the trigger guard and cylinder latch are marked in blue. (Photo courtesy of Brad Miller PhD)


First, check how much side-to-side space there is in the trigger. Insert the trigger and trigger pin. Now measure how much space there is between the trigger and the housing with a feeler gauge. This will assess how many trigger shims you need to reduce the side-to-side movement. If you have 0.005 inch of space you could go with up to four shims, two on each side. Apply the silicon grease on them to keep them slippery. Then insert the trigger and the pin. I didn’t need the shims because my trigger had only 0.0015 inch of space.

Next is the pawl and pawl spring, then the transfer bar. The pawl spring pin is in an awkward place and pushing it in was awkward. Consider doing this in a plastic bag to contain the pawl spring/pin because it is under a lot of pressure and a little slip will have you chasing parts. I clamped the trigger in a vise so I could use both hands, which made this tricky reassembly much easier.

Measure with Feeler Gauge
Measure the gap between the trigger and trigger housing with a feeler gauge and use the provided shims if necessary. (Photo courtesy of Brad Miller PhD)

Next is the trigger spring assembly with the new, lighter trigger spring. Once that’s done, put the trigger assembly back in the frame.

Put the hammer in place with the hammer pin and measure the space between it and the frame with the feeler gauge. I could just squeeze a 0.023-inch gauge between the hammer and the frame. That meant I could use all four of the 0.005-inch shims in the kit, two on each side. These shims are inserted with the hammer in place. Pull out the hammer pin and push the shims in with a feeler gauge blade until they align with the hammer pin hole. First one side, then the other. Insert the hammer pin and you’re all set.

Last is the hammerspring assembly with the new, lighter hammerspring. That’s it, you’re done!

Handheld Drill Bit Removes Burrs
Use a handheld drill bit to remove burrs from the trigger return spring tunnel. Be careful to not damage the lip at the end of the tunnel (red arrow). (Photo courtesy of Brad Miller PhD)

Safety First

After full assembly, do a safety check to make sure the polished hammer and trigger sear edges are still safe by cocking the hammer single action then pushing on the hammer to see if it will push off. It shouldn’t. If it does, that means the sear edges have been rounded off and would have to be recut. This is one reason why you don’t want to remove metal; you want to just polish it.

How did my D-I-Y action polishing turn out? Well, it wasn’t as smooth as butter. There is a “stagy” feel to the trigger pull, but the staginess is not caused by hammer/trigger movement. It was smooth.

I tracked it down to the cylinder rotation in the window. The cylinder has a slightly jerky rotation when in the window, but not when it’s outside the window. Something cylinder-wise is rubbing unevenly on something, but I cannot determine what it is.

M*CARBO’s website claims a 50 percent reduction in trigger pull. On the video, the gun before the action job had a double-action pull around 10 pounds. After the trigger job, the double-action pull was around 4.5 pounds. That’s a huge improvement and an amazingly low double-action trigger pull weight. The website says that a stock double-action 10-pound trigger pull can be reduced down to 5.5 pounds, with average results varying up to +/- 1 pound.

My revolver’s starting double-action trigger pull of 12 pounds was reduced to 9.25 pounds (measured with a mechanical Timney Triggers gauge). My gun’s single-action pull went from 4.25 pounds to 3.5 pounds.

Feeler Gauge Also Used to Push Shims in Place
A feeler gauge is used to measure the space between the hammer and frame and to push the shims in place. (Photo courtesy of Brad Miller PhD)

My gun’s double-action trigger pull is not as light as I had hoped, but more importantly, it is reliable, consistently setting off primers. Ignition is related to hammerspring strength (and hammer weight). For this test, I used the lightest hammerspring I could find. Wolff Gunsprings’s lightest SP101 hammerspring weight is 9 pounds, which is slightly less than M*CARBO’s 9.5 pounds.

I tested ignition with pistol and rifle primers. I included rifle primers because the .327 Federal sometimes calls for rifle primers in load data. I tested CCI 500 (standard) and 550 (magnum) Small Pistol primers (count = 54 and 57 respectively), CCI 400 Small Rifle (count = 30), Winchester WSR Small Rifle (count = 40), and Federal 205 Small Rifle (count = 13). Of these 194 rounds, I experienced two misfires, a CCI 550 and a CCI 400. Both fired on the second attempt.

Wolff’s 9-pound hammerspring is on the edge of reliable ignition. The slightly stronger M*CARBO hammerspring should be more reliable. Wolff makes 10-pound and 12-pound hammersprings if there are still ignition issues. Whatever hammerspring you select, test it before you trust it!

M*CARBO’s kit and instructions make a do-it-yourself trigger job for the SP101 (and GP100) a practical task for the intrepid amateur gunsmith. A smoother, lighter trigger pull is always welcome.

The retail price for the trigger job kit is $29.95. The polishing kit runs $15.95. M*CARBO offers gunsmithing services and can install the parts for you. Parts and shipping are extra. They also will polish the parts. Whichever route you go, you’ll have a better trigger pull.

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