Clearences>Tolerances>Accuracy?


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Vegaslaith
December 24, 2008, 02:07 AM
Something has been bothering me lately. As I understand it clearances are the spaces between moving parts. Tolerance means the amount of variation between parts or between a gun's blueprints and the actual parts. Fine.

So, how do loose clearances, and therefore loose tolerances affect accuracy? If a cartridge is stripped from the magazine and brought into the chamber, who cares how loose the clearances are? The round ends up in the chamber, in the same place, ready to have its primer struck. Why should matter if the bolt is surrounded by a milled reciever or wiggles on a guide rail?:confused:

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stubbicatt
December 24, 2008, 09:27 AM
The entire system is designed to fire a projectile.

Accuracy is largely a function of consistency. Same bullet weight and design, same case capacity, same powder charge, same primer, bullet seated to same depth etc. Consistency is where accuracy starts.

The precision of the chamber, including neck dimensions on the tighter side get the cartridge to line up with the bore in the same manner. There must be some "give" in the chamber for the system to extract, so the cartridge tends to lay in the bottom of the chamber, canting the projectile ever so slightly. A consistent bolt lockup tends to assist the cartridge in lining up straight with the bore.

The greater the size of the chamber, the more the cartridge is going to lay off center of the bore. The larger the chamber neck dimension, combined with the larger chamber, the more likely the bullet will cant at the moment of firing, and under the extremely high pressures, enter the bore with the nose of the projectile off center. This induces yaw or precession in the bullet flight, which adversely affects accuracy.

The semi auto mechanism, combined with a "sloppy" chamber can induce more variation in lockup, canting the cartridges every which way, which leads to inconsistency.

The material of the barrel, and receiver, vibrates and stretches when the cartridge is fired. The firing of the cartridge creates vibrational forces in addition to linear, or stretching forces. If there is a great deal of elasticity in these materials, the vibrational nodes will be further apart, leading again to inconsistency, and inaccuracy.

There is a scene in "Lord of War" in which an Afghani tribesman is firing off a Kalashnikov in automatic, while the protagonist looks on. The images go to slow motion, with the sound of a cash register ringing at each cartridge being fired. Look at the amount of flexing going on in that system as it is being fired. The receiver bows so much that the top cover is moving fore and aft. --An example of inconsistency. As the bullet is traveling the bore, the terminus of the bore is moving around relative to the target due to the flexible materials of that rifle, leading to inaccuracy, as the bullet will leave the bore at a random vector.

Clearances are necessary to get the firearm to run properly. One must trade off these clearances for dirt, oil, impurities such as accumulated carbon etc., with precision. As tolerances increase, variables also increase, which introduces inconsistencies. Inconsistency leads to inaccuracy.

HTH.

heron
December 24, 2008, 01:15 PM
Stubbicatt, you've won the Technical Writing Merit Badge for today. Well done!

Vegaslaith
December 24, 2008, 06:58 PM
Great answer! Thank you.

Art Eatman
December 24, 2008, 08:20 PM
One reason for only neck-resizing in the reloading process is that the case is fire-formed to that particular rifle, which is as consistent in chambering as can be had.

Modern machinery makes it possible to hold a barrel to 0.0001" tolerance in the bore diameter. Probably difficult to do better than that.

SFAIK, about the loosest design tolerance which could affect accuracy is the headspace for a bottleneck cartridge. There are three separate pieces to machine and have fit up properly--receiver, bolt and barrel.

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