How Does The Bolt Unlock?

**prepare** · 04-03-21, 17:33

I know gas unlocks the bolt but it appears the gas enters the carrier right at the gas rings so does the gas pressure actually move the carrier rearward first in the cam track and then the bolt unlocks?

**AndyLate** · 04-03-21, 17:57

Yes, the carrier moves back. The bolt carrier track causes the cam pin to rotate (unlock) the bolt.

Andy

**prepare** · 04-03-21, 18:18

Ok, thats why the bolt tail seal is critical!

**Clint** · 04-04-21, 10:23

The gas rings, bolt tail and bolt carrier form an expansion chamber.

The gas feeds into the middle of this chamber and pushes the bolt and carrier in opposite directions.

The bolt, being much lighter, moves forward slightly taking up any extra head space and is stopped by the cartridge, chamber and the whole mass of the carbine.

The bolt carrier, being lighter than the rest of the weapon, now accelerates rapidly rearward.

The carrier gains most of its velocity in the first 1/8" of travel, which also corresponds to the "free" travel in the cam track path.

The angular portion of the can track begins after the free travel straight portion.

It this point, the carrier moves the bolt rearward slightly, where the bolt lugs react against the barrel extension lugs and the bolt rotates and unlocks along the cam path.

The carrier gains a little more velocity over the remainder of the .325" total travel and the gas vents are uncovered.

At the end of the cam travel, gas pressure starts to vent out and the carrier drags the unlocked bolt rearward together compressing the action spring.

**lysander** · 04-04-21, 14:20

Originally Posted by Clint

The gas rings, bolt tail and bolt carrier form an expansion chamber.

The gas feeds into the middle of this chamber and pushes the bolt and carrier in opposite directions.

The bolt, being much lighter, moves forward slightly taking up any extra head space and is stopped by the cartridge, chamber and the whole mass of the carbine.

The bolt carrier, being lighter than the rest of the weapon, now accelerates rapidly rearward.

The carrier gains most of its velocity in the first 1/8" of travel, which also corresponds to the "free" travel in the cam track path.

The angular portion of the can track begins after the free travel straight portion.

It this point, the carrier moves the bolt rearward slightly, where the bolt lugs react against the barrel extension lugs and the bolt rotates and unlocks along the cam path.

The carrier gains a little more velocity over the remainder of the .325" total travel and the gas vents are uncovered.

At the end of the cam travel, gas pressure starts to vent out and the carrier drags the unlocked bolt rearward together compressing the action spring.

A few additions and clarifications:

The free travel of the piston in the cam track is only 0.070" (0.00178 meters). You can see that after that much travel the acceleration of the piston (the slope of the velocity curve) begins to drop after that much travel. The actual helical travel is only 0.213", the remaining 0.042" being the straight section of the on the unlocked end of the path.

Venting starts prior to completion of unlocking.

Rearward bolt motion starts at the end of piston motion. Since the pressure inside the carrier cavity is pushing the bolt forward and the carrier rearward, even thought the cam pin hits the forward slope of the cam path, there is no net rearward force yet to push the bolt to the rear. It is not until the cam pin bottoms out in the cam pin track (end of piston motion) is the forward force on the bolt removed, and this allows the bolt to begin moving to the rear.

The maximum bolt/bolt carrier velocity is attained at the end of piston motion, after that the spring, drag from the friction, extraction, and over running the hammer and rounds in the magazine begin to rob the carrier of velocity. The optimum carrier velocity for an AR is 5.5 to 6.5 meters per second.

**MistWolf** · 04-04-21, 17:24

Just a small correction-

Originally Posted by Clint

The bolt, being much lighter, moves forward slightly taking up any extra head space and is stopped by the cartridge, chamber and the whole mass of the carbine.

During combustion, peak pressure is achieved as the bullet enters the lands of the rifling and the brass case expands in diameter and length to seal the chamber, pressing against the bolt face (bolt thrust) and chamber walls and remains expanded until pressures drop enough to allow the brass to shrink enough to release it's grip on the chamber walls. This means there will be no extra headspace to be taken up by the bolt being pushed forward. The exception to this is when a case is so underloaded that it expands against chamber walls before the shoulders can push it back against the bolt face. When this happens, it's common for the primer to back partially out of its pocket.

The pressure in the expansion chamber would have to be greater than the pressure in the bore in order to push the bolt and the case forward. The most the pressure can be in the expansion chamber is equal to the pressure in the bore. It's more likely to be less.

**lysander** · 04-04-21, 19:11

Originally Posted by MistWolf

Just a small correction-

The bolt, being much lighter, moves forward slightly taking up any extra head space and is stopped by the cartridge, chamber and the whole mass of the carbine.

During combustion, peak pressure is achieved as the bullet enters the lands of the rifling and the brass case expands in diameter and length to seal the chamber, pressing against the bolt face (bolt thrust) and chamber walls and remains expanded until pressures drop enough to allow the brass to shrink enough to release it's grip on the chamber walls. This means there will be no extra headspace to be taken up by the bolt being pushed forward. The exception to this is when a case is so underloaded that it expands against chamber walls before the shoulders can push it back against the bolt face. When this happens, it's common for the primer to back partially out of its pocket.

The pressure in the expansion chamber would have to be greater than the pressure in the bore in order to push the bolt and the case forward. The most the pressure can be in the expansion chamber is equal to the pressure in the bore. It's more likely to be less.

Not quite.

The pressure in the cavity does not have to be greater than the chamber pressure. If you look at the graphs of the piston displacement and note the time at which unlocking starts, 1.3 ms. Now, look at the pressure time curve below and note the pressures in both the chamber and carrier cavity at that time.

Chamber pressure - 5000 psi
Cavity pressure - 2750 psi

HOWEVER - If you look at the areas these pressures act on:

Chamber - 0.0989 square inches
Cavity - 0.1963 square inches

So, the actual forces acting on the bolt from opposite ends are:

Chamber force (pushing backwards) - 495 pounds
Cavity force (pushing forward) - 540 pounds

The cavity force is larger by 45 pounds, so the bolt moves forward. Yes, this is based on a single rounds pressure time curve, but the ratio of the two areas is fixed, and the ratio of the two pressures is generally the same being governed by the gas system geometry, so it's not necessarily 45 pounds but it generally equal or larger. That's one of the neat things about the AR gas system, the bolt lugs are not loaded or at worst very lightly loaded when it's being turned, the bending loads on the lugs is lower than with traditional gas systems.

EDIT:
Further, at this point on the firing cycle, you see that the internal pressure in the case is only in 5000 psi, that is well below the inflation pressure of the brass case, so the case is in its relaxed position.