Firing pin strike

[cw308]

Two 308's used only for target shooting. I noticed that one rifle the firing pin strike is dead center on the primer, the other is off center. Would this effect the accuracy. dead center is the 700 Rem. off center is the Savage 12 seriers.

[Jimro]

Shoot them and find out.

Jimro

[Scorch]

I agree with Jimro. I am sure that someone will pop up and say that theoretically the one that strikes dead center would be more accurate, but I say shoot them and find out for yourself.

[Bart B.]

Considering the fact that a rifle whose bolt face aligns dead center on the chamber when the bolt's closed and the hole in the bolt face is dead center on the bolt face, the external extractor will push the case off center in the chamber and on the bolt face anyway. This automatically puts well centered primer pockets on case heads a bit off center relative to the chamber.

But the direction and amount's very repeatable from shot to shot. Good accuracy's guaranteed. . .if all else is good, too.

And if the pressure ring on the case is bulged out quite a bit at one point and that is at the same point were the extractor presses the case off center, that may well end up centering the primer on the firing pin/bolt-face-hole.

Stranger things have happened. But the little details of how all this stuff works has been facinating to me.

[Slamfire]

I am of the opinion that a manufacturer should build his firearm so the firing pin strike is in the middle of the primer.

While Bart has addressed concentricity and accuracy, something that is not well understood is that primer sensitivity changes due to off axis hits. The further the firing pin impact is from the center of the primer the more likely you are to get misfires/squibs. I believe this has other effects as well, based on the assumption that the ignition characteristics of the primer vary on firing pin strike.

If you want the most consistency from your ignition system the first place to start is a firing pin strike in the middle of the primer.

[Picher]

If you don't have misfires or noticeable vertical stringing and groups are sastisfactory, don't worry; be happy!

[Bart B.]

Slamfire's of the opinion that a manufacturer should build his firearm so the firing pin strike is in the middle of the primer. That would be great.

But it also means case makers need to have their primer pockets exactly in the middle of the case. And case wall thickness be exactly the same all the way around. Plus zero fit tolerance in the bolt body to the receiver boltway.

Then the barrel tenon threads have to be exactly centered with the boltway axis. The hole in the bolt face has to be dead centered there. And the bold shroud around the bolt face has to be exactly the same diameter of the case rims used.

Finally, the case body diameter in front of the extractor groove has to be the same diameter as the chamber at that point so it centers perfectly at that point.

I may have forgot one or other dimensional issue that has to have zero tolerance, if so, sorry 'bout that.

[Jimro]

I'm honestly not worried about an off center primer strike. The firing pin hits the primer cup, crushing the primer compound between the cup and the anvil (built into the primer for Boxer primers, built into the brass for Berdan primers).

I honestly can't see a significant difference between a direct on strike and an off center strike, the resulting detonation is going to happen REALLY fast. The hot gasses will then push through the flash hole (or holes in a Berdan primed case) to ignite the powder. Simply due to physical differences the resultant gas jet will have slightly different vortices due to the difference in firing pin impact.

Whether that translates into difference in accuracy I can't say, but I don't think it does. I think the difference in primer strength between primers even of the same brand and lot, are enough to have a bigger effect on accuracy than where the firing pin lands.

Jimro

[Slamfire]

Slamfire's of the opinion that a manufacturer should build his firearm so the firing pin strike is in the middle of the primer. That would be great.

But it also means case makers need to have their primer pockets exactly in the middle of the case. And case wall thickness be exactly the same all the way around. Plus zero fit tolerance in the bolt body to the receiver boltway.

Then the barrel tenon threads have to be exactly centered with the boltway axis. The hole in the bolt face has to be dead centered there. And the bold shroud around the bolt face has to be exactly the same diameter of the case rims used.

Finally, the case body diameter in front of the extractor groove has to be the same diameter as the chamber at that point so it centers perfectly at that point.

I may have forgot one or other dimensional issue that has to have zero tolerance, if so, sorry 'bout that.

If we lived in a perfect world all would be perfect. The offset does not have to be less than the wave length of an electron. For me, if the strike appears to be in the center, that is good enough for me. If the strike is obviously off center, that is too much.

So how far off center a firing pin strike is acceptable to you?

[Bart B.]

So how far off center a firing pin strike is acceptable to me?

.010" or more up to .015" has not shown any accuracy problems. Estimated offsets as I've never measured any for exact dimensions.

[Slamfire]

So how far off center a firing pin strike is acceptable to me?

.010" or more up to .015" has not shown any accuracy problems. Estimated offsets as I've never measured any for exact dimensions.

Thanks for the data point.

[Jim243]

So how far off center a firing pin strike is acceptable to me?

As long as it sets off the primer, you are good to go. The barrel and chamber have more to do with accuracy than the primer strike.

None of you shoot bullseye in 22 LR do you?? Talk about off center strikes. How about the wind down range, how about the bullet shape, how about the amount of powder in the case, how about setting the bullet straight in the case, how about making sure the mouth of the case is trimmed properly.

NO, you have to be worried about something crazy as a .02 inch difference as to where the firing pin strikes the primmer.

By the way you Savage is more accurate than your Remington, not because of the primmer strikes, but because of the way the barrel and chamber were made.

Jim

[Bart B.]

I believe most rifles' accuracy benefits from 15% stronger firing pin springs. And replace them every couple of years. Seen many instances of vertical stringing at long range fixed with a new spring.

[Jimro]

Bart B., you aren't the only one to observe that a weak firing pin can open up groups, I've not seen it myself (don't shoot Palma though). A weak firing pin spring can delay ignition by a few milliseconds, which is time for the shooter to wobble. I know of a couple Palma shooters who replace firing pin springs before the start of the season.

Jimro

[Bart B.]

