TC Encore Headspace How-To

Thompson Center Encore Headspace Measurement and Correction

and Bullet Seating Depth


Getting Started Measuring

The Thompson Center Encore is a break action single shot rifle or pistol. It features easily changed barrels for many numerous chambering options. However due to tolerance stacking in the machined surfaces of the barrel and frame assembly you can run into trouble with reliability and accuracy.

One of the corrective actions you can take it to accurately measure and adjust your headspace to help in reliable ignition, accuracy and optimal brass life. Due to the break open design and the inherent flex in the action when fired with more flex in higher pressure rounds, you cannot merely take a rimless bottleneck cartridge or even rimmed bottleneck cartridge and measure it as you would a bolt action gun. This is because as we will show below the case can actually grow larger than the space provided under flex. This over sized case would be difficult to chamber and result in differing barrel lockup tension and could hurt reliability especially in the field and even accuracy.

To measure you will need a depth Micrometer w/adapter and Feeler Gauges.

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Depth Micrometer with Bellm Adapter
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Depth Micrometer  with Bellm Adapter and Tip Removed

You should ideally Remove the extractor from the barrel for most precise measurement.

With a fired case in the chamber we can see it protrudes from the barrel Chamber 0.0012″.

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Protrusion of Fired Case

The unfired case in this instance measured a depth of 0.005-6″ into the Barrel.

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Depth to Unfired Case

The next measurement is the barrel to frame gap. This is actually the Barrel to Firing Pin Bushing. Firing pin Bushing is show in great detail below as this is also our correction point. In this case the 0.002″ was free to move but 0.0025″ locked in place. so 0.002″ is a good measurement.

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Measring Barrel to Frame Gap

Calculating Headspace

OK, now we are here. All you have to do is take the unfired depth or protrusion and subtract it from the Barrel to frame gap.  So our unfired brass has a protrusion measurement of -0.005″ while the barrel to frame gap was 0.002″

0.002″- (-0.005″) = 0.007″ of actual headspace.

This is in excess of the SAAMI maximum allowable of 0.006″. Ideal is 0.001″

To correct this we would  have several options. If this was your ammo of choice and you did not reload (I know we are The Reloaders Network, that would be heresy) it would actually behoove you to have 0.005″ faced off the end of the barrel. This larger gap would actually not change headspace at all as the bottom of the case would not have moved in relation to the Firing Pin Bushing, but would allow space to shim the Firing Pin Bushing forward to the brass 0.006″ to provide an ideal headsace of 0.001″.   0.007″-0.006″=0.001″.

Now as a reloader the better option would be to take the fired brass which is 0.012″ protrusion which is just a bit bigger than the 0.002″ between the barrel and frame, and then resize the brass till you get 0.001″ protrusion on the depth Mic. That as you can now see would leave you 0.002″-0.001″=0.001″ or ideal headspace. save this setting on your die and you have you setting for this frame and barrel combination.

To install the Shims behind the Firing Pin Bushing to adjust headspace you need to measure and make sure you have the right one(s) to make up the measurement you need and cut away with scissors the section to allow the Machine Allen Mounting Screw to reattach the bushing to the frame. They actually cut easily with sharp scissors.

To better understand what the firing pin bushing is and where it is the series of pictures below should help clarify.

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Firing Pin Bushing in Frame

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Firing Pin Bushing Removed from Frame


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Firing Pin Bushing, Firing Pin, Firing Pin Return Spring, Mounting Machine Allen Screw

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Firing Pin Hammer Side in Frame


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Firing Pin Bushing Kit

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Encore Firing Pin Bushing Shims


Bullet Seating Depth

No pictures provided but simply drop the bullet into the barrel throat and use the depth gauge from your calipers to measure to the back of the bullet to the barrel breach face (Chamber opening). Measure that Bullet length. COAL for that bullet to the lands is simply the Bullet Depth+Length of Bullet+Cartridge Protrusion. Subtract the jump you would like and you now have a Cartridge Overall Length… For consistency cartridge to cartridge due to bullet variance. You can take the Cartridge Base to Ogive like you would for any other gun. Now duplicate.

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