Firing pin fall --a discussion

[Mulligan]

With all the research Mr. Walker did while developing the 700 ignition system and, what we have learned since then, how much firing pin fall is the bare bones minimum for an extreme accuracy rig? Is there a measurement of firing pin fall, that most agree is the minimum? Going off memory, I believe Jim Kelbly wrote in a thread on Benchrest Central that 0.240" was the absolute minimum. When I called and ask the same question, I got the same answer from Jim. At that time, I had a Panda that had about 0.220" fall and I asked Jim what trigger hanger he recommended, he said, " 0.030", that will get you to 0.250", remember, 0.240" is the minimum". I did put the new trigger hanger in the rifle and it has shot smaller aggs ever since. Yes, I did end up with a little more cock-on-close. The added cock-on-close isn't really noticeable and does not cause me difficulties in tracking or rocking in the bags, that I can see.

We have trigger hangers from several actions companies and aftermarket venders that help with the adjustment fore and aft of the trigger in relation to it's assigned place in the action. All of which moves where the cocking piece sits in relation to it's distance from the fired position. This movement either is adding to or subtracting from the amount of cock-on-close the system has. Cock-on-close is many times discussed as a negative. Can cock-on-close be a good thing? To what extent do we go to remove cock-on-close?

I assume, a mill and a bushel basket full of intestinal fortitude, are at least a couple of the items required to actually tune up the bolt body correctly if one wants to remove all cock-on-close on an action that has this infliction?

Thanks for your time.
CW

 

[WSnyder]

When I called and ask the same question, I got the same answer from Jim.

Years ago I had the same conversation with him. This was way back early in BR Central days and early discussions of ignition were high. There were lots of differing opinions (most were speculation with nothing to back it up) so I chose to call and ask the leading BR action manufacturer of the day and ended up with Jim. He very graciously went through the Walker/Remington history and why they (Kelbly's) made their actions like they do. That information has held up pretty well over the years.

Speaking towards Rem 700 and clones as to cock on close. My opinion on whether to take the time and expense to remove it or not depends on the application. Is this a hunting rifle or BR style gun shot off bags? If it's a hunting rifle shot off a bipod or pack or offhand in the field I see no reason to spend the extra money or time to remove cock on close unless you just want to. Some actions and combinations of parts are easier to remove cock on close so factor that in. Most 700 style bolts will need the cocking helix modified to be able to remove all cock on close. If you replace the bolt with a PTG it's helix is slightly deeper than the factory bolt and you can remove the cock on close without modifying it. So now you're ahead on time but just spent more money. If you go with say a BAT action they are easily modified to gain pin fall and have enough room (with most triggers) to avoid cock on close while maximizing pin fall.

 

[Walt Krafft]

I assume the amount of firing pin fall is based on an ideal amount of firing pin protrusion from the bolt face. What is this dimension.

 

[riflewoman]

What you’re dealing with is dynamic acceleration using springs. There is a minimum energy that primers need to fire. That energy has to come from the mass of the firing pin and the resulting energy transferred to the firing pin. The spring has to fit inside the bolt and outside the firing pin. In a typical bolt design, the optimal striker fall appears to be about 1/4 inch. It can certainly be made smaller, but that comes at a cost of a spring becoming much larger. This forces the bolt diameter to be larger but there are limits like overall weight of the action/rifle come into play.

 

[Pawnee Bill]

Isn't firing pin fall and "lock time" the same thing?

 

[Mulligan]

Isn't firing pin fall and "lock time" the same thing?

Nope,

Pin fall is the measured distance the firing pin moves forward on an empty chamber.
Lock Time is the amount of time between the trigger pull and the primer strike.

CW

 

[DickE]

How and what do you do to the cocking helix?

 

[BoydAllen]

With all the research Mr. Walker did while developing the 700 ignition system and, what we have learned since then, how much firing pin fall is the bare bones minimum for an extreme accuracy rig? Is there a measurement of firing pin fall, that most agree is the minimum? Going off memory, I believe Jim Kelbly wrote in a thread on Benchrest Central that 0.240" was the absolute minimum. When I called and ask the same question, I got the same answer from Jim. At that time, I had a Panda that had about 0.220" fall and I asked Jim what trigger hanger he recommended, he said, " 0.030", that will get you to 0.250", remember, 0.240" is the minimum". I did put the new trigger hanger in the rifle and it has shot smaller aggs ever since. Yes, I did end up with a little more cock-on-close. The added cock-on-close isn't really noticeable and does not cause me difficulties in tracking or rocking in the bags, that I can see.

We have trigger hangers from several actions companies and aftermarket venders that help with the adjustment fore and aft of the trigger in relation to it's assigned place in the action. All of which moves where the cocking piece sits in relation to it's distance from the fired position. This movement either is adding to or subtracting from the amount of cock-on-close the system has. Cock-on-close is many times discussed as a negative. Can cock-on-close be a good thing? To what extent do we go to remove cock-on-close?

