Firing Pin Stop

Wayne Shaw

Wayne Shaw

Active member
When a firing pin hits the primer, does it push into the primer to the point the pin bottoms out? (.050"-.055") Or does it hit and the primer is tough enough to hold it back to a degree?
 
I believe that the primer stops the pin, but that in the case of the "warmish" loads, that are typical in Benchrest, that the depth of the indent may be pushed back out a little by the peak pressure after ignition. A friend, who is retired from the firearms industry, stets the firing pin protrusion of his Savage firing pins (adjustable on Savage bolt actions) at .035, and does not have any ignition problems. I guess that one test that could be done would be to reprime a fired (to minimize longitudinal clearance in the chamber) and fire it, with no powder or bullet, and compare the depth of the indent to that of a fired case from a previously loaded round.
 
A small rifle primer measures from about 0.108 to 0.113 inches deep. The cup thickness ranges from about 0.019 to 0.025 inches.

A large rifle primer measures about 0.117 to 0.121 inches deep, with cups running about 0.027 inches thick.

The anvil is probably around 0.010 inches thick, I don't know, I've never measured one to find out. Plus, the legs of the anvil are bent up to trap the priming compound.

A firing pin that bottomed with all the parts crushed into the bottom of the primer pocket in the absence of chamber pressure would have a large protrusion, but maybe not as great as first impressions from looking at the cup depth only might imply. Consider an "average" SRP with a depth of 0.110, thickness of 0.022, and anvil of 0.010 inches; 0.110 - 0.022 - 0.010 = 0.078 inches crush depth, and if the anvils are 0.020 inches thick, that changes to 0.068 inches. So a "conventional" 0.060 inch firing pin protrusion seems like a good place to start, leaving a little space and forcing the anvil to do its job.

I expect this (the benchrest) crowd is more interested in using no more protrusion and firing pin travel than absolutely necessary to obtain sufficiently reliable ignition.

Now I have to measure a few anvils and the depth from the open end of the primer cup to the exposed side of the anvil.
 
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Wayne, the pin bottoms out on its mechanical stop on most centerfire bolts. What it does after that depends on the pressure on the other side.
 
OK, What should bottom out? Should it be the major diameter of the pin that would stop at the "bottom" of the bolt body, or should it be the striker end hitting the end/bottom of the cocking ramp?
 
The fwd surface of the f-pin shoulder is the stop & acts as a gas check in case of a blanked/pierced primer.

The striker/cocking finger is the f-pin stop on a 40XR rim fire f-pin assy.

I've toyed w/alum/brass/delrin/ Oring as stops to cushion f-pin assy's-
I'm back to steel on steel.
Lighten & Time the pin & verify protrusion.

MIM'd/sintered aftermarket strikers will break when least expected.
 
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So, on a typical BR centerfire action, the pin should bottom out on the inside bolt body surface? Hmmmm.....
 
On factory actions, one thing that comes up from time to time is that the bolt body is out of time with the bolt handle, causing the cocking piece to drag on the opposite side of the cocking cam cut, and hit the radius where the cut terminates at the bottom of the pin fall. This can cause the bolt handle to jump. You can see that this is the cause by picking up the bolt handle a little and then dry firing with it in that position. The fellow that designed the cut on my custom action's bolt avoided the problem of this sort of drag by moving the straight side of the cam cut over to create clearance, since the shroud is guiding the cocking piece.
 
Wayne Shaw said:
So, on a typical BR centerfire action, the pin should bottom out on the inside bolt body surface? Hmmmm.....
Click to expand...
For example, in the Remongton/Panda/etc bolt design the firing pin has a shoulder just back of where the tapered "pin" part ends. This shoulder on the pin bottoms out on a shoulder in the bolt body. That is why it is important to keep the bolt body washed out.

Have you never blown a primer then the next round would fail to fire, then you pull the pin/spring assembly and dump that little disc out?
 
As stated, the firing pin should stop because it hits inside the bolt body and the cocking piece should NOT bottom out. On a 700 the pin has a larger diameter stop around an inch from the tip that hits inside the bolt body and on a Savage the firing pin stop, which is the adjustable piece that set protrusion, hits the bolt head. The pressure generated by firing partially drives the dent in the primer cup back out. The mass and spring pressure of the firing pin assembly keeps the primer from blowing, if the firing pin magically disappeared, the primer would blow instantly. Sometimes it happens when people dick around with their firin gpin springs to improve bolt lift. The dents you see are never as deep as your protrusion, UNLESS you have a misfire. Ever notice they are very deep on a misfire? When the firing pin is a loose fit, primer cup material forms a crater look as it molds around the pin and tries to enter the hole. The radius of the firing pin tip influences how the brass flows. A small radius, aka sharp tip, allows the brass to thin at that sharp point and more material is available to move further along the pin and enter the hole. If the tip has a large radius, the brass forms more evenly around it instead of thinning out at that point and as a result there is less material sliding past the tip.
 
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JerrySharrett said:
For example, in the Remongton/Panda/etc bolt design the firing pin has a shoulder just back of where the tapered "pin" part ends. This shoulder on the pin bottoms out on a shoulder in the bolt body. That is why it is important to keep the bolt body washed out.

Have you never blown a primer then the next round would fail to fire, then you pull the pin/spring assembly and dump that little disc out?
Click to expand...

You bet I have. I've sent you a PM.
 
Some bottom on CP

Two designs exist--most bottom inside the bolt at front of firing pin stop-but they do not stop there when rifle is fired. The other design (that Farley used to use) was to have Cocking piece bottom out in bottom of cocking cam.

But in either case--the firing pin does not actually bottom out when rifle is fired. Bob Greenleaf, Bill Davis and others did experiments to determine that.

Jim
 
Jim Borden said:
Two designs exist--most bottom inside the bolt at front of firing pin stop-but they do not stop there when rifle is fired. The other design (that Farley used to use) was to have Cocking piece bottom out in bottom of cocking cam.

But in either case--the firing pin does not actually bottom out when rifle is fired. Bob Greenleaf, Bill Davis and others did experiments to determine that.

Jim
Click to expand...

But don't most of these Rem/etc stop on that shoulder if the chamber is empty?
 
Yes

Jerry

Yes the Remington design (Borden, Stolle, BAT, Stiller) stop inside front of bolt on empty chamber--however, the Farley's I have had here do not--they have stopped on the CP

Jim
 
Jim Borden said:
Two designs exist--most bottom inside the bolt at front of firing pin stop-but they do not stop there when rifle is fired. The other design (that Farley used to use) was to have Cocking piece bottom out in bottom of cocking cam.

But in either case--the firing pin does not actually bottom out when rifle is fired. Bob Greenleaf, Bill Davis and others did experiments to determine that.

Jim
Click to expand...
I can tell you with absolute certainty that, with my protrusions set at .035" - .040", the firing pin hits the bolt when fired on a chargeless and projectiless case. If the round was loaded, I would think it does as well but perhaps the explosion is so fast it reverses the firing pin travel before it hits home.

Since we are on the topic and Mr Greenleaf came up, I thought of this tidbit. If my recollection and highly regarded source is correct, Bob Greenleaf stated that his action design, the Savage 110, required 22lbs of firing spring pressure to ensure consistant ignition.
 
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