Physics of shooting a rifle

[stagpanther]

Yes. IF that's true (and the crown is undamaged), then after the transients are damped, the bullet should still be on the proper trajectory. But, as I said: " This is, in fact, why the muzzle crown is so important. If the force of the gases on the bullet as they escape is not reasonably symmetrical, the result can be that the bullet is displaced/reoriented after the initial small, transient, cyclical motion dies out." The graph you posted shows a case where that happens. You can read the graph and see that the position on the displacement axis is different at the end of the graph than at the beginning, right?

I'm not trying to argue-- the subject of muzzle crowns has always been of great interest to me, other than the throat I obsess more over crowns than any other part of a rifle's bore. More to the point, my impression has always been that a deformed crown (most of the time I observe damage in the form of a ding or "tear" at the edge of a groove, but not always) tears the bullet and that would have more of a destabilizing influence than the gases. Also, many muzzle devices quickly accumulate residue and can develop fire-etched baffles--how does that compare to gas exiting the crown?

 

[JohnKSa]

If a defect in the crown physically damages the bullet, that's going to cause accuracy problems. But you can also get inaccuracy from a damaged crown that doesn't touch/damage the bullet if it causes the gas to escape asymmetrically and bounce the bullet around in ways that leave it displaced/misaligned from its original trajectory after the transients settle.

Think about it. If a 10mph wind can cause a bullet to go off trajectory, imagine what a 3000mph wind (albeit one that affects the bullet only for a brief instant) can do.

Edit--added a missing word.

 

[tangolima]

I think the effect of an asymmetrical (damaged) crown has two folds. It steers the bullet, and it makes the exiting gas jet asymmetrical. All that, plus the instability of the bullet during transitional ballistic period, even worse the nonideality of the bullet's rear end, is not good for business.

-TL

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[stagpanther]

Damaged crowns are easy to spot because (at least in my experience) they accumulate copper being torn from the bullet. I can see that the gas pressure behind the bullet probably aggravates that action.

If a 10mph can cause a bullet to go off trajectory,

Ummmm, no.

 

[davidsog]

Yes. IF that's true (and the crown is undamaged), then after the transients are damped, the bullet should still be on the proper trajectory.

Good. I think we can agree then that rule is muzzle gases have no real influence on the flight of a bullet in a properly set up rifle.

We can call that the rule. Are there exceptions to the rule or situations where the effects of the rule are hidden? Of course.

More to the point, my impression has always been that a deformed crown (most of the time I observe damage in the form of a ding or "tear" at the edge of a groove, but not always) tears the bullet and that would have more of a destabilizing influence than the gases.

I would tend to agree. While the muzzle gases do have lots of energy inside the barrel these lose most of it almost instantly upon exit. The moment the bullet encounters the atmosphere as it exits, it forms a normal shock. The exit gas would have a decent amount of energy but the time they have to exert influence is very small. It will have an influence but I would think an asymmetrical collision of the bullet with the torn crown would impart a much larger influence. I would have have to see the math though. It's a tough call, lol.

 

[davidsog]

If a 10mph wind can cause a bullet to go off trajectory,

Frame of reference issue that is leading a fundamental misunderstanding of what is happening. You are a very intelligent guy, John. I sense your frustration with this too.

You have a good grasp of this but the separating your frame of reference and being able to think from one frame to the other causes the wheels to fall off the bus on occasion.

 

[tangolima]

Damaged crowns are easy to spot because (at least in my experience) they accumulate copper being torn from the bullet. I can see that the gas pressure behind the bullet probably aggravates that action.

Ummmm, no.

If the damage is smoothly fairing away from the bore, instead of encroaching into the bore like a burr, it may not tear on the bullet.

My SKS shot pretty lousy after exhausting all the tricks in the bag. I examined the crown with magnifier and couldn't catch anything wrong. It was the rounded military profile. As a last resort before giving up, I recrowned it with 11 degree profile. It made day and night difference. Perhaps the original crown was cut uneven smoothly, so visual inspection turned up no clue.

-TL

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[stagpanther]

My SKS shot pretty lousy after exhausting all the tricks in the bag. I examined the crown with magnifier and couldn't catch anything wrong. It was the rounded military profile. As a last resort before giving up, I recrowned it with 11 degree profile. It made day and night difference.

You mean it shot 5" groups at 100 yds instead of 6"?

 

[tangolima]

You mean it shot 5" groups at 100 yds instead of 6"?

More like 6"-8" before and better than 2" after. Believe me, I went out of my way to make it shoot, including glass bedding the action, a trigger job, and carefully handloading the rounds.

-TL

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[JohnKSa]

Good. I think we can agree then that rule is muzzle gases have no real influence on the flight of a bullet in a properly set up rifle.

Correct. That's why I said this: "This is, in fact, why the muzzle crown is so important. If the force of the gases on the bullet as they escape is not reasonably symmetrical, the result can be that the bullet is displaced/reoriented after the initial small, transient, cyclical motion dies out."