I assume, a mill and a bushel basket full of intestinal fortitude, are at least a couple of the items required to actually tune up the bolt body correctly if one wants to remove all cock-on-close on an action that has this infliction?

Thanks for your time.
CW

If you make the cocking notch deeper, you will have to modify the FP to have more fall. If the CP is pinned. There are a coupe of ways that you may be Abe to use to pick up a few thousandths.

 

[Mulligan]

I assume the amount of firing pin fall is based on an ideal amount of firing pin protrusion from the bolt face. What is this dimension.

Walt,
I am not sure what the magic number is for firing pin protrusion, I THINK it is in the neighborhood of .035-.055", but that is no more reliable than a rumor told the 5th time. I am hoping folks that know will help us out here.

The firing pin fall is not based on pin protrusion. Adjusting pin protrusion can impact pin fall, but I am not sure if it works the other way around?
CW

 

[jackieschmidt]

The best shooting Rifle I own is my 30BR Farley VFS that I modified the trigger hanger to allow .250 firing pin fall. And 24 lbs of static spring pressure.

I figure 15% of this is “cock on close”.

Shooters get so ate up on that “flip the bolt open and close” that they forget the rifle doesn’t agg worth a darn.

Years a very good friend of mine had a brand new Farley 6PPC built. It was slick. He marveled at how smooth and with ease the bolt opened and closed.

The darned thing would’ve shoot much better than .300 groups.

We took it to my shop, I offset bushed the trigger hanger so it had .230 inch travel. I made a spacer to get the spring pressure up to 23 pounds.

Yes it had a small amount of “cock on close”. The very first group he shot after the modification was about a .150. He won quite a few plaques with that rifle.

Kelbly has it right.

 

[BoydAllen]

Firing pin fall is measured on an empty chamber. The front of the firing pin flange is what stops the pin fall when dry firing on an empty chamber. With a Remington style striker assembly, I believe that one would have to remove material from the front of the flange in order for the pin to fall farther. This would also require modifying the pin tip for a couple of reasons. First of all the taper behind the tip diameter might contact the bolt, keeping the pin from falling to the point where the front of the flange contacted the inside of the bolt. Secondly the increased fall would increase pin protrusion in direct proportion to the increase in FP fall. The usual standard for pin protrusion is about .055 but when the pin falls on a live primer in a properly sized case it is stopped by the primer, well short of that. You can make your own measurement on a Remington by comparing how far down in the shroud the back of the cocking piece is after dry firing, and comparing it to how far down it is after firing a live primer. With that difference and the actual measured FP protrusion, you can see how much of the excess protrusion might be, keeping in mind that you do not want the front of the flange to bottom when actually firing a round. Years ago Bob Greenleaf (retired engineer from Savage) toldme that he set his personal rifles for a FP protrusion of .035. Bob was with the company for 25 years. For those who are not familiar the model 110s had adjustable FP prutrusion. To get a better understanding of the whole protrusion thing, Imagine a pin that had a tip that was so long that it was flush with the bolt face with the action cocked, which would make the protrusion equal to the fall. In that case, the pin would only be able to fall the distance to the primer from the bolt face plus the depth to which it deformed the cup. You need to use an empty case for these tests because I believe that in the case of a hot load the primer dent may be pushed back a bit. from where it would be if the primer alone was fired. Excess protrusion actually reduces the effective pin fall and reduces the energy delivered to the primer.

 

[Alex Wheeler]

I would agree, 240-250 is where I want to be. Whether you do it with or without cock on close doesnt matter. You can design it either way based on your preferences in how you want an action to run.

 

[Ledd Slinger]

I personally despise cock on close. Just my preference.

There is a way to get more pin fall in a 700 that does not require any internal bolt body modification to let the pin to fall farther forward. The modification results in the pin being cocked farther rearward. It’s a modification that needs to be done on a milling machine with careful measurements and quality tooling.

The main problem with Remington 700s is the large amount of inconsistency in the bolt and receiver trigger mount machining. With one 700 you might be able to get really close to .240” pin fall doing the modification after timing a custom trigger for no cock on close (.238” is my best and the rifle shoots lights out). Then the next 700 you take measurements on, you find it won’t have a snowball’s chance in hell of getting more than .215” of pin fall at very best after timing a trigger and modifying the pin fall. In that case you are better off not timing the trigger and just dealing with cock on close to maintain better pin fall.

 

[Mulligan]

I had about .210”pin fall on a BR action….. no cock-on-close. By adding a .030” trigger hanger to the system I gained some cock-on-close (not the full 30, but some) and I gained a lot more reliable ignition system.

