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General Shim Instructions
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Gun Shim Measuring and Installation Instructions
These instructions explain how to measure and install trigger shims, hammer shims, hammer dog shims, and cylinder endshake shims using a feeler gauge. The procedures were developed around Ruger Double Action Revolvers but apply to many firearms and firearm actions.
These General Measurement Instructions were developed using Ruger Double Action Revolvers including the GP100, SP101, Redhawk, Super Redhawk, Alaskan, Security Six, Speed Six, and Police Service Six. The same basic measurement principles apply to most firearms.
Trigger and Hammer measurements are generally performed in the same manner. In the video example below, I demonstrate how to measure Hammer Clearance for fitting and installing Hammer Shims. Trigger Shims are measured and installed using the same basic procedure.
Trigger Shimming
Use a Feeler Gauge to measure the total side play between one side of the Trigger and the Frame. In this example, the Trigger has approximately .009" of side play, although your firearm may vary.
Subtract .002" to .004" for the desired operating clearance and divide the remainder by two.
Example:
- .009" - .003" = .006"
- .006" ÷ 2 = .003"
Install two .003" shims, one on each side of the Trigger.
The shims create bearing surfaces that reduce friction, limit unwanted side movement, and help maintain more consistent Trigger-to-Hammer contact.
If Trigger-to-Frame clearance is less than .006", a single .002" or .003" shim installed on the left side of the Trigger can still provide benefits to a right-handed shooter.
After installing the shims, verify that the Trigger moves freely with no binding. I do not recommend reducing Trigger clearance below .002".
Hammer Shimming
With the Hammer Main Spring removed to relieve tension, measure the total clearance between one side of the Hammer and the Frame.
Unlike the Trigger, the Hammer will usually have slightly more side clearance. This allows the Hammer to contact and drag against the Frame.
Inspect both sides of the Hammer. Arc-shaped wear marks indicate that the Hammer has been rubbing the Frame.
Hammer drag reduces Hammer speed and energy, and in extreme cases may contribute to light primer strikes.
Installing Hammer Shims reduces side play, creates bearing surfaces, and helps prevent the Hammer from contacting the Frame.
Use the same formula described for Trigger Shimming.
Example:
- .014" - .004" = .010"
- .010" ÷ 2 = .005"
Install two .005" shims, one on each side of the Hammer.
The easiest installation method is to install the Hammer first, then slide the Shim down the side of the Hammer. A Feeler Gauge blade can be used to help guide the Shim into position.
After the Hammer Pivot Pin is installed, verify that the Hammer moves freely and does not bind. I do not recommend reducing Hammer clearance below .003", however .005" ending clearance is more practical.
Ruger Double Action Hammer Dog Shimming
Shimming a loose Hammer Dog can improve Double Action Trigger Pull consistency.
Measure the total clearance with a Feeler Gauge and maintain a minimum of .002" operating clearance after installing shims.
Hammer Dog Shims are available in thicknesses from .002" to .010".
I do not recommend reducing Hammer Dog clearance below .002".
Barrel-Cylinder Gap and Endshake
The gap between the rear of the Barrel and the front of the Cylinder is known as the Barrel-Cylinder (B-C) Gap. The front-to-back movement of the Cylinder is called Endshake.
A Feeler Gauge is the easiest and most accurate tool for measuring these clearances.
To measure Barrel-Cylinder Gap:
- Hold the Cylinder fully rearward.
- Insert the thickest Feeler Gauge blade that will fit between the Barrel and Cylinder.
- Record this measurement.
For most Ruger Double Action Revolvers, the Barrel-Cylinder Gap should generally fall between .004" and .008" but can be as much as .012"
Next:
- Push the Cylinder fully forward.
- Measure the Barrel-Cylinder Gap again.
- Record this second measurement.
The difference between the two measurements is the amount of Cylinder Endshake.
Most Ruger Double Action Revolvers should have approximately .001" to .004" of Endshake, although specifications may vary among manufacturers.
TriggerShims Cylinder Shims can be used to adjust both Barrel-Cylinder Gap and Endshake.
Available shim thicknesses include:
.002", .003", .004", .005", .006", and some models available .007", .008", .009" and .010".
v.1t Item # FG161D
TECH TOOL TIP: USA Feeler Gauge
The easiest way to Measure your Trigger or Hammer assembly or your Cylinder Endshake is with a simple blade type feeler gauge.
I sell a Nice U.S.A. Made set you can include in your orders.
It comes with a Manufacturers Lifetime Warranty
This 32 Blade Gauge will measure from .0015" up and is perfect for general use.
Price $18.95 includes Free Shipping and Tracking / Insurance
~ USA Only ~
~ Sorry No International Exports ~
Why Use a Feeler Gauge?
