rifle movement with dry firing

D

docsleepy

Guest
What makes the point of aim move when you dry fire? I finally recognized this as a possible large contributor to my slowly-improving groups on a savage accutrigger action.

Is it finger pressure on non-2-oz trigger, and resulting movement?

Is it momentum from internal movements of trigger mechanism or firing pin?

Is it (rotational) torque from the spring when released?

Or some combination of all of the above?

Do benchrest rifles with 2oz triggers still change point of aim significantly when dry fired, and if so, what do you do to minimize it?

thanks,
gordon
(saving up for improvements or replacements)
 
Movement

Well, you do have about 20+ pounds of firing spring energy being unleashed.

If you notice, if you put a case in the chamber with spent primer, the shock and vibration falls off drastically.

Have you ever heard the old joke: "A man goes to the Doctor and says, "Doc, it hurts when I do this." The Doctor responds, "Then don't do that".

Stop looking through the scope when you let the firing pin fall on a empty chamber.:rolleyes:
 
Thats the reason that a quick lock time is essential!
Mark
 
jackie schmidt said:
Well, you do have about 20+ pounds of firing spring energy being unleashed.

If you notice, if you put a case in the chamber with spent primer, the shock and vibration falls off drastically.

Have you ever heard the old joke: "A man goes to the Doctor and says, "Doc, it hurts when I do this." The Doctor responds, "Then don't do that".

Stop looking through the scope when you let the firing pin fall on a empty chamber.:rolleyes:
Click to expand...

I'll try that! I never tried it with a spent primer!
thanks!
gordon
 
I have a couple of high end air rifles and they will do the same thing when 'dry firing'. It's disconcerting but it doesn't keep them from driving finishing nails :D when you actually get around to shooting them.
 
We make good use of dry firing when working with shooters in the tunnel. Shifts in point of aim when the striker falls indicates problems with rests, bags and rifle handling. The subject is pretty involved and it is difficult to accomplish much here on the forum with the written word but if you will load up your equipment and make a trip to Odessa, TX, I'm sure we can works things out.

Keep dry firing on an empty chamber to an absolute minimum. When dry firing for practice, always chamber a fired case with the spent primer in place. This will cushion the firing pin blow and better simulate what happens when the firing pin strikes a live primer. Dry fire only once on a fired case.

Hope this helps. :)

Gene Beggs
 
One time when I was looking at this situation, I noticed that with one rear bag I got cross hair movement and with another I did not. currently, I am using a lighter pin and heavier spring because there seems to be less reticle disturbance, and there doesn't seem to be an ignition down side, and it may shooting slightly better...very subtle...hard to prove.
 
Great input, thanks.
I'll switch to using spent shells.
When I noted this, I practiced trigger control until the movemment was dramatically reduced and I think that helped remove some unexplained fliers. There were clearly better and worse ways to hold/pull rifle/trigger. They were not what I would have expected. Simply touching ONLY trigger was not the solution.

Bags/rests are still suspects and will continue to try things there. I think my front rest bag insert is too skinny and has too much sand in it (too hard, despite removing some sand). Will try changes there. Also got myself on a gunsmith's list for a lighter weight trigger and some other improvements.

Never ending list of errors to minimize, in this hobby!
 
There shouldn't be anything mysterious about this. It's simply a case of a weight (the firing pin) moving inside of another weight (the gun) - starting and stopping very suddenly. The gun (and scope) is moved very slightly and this is what you see when looking through the scope. It's affected by "lock time" only in that a lower lock time means you have probably applied a force (firing pin spring load) that made the pin start and stop more quickly over a shorter time span. It's the impact force between the pin and bolt that causes the gun to move slightly. If you apply a force, no matter how slight to any weight, you will move it!

If you know all the variables - weight of pin, weight of gun, spring load, travel of pin, restraining force on gun - you could calculate just how much your gun will move when you dry fire it.
 
I have seen bolt turn position as a factor here, even with some of my custom actions. Watch your bolt handle on dry firing. Does it jump up or down?
If so, it's a random injection of pretty evil energy[That's what she said].

Always surprised me that bolt closure doesn't click into a detented & proper position. A Savage action is the easiest I've seen to modify for just this(due to the rear collar), and I think it improved mine.
I'll never know though as some bastard Canadian gunsmith stole it years ago...
 
Gene (Et al)- What about snap caps? I didn't see mention of using them. No good?

Pardon my ignorance but what's "The Tunnel"? You're about a 5 hour drive from here. What kind of shooting do y'all do over there? Any good gun shows come to town? Thanx.

Cheers,

Mark
 
never thought of snap caps. Dunno if they make one for 6PPC, but I could probably take a trimmed pencil eraser and stick it in the primer pocket and have one.

Thanks for the ideas!

I think pacecil has an interesting point, but I'm not sure how Newton's 3rd Law fits in. When the spring pushes on the firing pin, the firing pin pushes back on the spring (and attached gun), momentum total is conserved. Since the firing pin now has momentum toward muzzle, the gun has momentum toward shoulder. But when the firing pin hits the primer, the situtation exactle reverses, I think. This makes my head hurt. And I teach physics! the configuration of the system DOES change. There's a torque released also; potential energy is converted to kinetic energy (rotational movement of spring) as it is released.... I'm going to have to think more about that.

And I think I'll try the eraser idea.
thanks
 
Maybe this will help explain...

When the gun is cocked the spring load is against the gun in the rearward direction and also against the pin, and thus to the gun through the sear in the forward direction - so nothing is happening. When the sear releases the pin, the pin starts to accelerate forward. At this instance there is still the spring load back against the gun but the forward spring load is now acting only on the pin. The gun will begin to "recoil" backward as a result of the spring load acting only in the backward direction on it.

The pin is moving forward and will either be stopped by the impact with the shoulder in the bolt , or it will be stopped by impact with the primer. Impact with the shoulder (which is in essence against the gun) will stop the pin in a very short distance (high deceleration) which requires a high force. Impact with the primer (which is in essence also against the gun) stops the pin over a longer distance (lower deceleration) which means a lower force. Once the pin is stopped then the spring load is once again applied in both the forward and backward direction against the gun so everything will come to rest.

So there you have it; the pin may hit the shoulder and thus cause it to exert a high impact force, or it might hit the primer and thus be cushioned so it exerts a low impact force. This is what causes a big disturbance to the gun, or a little one.
 
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