I don't want anyone here to think I come here to yank chains but I read more theory here that makes sense (to me anyway) than in some books and on most boards. Here goes my question. Premise #1 The brass cart is a ballooning , chamber filling, -self sealing powder container. #2 the cartridge is the bullet holder and tensioning device. #3 The bullet moves AFTER the cartridge has expanded to full chamber dimension. If all of this is true (I hope) then Accuracy can not be affected by any adjustment made behind the shoulder of the chamber. The fact that the shoulder seals the gasses in the chamber even compensates for bad headspace. Polishing the chamber, squaring the bolt face, polishing the lugs. All of these tweaks effect the balloon before liftoff. Even a cartridge lying in the bottom of a grossly oversized chamber will fill the rise to the occasion. Correct? So are all of these behind the shoulder accurizing issues like vacuuming your trunk to make your car will run better???? Inquiring minds want to know! (you don't want to look in my trunk) thanks in advance, JB

JB - Interesting thoughts. I don't think it can be simplified as much as you've done, however. For instance, demonstrable accuracy improvements have been proven too many times by squaring the action, sleeving the action, lapping the lugs, etc.., in before/after tests of too many rifles to suggest that those changes should have no effect on accuracy.

I will grant you that your Premise 3 does make sense in that bedding definitely will influence accuracy more than anything else.

I still think we have to approach accuracy tuning by treating the firearm as a "system", where every component in the system works either with, or against, every other one.

Man, this requires too much thinking for so late in the week.

R-WEST

Close, JB, but no cigar.

The chamber and leade are always oversized (by definition), therefore the shape and concentricity of the unfired cartridge can influence the orientation of the projectile as it begins its forward motion into the rifling. This can have an effect on how and in what condition it exits the muzzle (and ultimatly on where it strikes the target).

To a lesser extent, the amount and timing of the expansion of the case to fill the chamber (and you describe it well) can have an effect on the harmonics of barrel vibration, which in turn, can affect the exit of projectile from the muzzle. I'm not sure I would agree that the bullet does not begin to leave the case neck prior to full expansion of the case, but that is splitting hairs and I know of no way to prove it empirically.

I do agree with you that these effects are much less than many presume. Various posters on this board will swear that they can tell the difference in pressure and velocity between neck-sized and full length sized brass with the same load in the same gun (curiously, some of them think neck sizing increases p & v while others subscribe to the opposite therory).

stonecreek, I agree about concentricity of the neck in the leade and that is why my comments are from the edge of the shoulder back. And I guess my thought is that the shoulder holds the gasses even in a light load that would soot the neck. And I do not think that detonation would affect barrel vibration as would the torque of the bullet driving through the rifling (this is pure speculation). And I started to think about this becaus of the System concept that R-West speaks to. If you were holding a glass of water while riding a bike and the surface of the water is vibrating I think it matters little what the sruface looks like when you hit a pothole. The new stronger dynamic sets up a new vibration pattern. I guess what I am trying to say (poorly) is if you tap a steel rod and get a tone and tap it again quickly and harder dosen't it cancel the first vibration pattern and tone immediately? Isn't it magnitude of force and not a cumulative issue of forces (lets the dampening of poor bedding out of the discussion) that set up the barrel vibration? firing pin vibration and ignition and bolt set back should have occured before bullet release and "jump" to the lands. I my mind the energy of the bullet digging into the rifling and the energy to push and twist the bullet is where ALL of the torque is generated. Proof here I think comes from different results with molly (slicker) and cast (less tourque) bullets. same bolt, same firing pin but less barrel time or less energy transfered to the barrel by friction. As you can see I am bordering on becoming a heritic (God Forbid) But I will tell you how my head came to this place. I read about gunsmiths taking the barrel off the action to true the face and re-asemble the gun and collect their fee to make the gun more accurate. And my question/ comment is that the action is simply the baloon feeder. Once the whole mess is jammed into the chamber who cares what is going on back there. All the real energy is released / generated in the lead. Well forward of any out of square or pooly lapped bolt lugs.

In my mind I should be able to take a chamber cast, built some ammo up where the neck of the round just fits in the barrel with no slack to speak of, have the bullet set just off the lands and I could put any sorry mess behind that and it should shoot well. JB

JBMauser,

I think there at 2 answers and it depends on whether neck size cases are used or whether cases are sized to have about .003" or so of headspace.

With neck sized cases, if the case is out of alignment with the bolt because the bolt face is not square or the barrel threads are off then that impacts on chambering a loaded round that have been previously fired.

Full sizing for a few thou headspace is quite a different set up.

My own experience with bench style rifles in calibers like 270 and 300 Win using full lenghth sized brass for about .003" headspace has been that the action "quality" does not seem to play any part in the result. Well, it would play a part, but I have not seen it and it would probably a lot of rifles to show up that difference. It is probaly similar to weighing powder as opposed to using a measure. Obviously if every load has exactly 58.5 grains of powder X that must be better than loads ranging from 58.2 to 58.8 grains, but it might take a lot of shooting to for any gain to show up.

