tilting breechblock(FN FAL) and rotating bolt difference??

[technical]

in an fn FAL:the locking mechanism is what is known as a tilting breechblock. To lock, it drops down into a solid shoulder of metal in the heavy receiver much like the bolts of the Russian SKS carbine and French MAS-49 series of semi-automatic rifles.

in a rotating bolt(eg. AK47) http://en.wikipedia.org/wiki/Rotating_bolt



which one is more effective??? or is one better for fully automatic fire and the other for semi automatic fire??

if anyone can find diagrams or videos explaining any of these mechanisms please post


thank you,
regards,
technical

[AnandNair]

Tilting breech block, most commonly used in FN FAL (the Old Indian SLR rifles)

In a tilting block type mechanism, the rear section of the bolt drops into a recess in the receiver and locks it into place during firing. Once the round is fired and the pressure is significantly reduced, the movement of the gas piston rearward pushes the breech block rearwards which releases the bolt. Till the time the breech block is in fwd location, the lock is maintained. However the degree of accuracy with this type of system is directly correspondent to the tolerance of the bolt carrier to the receiver, where it mates at the rear of the bolt carrier and the recess in the receiver. The tighter the lockup, the more accurate the setup. And since a higher lvl of tolerance is required, this type was generally used in slow firing weapons.

Rotating Bolt

On a rotating bolt system,the bolt rotates as it follows the grooves in the receiver and drops into the recesses when it locks. The Bolt has lugs that fit into the grooves in the block and during the fwd movement rotates to acheive the lock. This is a simple yet effective mechanism and the lock is more secure. almost all artillery guns use this type of lock since it withstands high pressure, and provides a more secure lock and at the same time is simple to construct. Because of its secure and simple lock mechanism, almost all high power, fast firing rifles have this.
http://50ae.net/VZ-vs-AK/ak-4-c.jpg
The grooves in ak breech block receiver
http://50ae.net/VZ-vs-AK/ak-5-c.jpg
The lugs in front of the ak bolt.

Both mechanism can be used for fully auto weapons, however since a more secure positive locking is provided by the rotating bolt, this mechanism is preferred in high speed weapons and high power weapons. (even the single shot high power rifles uses the rotating bolt)

[technical]

@anandnair, thanks a lot

[TwoRivers]

Tilting breech block, most commonly used in FN FAL (the Old Indian SLR rifles)

In a tilting block type mechanism, the rear section of the bolt drops into a recess in the receiver and locks it into place during firing. Once the round is fired and the pressure is significantly reduced, the movement of the gas piston rearward pushes the breech block rearwards which releases the bolt. Till the time the breech block is in fwd location, the lock is maintained. However the degree of accuracy with this type of system is directly correspondent to the tolerance of the bolt carrier to the receiver, where it mates at the rear of the bolt carrier and the recess in the receiver. The tighter the lockup, the more accurate the setup. And since a higher lvl of tolerance is required, this type was generally used in slow firing weapons.

Rotating Bolt

On a rotating bolt system,the bolt rotates as it follows the grooves in the receiver and drops into the recesses when it locks. The Bolt has lugs that fit into the grooves in the block and during the fwd movement rotates to acheive the lock. This is a simple yet effective mechanism and the lock is more secure. almost all artillery guns use this type of lock since it withstands high pressure, and provides a more secure lock and at the same time is simple to construct. Because of its secure and simple lock mechanism, almost all high power, fast firing rifles have this.
http://50ae.net/VZ-vs-AK/ak-4-c.jpg
The grooves in ak breech block receiver
http://50ae.net/VZ-vs-AK/ak-5-c.jpg
The lugs in front of the ak bolt.

Both mechanism can be used for fully auto weapons, however since a more secure positive locking is provided by the rotating bolt, this mechanism is preferred in high speed weapons and high power weapons. (even the single shot high power rifles uses the rotating bolt)

Try that one again. In both systems the bolt carrier is pushed by the operating rod, or driven by direct gas impingement. In the tilting block system the carrier lifts the block up out of locked position by means of angled surfaces and carries it to the rear. Driven forward by the return spring the carrier carries the block forward and again drops the rear of the block against the shoulder in the receiver bottom to lock it.Tolerances between the carrier and receiver have nothing to do with accuracy.

In a rotary bolt system the bolt carrier rotates the bolt out of its locked position by mens of a cam slot and stud to unlock the bolt, then carries it back. The raceway in which the bolt travels is straight, not curved. Locking and unlocking is accomplished by a cam slot and stud, in bolt and bolt carrier.The bolt is not rotated into locked position until the end of its forward travel.

Both systems can be found in either "high speed and high power" weapons. It just depends on what the designer fancies, manufacturing costs, and which system passes acceptance tests. Theoretically, the tilting block should allow a higher rate of fire. Both systems are capable of higher rates of fire than desirable. Both systems are found in machine guns.