Jimro, weak firing pin springs don't detonate the primer compound as uniformly as strong ones do. Primers are much more uniform when they're smacked very hard and deep by the firing pin. Non uniform primer firing causes a greater muzzle velocity spread. And this is the reason why folks shooting competition at the longer ranges change springs often.

A 32-pound standard Rem. 700 firing pin that's weakened by 25% will have its normal 2.6 millisecond lock time increased to about 3.2 milliseconds. That's only about a 0.6 millisecond increase.

Win. 70's lock time with their 23 pound standard spring's about 3 milliseconds. I use 26 or 28 pound ones in my Winnies.

[Jimro]

Bart B.

Do you have a link for weak springs increasing velocity spread? I'd like to read about that.

Jimro

[Bart B.]

I've found a link:

http://www.6mmbr.com/verticaltips.html

This issue was first mentioned to me back in the late '60's by a Nat'l Champ. So I took my .264 Win. Mag out to the range with extra firing pin springs; each about 1/4 inch shorter with compression weights in the 22, 19 and 16 pound range. Its factory 23-pound spring had been replaced by a 26-pound one. I was given these weakened ones by a 'smith (Elmer Shook, who sold me the 26-pound one) who built match rifles, then cut them off myself. At 1000 yards, as each weaker spring was used to shoot 5 rounds, elevation shot stringing increased. Went from about .6 MOA to 1.5 MOA.

Later, in a friend's underground 100-yard range, we did some muzzle velocity tests with weak springs in a .308 Win. chambered Win. 70. Weaker ones produced greater muzzle velocity spreads.

At the 1992 World Palma Matches, two guys on the US Team had elevation problems with the ammo issued. Both were given a new 26-pound spring from my tool box for the Win. 70's, then they shot better scores.

Some rifles have enough room on the firing pin to put a spacer between the cocking piece and the back end of the firing pin spring. This "renews" weakened ones up to about factory specs. I've done it many times with Win. 70's with a 3/8" long spacer.

At the Nationals in the early '90's, I talked with a rep from Federal about primers. He said they all need a small range of firing pin impact force as well as dimple depth to perform consistantly for best accuracy. And if not right, it often is the reason why a given load that wins matches and set records in some rifles will not do well in others; the start-up pressure curve's not right and the poor primer ignition/detonation is the cause. It's another reason why there's hundreds (thousands?) of favorite loads for a given bullet in a given cartridge.

In the link above, it also mentions what happens when the spring's too strong. I think 20% over factory specs is about the limit. For the 23 pound spec on Win. 70's, the limit would be about 28 pounds.

[Slamfire]

So, I agree with everything being said about firing pin speed, the need for new springs to give a constant firing pin strike, and yet, after all this discussion,

Why then, as stated earlier and apparently agreed to in this thread, why are not off axis firing pin strikes detrimental to accuracy?

[Bart B.]

Slamfire asks why are not off axis firing pin strikes detrimental to accuracy.

Because, if they're not too far off, they're very repeatable from shot to shot. The flame output and temperature paramaters are, for all practical purposes, the same from shot to shot.

If someone want's more details and the nitty-gritty technical reasoning, contact a primer making company then ask them. I've only offered my own experiences and reasoning.

[tahunua001]

off center primer strikes doesn't affect a dang thing. the primer still ignites the powder and the powder burns at the same rate and pushes the bullet the same way.

[Slamfire]

What these assertions show is just how little even experienced shooters know about primers.

Of course this has been studied, and it takes equipment far more complicated than a new mainspring.

Ammunition characteristics are very important for revolving cannon. Issues such as ignition dwell, hangfires, which won’t create a catastrophic event in a single barrel gas or recoil mechanism, will positively cause problems in a externally driven multiple barrel/chamber mechanism.

And what was found was that misfires increase with increasing off axis hits and off axis firing pin hits affect the time to maximum primer flame.

These effects were lesser issues compared with varying other primer characteristics, but they are still there.

I suppose Palma shooters are a very tolerant lot of imperfection even though I would have expected otherwise. In competitions, I want the maximum performance out of my equipment, I expect the firing pin to hit the center of the primer, the hole to be in the middle of the barrel, the chamber to be concentric, the receiver ring to be machined so that the cartridge is in the center of the action. There are always tolerances, but I want as close to perfection as possible. It may not make a difference, but on the other hand, it may. So it would bother me to think I spent a weekend at a match, or flew to South Africa for a Palma meet, only to have my score reduced by inconsistent ignition, sloppy firearm components, or sloppy assembly. If we don’t insist that manufacturers make precise shooting fire arms then we must accept the occasional dropped shot due to poor equipment. There may be those who want to live according to the philosophy of “Don’t Worry, Be Happy” , http://www.lyricsfreak.com/b/bob+mar..._20726616.html and to each his own.

[Jimro]

Slamfire,

A precision bolt action rifle is not an "area affect weapon" designed to give a cone of fire and produce effects on the beaten zone. Unless you can demonstrate that anything Bart B. wrote is categorically untrue, you might want to rethink your assertion that extremely consistent ammunition for an M134 Minigun has anything to do with accuracy. There is a reason that M80 machinegun ammunition was redesigned for the M134, and that Mk316 Mod0 was designed for precision rifles. One is consistency of ignition, the other is consistency of precision.

I would like to remind you that we've been over this before, so it isn't like anyone here is ignorant of how primers work, or how to get maximum accuracy by choosing a particular primer over another for a particular powder.

Read these again, http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA341390 and here http://www.dtic.mil/ndia/2009infantr...ioniii8524.pdf and http://riflemansjournal.blogspot.com...mer-study.html for a graphic illustration of why a hotter primer (faster ignition) may in fact provide more consistent ignition but worst accuracy (more consistant pressure curves).

Jimro