Mike

Well, JB, you make a compelling argument. I guess the defect in your assumption is that a body which is already vibrating will vibrate the same after subjected to a greater force than one which is static when subjected to the same (greater) force. In other words, I'm speculating that a steel rod vibrating at, say, 4000 cps, but at a very small amplitude, will NOT be vibrating the same way as a similar but static rod if both are impacted by a 500 cps force at a greater amplitude (harder hit). By extension, two similar rods vibrating at different frequencies/amplitudes will also be acting slightly differently after being subjected to the greater force.

Theoretically, the slopiest fit of action, poorest bedding job, and least concentric chamber should shoot as well as the most precision, because each would react consistently with itself with each shot. That's obviously not the case.

Seemingly small variables can make large differences in shooting. I agree that the fit of the very elastic case in the chamber would seem to be of such insignificance as to be de minimus, but you can never tell until the group is on the target.

JB, I tend to agree with you. I find that of the metal components which make up the rifle, the barrel accounts for about 90% of the inherit accuracy of the rifle. Bedding is I believe even more important as it affects the system damping of the barrel, hence the position of the barrel's muzzle at the instant the bullet leaves the barrel. I have seen rifles with horrible amounts of excessive head space shoot very accurately indeed. By excessive I mean you got one shot out of the brass. This was a Brit 303, which would shoot better than MOA at 200 yards. Scary isn't it?

Axel

stonecreek, I do not think the vibration before bullet movement and after will be the same. I feel it will be very different and that the second "rap" as it were will cancel the first and set up it's own "tone" as it were. I think the bullet will create a twist force to the "rap" which will be greater than the force of the firing pin vibration and the ignition "tap". There is no way that ignition which is equal force or the firing pin which is lateral force can stress a barrel along its length and twist the barrel slightly as the bullet is pulled out of lateral inertia. Also I know there used to be a kind of pipe or whistle the when a plunger was pulled out as it was blown the pitch of the sound rose. I think the the harmonics of the barrel change at each point until the bullet exits. just like the whistle. And... if this is correct then my initial premise still holds water (can it?) that the action goes along for the ride. the bullet holds/tansfers all the energy until the recoil of the muzzle jet blast which we all agree has no effect on accuray. The bullet like elvis has left the building... still doubting ... JB

The pressure exerted on the walls of the barrel also stiffen the barrel therefore dampening vibrations. So that means that powder burn rate, pressure rise, will have a significant impact on the barrel harmonics. Again, the action is not doing anything to effect this phenomenon.

Axel

If the bolt lugs don't bear evenly, then the bolt is going to bend a little due to the forces being concentrated on the side where the lug is bearing, and none going to where the lug isn't bearing. This causes the bolt/lugs to behave like a spring, and affects the vibration patterns of the barrel. Worst case is where this bending causes the lug that isn't bearing to start bouncing off of its' seat on the action.
Poorly formed (eggshaped) chambers would also have a very small effect as the pressure waves in the powder gasses would be different from a perfectly concentric chamber; putting slightly more pressure on one side of the bullet base. This would cause the bullet to "cock" slightly in the barrel and end up being less accurate.

If there is already a vibration pattern in a piece of steel (like a barrel), there is energy involved. If you strike a vibrating piece of steel you are introducing more energy, but the old energy isn't canceled out. They vibration patterns will combine and form a new, more complex vibration pattern. Where the vibration wave patterns "cancel", the barrel is actually heated slightly as the energy can't be destroyed, only transformed into a different form. The default of this transformation seems to always be heating of the substance, but I don't think that physicists have proven that this HAS to be the result.

I don't buy the bolt lugs bearig evenly arguments for two reasons.

1.) I have a M70 which when new only beared upon one lug. The rifle shot 1.5" groups at 100 yards. I had the action lapped to get both lugs bearing evenly and the head spacing corrected the the rifle still shoots 1.5" groups at 100 yards.

2.) The lug races in the action have different stiffnesses. So the thrust force of the cartridge will cause the lug races to flex differently, top to bottom. This would produce the same effect as a lug that did not bear. Albiet the unbearing lug may be somewhat of a more worse case scenario.

Irregardless I do not believe that the lug bearing has a significant effect on accuracy. Now before you all start to flame me on this becasue bench rest rifles are this way. Consider this how many accurate rifle are built with only one tweak made at a time to see what effect this variable, the tweak, has on the rifle's accuracy. None to my knowledge. You get awhole bag of tricks, pays your money, and goes shootin'.

I personally believe that accuracy is best improved with the following rifle tweaks, in descending order:

1.) Bedding
2.) Barrel quality
3.) Chamber to bore concentricity.

Load development is not a rifle tweak. I consider an ammunition tweak.

Regarding the statements made about what pressure is required to unseat the bullet. The primer is all that is required. The primer ignition alone will expand the case as well to lesser or greater degree depending upon case size.

Pressure will not vary appreciably because of an egg shaped chamber either. The stresses acting upon the barrel shank in the chamber area will vary, but the pressure will remain the same. This will not "cock" the bullet as it travels down the bore. The pressure is evenly distributed across the entire volume of the bore.

Axel

[ 09-21-2002, 15:56: Message edited by: Axel ]