My double-set-trigger TCR-83 had a problem so I sent it in to Thompson-Center/Smith & Wesson for repair. After getting it back I went to the range to sight in an essentially new .270 Winchester barrel on which I had just mounted a scope sight. Most of my hand loaded cartridges (with new brass) would not fire. The primers had dents in them but not as deep as I would have expected. My first thought was that the hammer spring had weakened or something was interfering with its fall. Then I got to thinking about how flattened the primers were on the cartridges that fired and wondered if there was a headspace problem. I had chronographed the loads, knew velocity was reasonable, and so pressure should also be reasonable.[size=78%] [/size]
I removed the firing pin bushing to see if there was a shim/washer behind it and there was not. I have occasionally removed TCR firing pin bushings because they get clogged up with material and interfere with movement of the firing pin. I was sure there was previously one for this receiver. I assume that the washer/shims were used in order to adjust for proper headspace of the actions upon their manufacture. That is, it would be difficult to manufacture them with an absolutely consistent distance between the hinge pin and breech face. By making that distance slightly large and using a firing pin bushing with washer/shims of varying thicknesses the effective distance between hinge pin and breach face could be made very consistent. Consistency in this dimension of the receivers would be very desirable as the interchangeable barrels would also have some manufacturing inconsistencies.[size=78%] [/size]
Excessive distance between the hinge pin and breech face would result in excessive head space but an insufficient distance (or a too long cartridge like sometimes when they are not full-length resized) prevents the action from closing all the way. The TCR actions must have some safety mechanism to sense this, for when that happens, the hammer will not fall when the trigger is pulled. I don’t have either a Contender or Encore so don’t know if the operate like this or even if the use firing pin bushings, but that’s how TCR’s work. I have four other TCR’s, all 87’s. One has a 0.003-inch thick washer/shim behind the firing pin bushing. Another does not have one. I could not check the other two, because with one, I could not get the firing pin bushing removed and the other is at a gunsmith getting fitted to a custom stock.[size=78%] [/size]
I had a barrel custom made for this action in .17 Ackley Hornet. The chamber was made very shallow and it would only fire on cartridges made with Remington brass (and only with this action). My other actions had insufficient hinge pin to breech face distance to allow this barrel to close sufficiently to let the hammer drop. Even with the TCR-83 receiver, the rims of cartridges made with Winchester brass were too thick to allow the action to close sufficiently for the firing pin to drop. After repair (and without a firing pin shim/washer) it would fire with cartridges made with Winchester brass with their thicker rims. To me, this was more proof that the headspace increased because the washer/shim was not re‑installed.[size=78%] [/size]
Thompson-Center was willing to fix the problem using the factory .270 Winchester barrel to determine the proper washer/shim thickness, but I was concerned that they might put in a thicker washer/shim then was previously behind the firing pin bushing and I would no longer be able to use my custom .17 Hornet barrel in that receiver. They only work on Thompson-Center factory or Custom Shop stuff. Anyway, I decided to get some shim/washers and fix it myself. Luckily I have a next door neighbor who works at a tool & die/machine shop and he made me a bunch of washer/shims in various thicknesses (0.001, 0.0025, 0.00035, 0.0005, 0.0010 and 0.015-inch).[size=78%] [/size]
The results of going to the range and trying out the shim washers were this: First, a 0.005-inch washer/shim would work for my custom .17 Hornet barrel, but a 0.005-inch washer/shim plus a 0.0035-inch washer/shim (total 0.0085-inch) would not work. That is, 0.0085-inch would not allow the action to close sufficiently to let the hammer fall with that barrel. Second, a 0.010-inch washer/shim would work for the factory .270 Winchester barrel (and all cartridges fired), but a 0.010-inch washer/shim plus a 0.0035-inch washer/shim (total 0.0135-inch) would not work. Again, 0.0135-inch would not allow the action to close sufficiently to let the hammer fall with that barrel. Third, a 0.010-inch washer/shim would work for a custom 6mm-06 barrel, but a 0.010-inch washer/shim plus a 0.0035-inch washer/shim (total 0.0135-inch) would not work. And again, 0.0135-inch would not allow the action to close sufficiently to let the hammer fall with that barrel.[size=78%] [/size]
The success of this story is that I will now use a 0.005-inch washer shim when using the .17 Hornet barrel and when using other barrels will use a 0.010-inch washer shim to minimize headspace. Also, I will experiment with my other TCR’s and see if I can put thicker washer/shims in them and reduce headspace a little. I was there when my gunsmith fabricated the custom 6mm-06 barrel, and know he reamed the chamber very gradually until the hammer would drop (in one of my TCR-87’s and cartridges would fire in it. I would guess the chamber is pretty shallow within the barrel, so there would not be excessive headspace with that barrel, the TCR-83 receiver and the 0.010-inch washer/shim. I am sure the reason the hammer does not fall with the .17 Hornet barrel with a total washer/shim thickness of 0.0085-inch is because of contact between the breech face and the cartridge. That is, the chamber was reamed so shallow that the cartridge case is not fully inside the barrel. Previously not firing cartridges with Winchester brass with their thicker rims was an indication of this.[size=78%] [/size]
However, I wonder about the factory .270 Winchester barrel. I guess the reason the hammer does not fall with a total washer/shim thickness of 0.0135-inch is that breech face/barrel contact will not allow the barrel to fully close. If the chamber was so shallow within the barrel that the back of the cartridges were even with the back of the barrel that would mean less than 0.0035-inch headspace with the 0.010 washer/shim in place, but I suspect the .270 Winchester chamber was reamed deeper into the barrel. That is, a missing 0.005-inch or 0.010-inch washer shim would only reduce the firing pin travel into the primer by that amount, which I think would be very minimal compared to how far the firing pin should indent the cartridge. I am thinking the chamber was reamed fairly deep and even with a washer/shim present the firing pin may just barely set off primers.