Frame of reference issue that is leading a fundamental misunderstanding of what is happening.

I'm not exactly sure what you're trying to say, but if it's that a 10mph wind or the gases that escape from the muzzle can't affect a bullet's point of impact (indicating that the trajectory changed from what was desired), that's obviously incorrect. If you're trying to imply that I stated they change the POI via exactly the same "method", I didn't say that.

My SKS shot pretty lousy after exhausting all the tricks in the bag. I examined the crown with magnifier and couldn't catch anything wrong. It was the rounded military profile. As a last resort before giving up, I recrowned it with 11 degree profile. It made day and night difference. Perhaps the original crown was cut uneven smoothly, so visual inspection turned up no clue.

Yup. It doesn't have to damage the bullet to have an effect. It just has to result in a significantly asymmetric release of the gases behind the bullet as it exits.

 

[tangolima]

The muzzle gas does affect the first inch or two of the bullet's flight, which is critical. Beyond that it has no effect.

There was another thread here on poi shift of the 1st and last round in the mag where a member maintained that the shooter could steer the bullet with muzzle gas. I don't believe it.

-TL



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[stagpanther]

More like 6"-8" before and better than 2" after. Believe me, I went out of my way to make it shoot, including glass bedding the action, a trigger job, and carefully handloading the rounds.

I have a soft spot for SKS's since I had a nice Norinco that I paid I think $75 for--came with bayonet and bayonet mount even. On a GREAT day I could get it to group 4" to 5" at 100 yds. I've known of guys who have done ultra-custom trick-outs with their SKS; met one at a range once and he let me shoot his, except for the basic action it was impossible to tell it was an SKS when I first saw it. It was a very, very cool rig though. It managed 1.5 to 2 MOA @ 100 yds which is truly exceptional for an SKS, this guy's was the only one I've ever encountered that could do that. You could probably go into business selling them if you can do that!

 

[tangolima]

I have a soft spot for SKS's since I had a nice Norinco that I paid I think $75 for--came with bayonet and bayonet mount even. On a GREAT day I could get it to group 4" to 5" at 100 yds. I've known of guys who have done ultra-custom trick-outs with their SKS; met one at a range once and he let me shoot his, except for the basic action it was impossible to tell it was an SKS when I first saw it. It was a very, very cool rig though. It managed 1.5 to 2 MOA @ 100 yds which is truly exceptional for an SKS, this guy's was the only one I've ever encountered that could do that. You could probably go into business selling them if you can do that!

Mine is a yugo. Not the newer type with grenade launcher, but the original type. Got it from classic arms for $200 or so.

I truely did it. I spent months to tinker it. Getting rid of the grease from everywhere, especially the wood, alone was already quite a task. Modified a lot of things, but didn't change its original military configuration. The trigger had dangerous negative sear engagement, and it was the first thing to be corrected.

If I ever do this to make a living, I will end up being homeless. Nobody is going spent the money to pay for it. I was once a paid part-time gunsmith. The work was fun, but hard to make a living, at least so the way I like to do it. You will ruin a hobby as soon as you turn it into business.

I collect and shoot milsurp rifles. Improving it till it shoots better than 2moa (100yr R50 10-shot) is the goal. The original military configuration must not be affected. Glass bedding is acceptable as long as it is hidden. There have been a few rifles that I have to throw in a towel, mostly because the barrel is far gone and replacement is nowhere to be found.

I like the SKS, but never the AK.

-TL

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[stagpanther]

I like the SKS, but never the AK.

Sorta a Pepsi vs Coke argument in my book, though I far prefer a good AK over any SKS.

 

[JohnKSa]

Me too. But I think we would both have a different opinion if our primary reason for owning one was to get it to shoot under 2MOA for 10 shot groups without substantially altering the original configuration.

 

[tangolima]

Sorta a Pepsi vs Coke argument in my book, though I far prefer a good AK over any SKS.

It is far more than that for me. I'm a first generation immigrant from a communist country. I saw both there. SKS has redemption value as it fought the Nazis, however briefly.

-TL

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[czgunner]

Sorta a Pepsi vs Coke argument in my book, though I far prefer a good AK over any SKS.

VZ-58 for me.

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[tangolima]

Velmet from Finland is ok. Don't have that sort of money though. I do have a VZ-52. Different kind of sks.

Come on guys. Stop rubbing it in. I can't have any of those without installing the dorky fin on the pistol grip. I live behind enemy line in CA, and I refuse to look stupid.

My life is cursed. I have to keep running.

-TL

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[JohnKSa]

Can you own one of the Hungarian AKs (SA85M) that came in with thumbhole stocks? Other than the large-cap magazine, they don't have any of the "evil" features.

I was going to change out my stock, but it was nice wood (as far as wood on that kind of rifle goes) and very comfortable so I left it in place. That gun points really well for me. It comes up to my shoulder and the sights are pointing where I'm looking without me putting any effort in.

This is not mine, but it looks very much like mine.

 

[tangolima]

The tyrants have deemed thumbhole evil.

-TL

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