A feeler gauge directly measures the actual clearance between parts, making it the preferred tool for measuring Trigger, Hammer, and Cylinder Endshake gaps. Knowing the exact clearance helps you select the correct shim thickness the first time.
Bolt Shim Instructions
LANCE'S TECH TIP:
Rimfire Bolt Shim Installation
~ A General Guide ~
1. Understanding Rimfire Headspace
Unlike centerfire cartridges where headspace is measured from the shoulder, the .22 LR is rim-headspaced. The total dimension from the recessed bolt face to the barrel breech face determines how much "play" or clearance the cartridge rim has.
Rimmed cartridges utilize the thickness of the cartridge rim for head-spacing. A two-piece bolt requires a precise fit to hold the cartridge rim closely against the breech without crushing it. Installing our Bolt Shims between the two bolt halves eliminates excessive internal slop and rearward endplay, creating a solid mechanical setup that maximizes firing pin energy transfer to eliminate misfires and light strikes while enhancing accuracy.
Generally speaking, factory production rifles leave extra room between the bolt face and the breech to accommodate a wide variety of ammunition with varying rim thicknesses. You should first measure the depth of your bolt face pocket to ensure it meets SAAMI headspace minimum depth. Shimming between the two bolt halves ensures the bolt is seating the cartridge up gently against the breech with your chosen brand of ammunition.
2. How to Fit and Measure Your Shims
You can use our color-coded numbered shims like a gauge to find exactly when the bolt face touches the barrel breech face. From there, you can calculate your ideal bolt headspace depth based on your preferred ammunition's rim thickness.
- Fit by Feel: I cannot tell you which specific shim will work best with your rifle; you determine that yourself by fitting, feel, and live-fire testing.
- Measure in the Rifle: Installing Bolt Shims requires fitting by hand and checking with the bolt fully installed and locked down in the rifle action. Do not attempt to measure this with the bolt removed from the rifle.
- Use an Empty Chamber: To establish the proper bolt-to-breech fit, shims must always be fitted on a completely empty chamber.
- Extractor Tip: If possible, removing the extractors on some rifle models during the fitting process will give you a much better "feel" for the bolt face making contact.
- Finding the Stack: Start with a 4 or 5-pack of shims. Install the thinnest shim (.0015" or .002") and check how your bolt closes. If it closes normally, continue trying sequentially thicker shims (or stacking them if necessary) until the bolt handle begins to close harder than normal.
- Identifying the Slack: The exact point where your bolt begins to close hard indicates the total amount of mechanical slack that was present in your bolt layout.
3. Fine-Tuning the Fit
- Step Back One Number: Once you find the thickness that causes the bolt handle to close tight, remove that setup and step back by .001". For example, if a .005" Red Shim causes the bolt handle to close harder than normal, remove it and install a .004" Purple Shim.
- Avoid Undue Stress: If you have to force or put undue stress on your bolt to close it, you must reduce the shim thickness. Forcing the bolt closed puts premature wear on the shims and will not increase accuracy.
- Target Feel & Final Calculation: When you have the correct shim thickness installed, your bolt should close normally or just ever-so-slightly tighter than normal. At this point, you have successfully used the shims to tune the space available for your cartridge rim. By comparing your factory bolt pocket depth to your ammunition's rim thickness, you can determine exactly how much shim thickness to leave inside the bolt assembly to adjust your excess headspace.
Example:
- Your factory Bolt Pocket Depth measures a tight .038".
- Your favorite ammunition has a Rim Thickness of .041" (meaning the cartridge rim is physically .003" thicker than the factory pocket depth).
- By testing, you find it takes a .005" shim to completely eliminate internal slack and bring the bolt face up flush to the breech face.
- Because you have a .038" bolt pocket and a .005" shim to touch the breech, backing up by just .002" leaves a .003" shim inside your bolt assembly to completely address your headspace issue.
- Because of slight variations in rim thickness, it is better to be slightly loose rather than a slight crush, so trying the .003" shim and .002" shim for best fit is recommended in this example.
- Lubrication: For the longest shim life, apply a very small drop of oil or a tiny smear of grease to the shim before final installation.
4. Range Testing & Maintenance
Now you can go to the range. With the barrel pointed in a safe direction, check the fit with your favorite ammunition.
Due to natural variations in rim thickness across different ammunition brands, you may occasionally need to adjust your shim stack. Remember, you have already removed the excess mechanical headspace from the bolt assembly; your goal now is simply to fine-tune the clearance to find the "sweet spot" for your preferred cartridge rim thickness.
If you ever have safety questions or want to verify your dimensions, always use a dedicated rimfire headspace gauge to check your work.
Consistent chambering yields consistent results. Your satisfaction is guaranteed. If you feel you receive no benefit, you may send your shims back to TriggerShims for a full refund.
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TriggerShims Bolt Shim Instructions Copyright 2008 - 2026 V9-10/8/2026
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