Artillery as a rule uses sliding breech blocks, both vertical and horizontal; or a multiple lug, interrupted thread, rotary hinged breech plug. The latter common where the shell and powder charge are separately loaded.

[timmy]

A tipping block action is best typified by the Savage 1899 lever action rifle, except that in semi auto and automatic weapons, a gas operation replaces the lever. The idea is to jam the bolt between the barrel breech and a rear stop by tipping it. Another famous non-auto weapon that uses this system is the Winchester Model 12 shotgun. Many famous designs have used the tipping block, such as the Soviet Tokarev SVT38 and SVT40, the German StG44 (viewed by some as the first "assault rifle"), the Soviet Simonov SKS, and the FN49 and FN FAL.

A rotating bolt design operates in the same manner as Two Rivers pointed out. There are a few kinds of this. M1 Garand and its M14 derivative, as well as the Ruger Mini 14 are a rotating bolt design, but the bolt is very long and the action is essentially rear locking. The German MG34 machine gun was a revolutionary design for its time, the first "universal" machine gun, and used rotating lugs that locked into the barrel extension, rather than the receiver. (This was a trick that John Browning used in his A5 semi auto shotgun!) The MG34 was no slouch, as it could rip off 900 rounds a minute. The AK 47 and M16/M4 work with a rotating bolt, as well.

Another design is the roller locking mechanism, used by the Czech CZ52 pistol, but introduced by the German MG42 machine gun. This was an improvement on the MG34 and could rip off 1200 rounds per minute and was a very feared weapon in WW2. This is a short recoil system, as well, and only the rollers move in a direction that is not parallel to the bore axis, making it a very slick design.

A system that looks like a roller lock and is often mistaken for one, but is not a roller locking system, is the roller delayed blowback system. Like any blowback system, the bolt begins to move rearward as soon as the round is fired. There are recesses that rollers operate in, but they act against a tapered surface. The effect of this is to give great mechanical advantage to the spring and inertia of the bolt, as opposed to a straight blowback system. However, like a straight blowback system, the action does not lock. The latest developments of the MG42 machine gun and the StG45 assault rifle used this roller delayed blowback system. There is no gas piston or direct gas impingement -- the bolt starts moving back as soon as the gun is fired, but the resistance of the recoil spring and inertia of the bolt is increased by cam surfaces and rollers. After WW2, the Swiss Sturmgewehr 57 used a roller delayed blowback system, along with the Spanish CETME, and Heckler & Koch licensed this system from CETME.

Properly engineered, all of these systems have advantages. My own preference is for the tipping block and the roller lock, as I am a Browning disciple and like planar mechanisms. There is a unique tactile feel one obtains when operating them. I always wondered why people liked shooting those ugly SKSs until I got my own. Then the SKS suddenly became beautiful -- they are a joy to shoot, just like other fun planar mechanism guns.

[TwoRivers]

[quote="timmy"]

Tim: I'd have to disagree with the statements in the second paragraph. Neither of the rifles mentioned has a very long bolt. Compared to other semi-autos of the period, Garand's design, and clones, have the shortest bolts and receivers. The are front locking.

What you term the "the roller delayed bbowback system" Is in fact locked during discharge. Here we get into semantics, since these systems are neither fish nor fowl and do not fit established definitions. Put a stout rod, recessed for the firing pin, down the bore, and you will find that the bolt cannot be moved. The firing pin has to move back before the rollers can retract and the bolt can move.

[technical]

thanks a lot gentlemen

regards,
technical

[timmy]

Here is a diagram of the MP5 submachine gun from Heckler & Koch, a roller delayed blowback system:



(this link is for a much larger graphic of the same picture, which may be helpful: http://upload.wikimedia.org/wikipedia/c ... r_Koch.png)

In these roller delayed blowback actions, the barrel doesn't move -- it is attached to the frame. So this is not a recoil system. Nor is there any gas porting for a piston or a direct impingement arrangement. What is it that powers the bolt to move backward, cock the lock and eject the spent round, and cycle the system? There is no provision for the system to lock, and the bolt moves backward under the gas pressure of the fired round, just like any other blowback system.

What is different is that the rollers fit into recesses in the receiver ring, behind the barrel. They do not lock in these recesses, but the rear face of these recesses is shaped like a ramp. In order for the bolt to move backward, the rollers must move inward along that ramp, and at the same time, they pinch the tapered part of the bolt (#3). This arrangement of two cam surfaces interacting through a roller acts as a third class lever, where the high power - small movement of the short side of the lever acts against the lower power - long travel side of the lever, which is resisted by the inertia of the bolt and the recoil spring. The movement of the bolt is instantaneous after firing, since the bolt is not locked. The need to have a very heavy bolt and stiff spring is thus moderated by the use of these dammed surfaces, which provides the bolt and spring great mechanical advantage over the reward movement of the bolt.