I think a reliable ignition has got to be the first priority?
Cock-on-close sucks, no doubt about it. Especially when you have actions that a good smith has worked their magic on to compare to. However, a small amount of cock-on-close isn’t hurting anything on the above action and the aggs are smaller because of better ignition.

What percentage of smiths around the nation know how to properly address acquiring at least the minimum pin fall and little to no cock-on-close?

I am not trying to beat up on anyone here, just shining a light in a dark place and furthering my understanding of the ignition system.

CW

 

[jackieschmidt]

I personally despise cock on close. Just my preference.

There is a way to get more pin fall in a 700 that does not require any internal bolt body modification to let the pin to fall farther forward. The modification results in the pin being cocked farther rearward. It’s a modification that needs to be done on a milling machine with careful measurements and quality tooling.

The main problem with Remington 700s is the large amount of inconsistency in the bolt and receiver trigger mount machining. With one 700 you might be able to get really close to .240” pin fall doing the modification after timing a custom trigger for no cock on close (.238” is my best and the rifle shoots lights out). Then the next 700 you take measurements on, you find it won’t have a snowball’s chance in hell of getting more than .215” of pin fall at very best after timing a trigger and modifying the pin fall. In that case you are better off not timing the trigger and just dealing with cock on close to maintain better pin fall.

As I said in my post #10, I have done it on custom actions by off set bushing the pin holes in the trigger hanger so it moves the entire trigger to the rear the required amount.

Are you moving the trigger back on a Remington 700?

 

[WSnyder]

an ideal amount of firing pin protrusion from the bolt face. What is this dimension.

I think you'll find the most common stated dimension is ~.050-.055", I do ~.035" on my rifles but that's just me

The front of the firing pin flange is what stops the pin fall when dry firing on an empty chamber. With a Remington style striker assembly, I believe that one would have to remove material from the front of the flange in order for the pin to fall farther.

You can certainly do it that way (as you stated though it could result in more work needed to be done to make it work) but you can accomplish the same thing going a different direction resulting in less work.

How and what do you do to the cocking helix?

You figure out the tooling needed to re-cut the helix so it ends up deeper. That right there is likely to remain in pocket because it's not exactly easy to do on manual equipment. Some bolts and combinations of parts will only need the bottom cut deepened slightly leaving the helix alone. CNC 4th axis is the easiest to setup if you have the CNC or access to one and how many have that?

 

[JFrank]

After a few adjustments to pin fall I ended up with a touch of CoC that I’m certainly not crazy about however to eliminate it will take some skill that I do not have.

 

[Ledd Slinger]

As I said in my post #10, I have done it on custom actions by off set bushing the pin holes in the trigger hanger so it moves the entire trigger to the rear the required amount.

Are you moving the trigger back on a Remington 700?

No because that would make the firing pin cock on close. If you move the trigger rearward you gain pin fall, but then if you time the cocking piece to the new trigger sear location, you are right back to reduced pin fall. When I make the firing pin cock farther to the rear I gain pin fall regardless of trigger hanger position and whether I time a trigger or not.

I’ve only done this modification on actions that use the same ignition system as the Rem 700 so can’t say if it works on everything. I know Alex has done it on quite a few different custom BR actions and I gathered from talking to him that it’s not an easy task on some of the other ignition systems out there. Which is a big reason he talks to action manufacturers about making everything perfect right out of the box. Not fun spending a lot of money on a custom action then having to tear it apart to make necessary changes for proper timing and ignition.

I performed the modification on a Kelby Atlas a while back. When they first came out they had good pin fall but significant cock on close with the BnA trigger I like. Measurements showed me the Atlas had room for improvement so the modification would work. With the BnA trigger timed on the Atlas I fell down to around .218” pin fall. After modifying for more pin fall i got it back up to .242”. I talked to Ian at Kelby about my findings and I’m sure others did as well. I heard Kelby has done some modifications of their own to the ignition system to provide good trigger timing with proper pin fall. I haven’t tried one of the latest Atlas actions yet so can’t actually confirm that personally but hats off to Kelby for listening to input if this is true.

On my BAT HR, the BnA trigger is perfectly timed and has excellent pin fall right out of the box. No need to touch it whatsoever. Smooth as melted butter

 

[DangerRanger]

great thread

 

[jelenko]

I think you'll find the most common stated dimension is ~.050-.055", I do ~.035" on my rifles but that's just me

You can certainly do it that way (as you stated though it could result in more work needed to be done to make it work) but you can accomplish the same thing going a different direction resulting in less work.

You figure out the tooling needed to re-cut the helix so it ends up deeper. That right there is likely to remain in pocket because it's not exactly easy to do on manual equipment. Some bolts and combinations of parts will only need the bottom cut deepened slightly leaving the helix alone. CNC 4th axis is the easiest to setup if you have the CNC or access to one and how many have that?

Ah, so no dremel work.