Moving the bolt by pulling on a lever attached to it is thus easy, but pushing down the barrel on the bolt face requires a force of ~ 50,000 psi to move it rearwards.

Some of these systems have problems with extraction, since the gas pressure forces the cartridge case against the chamber wall. Flutes between the chamber wall and the cartridge case bleed some chamber pressure back to the outside of the case, counteracting the pressure inside the case and keeping the case from sticking in the chamber and jamming the action.

The patent diagram shows no locking with the firing pin, although locking of the firing pin in some actions may be necessary to prevent slam-fires. As for the movement of the firing pin unlocking the action, I would wonder, what moves the firing pin backward, unless the primer is pierced or is set back in the case? I don't see how this would work in practice, nor is it depicted here.

Here is the same principle in a Swiss Sturmgewehr 57:



Although the specifics differ, it is the same matter of rollers, acting against ramped cam surfaces in the receiver pressing against the tapered end of the bolt as the bolt head is moved rearward, multiplying the inertia of the bolt and the pressure of the spring resisting that rearward movement.

Here is a description of the process from Heckler & Koch: (below taken from http://www.hkpro.com/index.php?option=c ... cle&id=133)

As the leading manufacturer, we feel that the time has come to depict the bolt system of the G3 rifle in such a manner that, in addition to the circle of knowledgeable weapons experts, it can also be understood by technically interested weapons enthusiasts, without neglecting technical and physical accuracy. Numerous tests confirm the validity of the theoretical principles described below.

Let us assume it is known that the simplicity and ruggedness of automatic weapon bolts, comprised only of a mass and a spring cannot be surpassed.

For this reason, our discussion should begin with this type of simple bolt system (Figure 1), especially since the bolt system of the G3 rifle represents nothing more than an elegantly designed modification of an inertia bolt.

In an inertia bolt, the projectile momentum, i.e. the impact of the expanding gases, is transferred to the bullet (2) in one direction and to the bolt mass (4) via the cartridge case (3) sliding out of the chamber, in the opposite direction. The recoil spring (5) supports itself agains a mass (6) and returns the bolt mass to its initial position.

[ Image ]
Figure 1: Transfer of impact to the bolt mass
The ratio between projectile and bolt velocity is just as simple as this of the major components arrangement.

At any moment during barrel time,

(bullet mass + 0.5 x load mass) x bullet velocity = bolt mass x bolt velocity. The cartridge case obturation has been ignored in this case, as it is of no importance in considering the bolt system.

If a weapon with an inertia bolt were to be designed for 7.62 x 51 NATO ammunition, the bolt would have to weigh approximately 14 kg or 31 lbs.

To reduce this weight, much too high for practical purposes, without increasing the corresponding cartridge case extraction velocity (which could cause the cartridge to burst), the HK roller locked bolt is built in two sections. In such a manner that during a certain time, the bolt head, driven by the cartridge case as it slides out, imparts a velocity greater than its own to the bolt head carrier, connected to it by a roller transmission.

Thus the bolt head drives the bolt head carrier by means of an interposed transmission.

The lever transmission is especially well suited for explaining the bolt system of the G3 rifle.

[ Image ]
Figure 2: Transmission ratios in the G3 Bolt
The distance ratio a:b at the lever (6) is 1:4 in the G3 rifle, for example. With reference to the receiver (9), the path of the bolt head carrier (7) quadruples with respect to that of the bolt head (5). Because of the form locking connection between the two sections of the bolt, both sections travel their different distances in the same amount of time, i.e. during the duration of the effect of the impact (3), so that the velocities of both sections remain in the same ratio as the distance traveled. Bolt head and bolt carrier move longitudinally within the receiver; the lever (6)must support itself in point A to provide the distance and/or velocity ratio between the bolt head and carrier.

Because of this support in the receiver, the impact, (3) transmitted to the bolt head (5) through the cartridge case (4)and occurring while the powder burns, affect the bullet (2) in one direction and simultaneously the bolt head carrier and the receiver in the opposite direction. Meant by receiver or mass, are all parts which are rigidly connected with the receiver, e.g. barrel (1), and trigger assembly housing.

[ Image ]
Figure 3: Momentum diagram in the G3 bolt.
If the lever ratio a:b = 1:4 is used as the distance and/or velocity ratio for the receiver mass m3, the ratio a:c = 1:3 applies for the distribution of momentum, i.e. 1/4 of the bolt head face momentum drives the bolt head carrier and the remaining 3/4 drives the receiver and the parts rigidly connected to it.

Applied to the bolt system of the G3 rifle, this means the following:

The velocity ratio R = a:b between bolt head and bolt carrier, characteristic of this rifle's bolt system, related to the receiver, permits a reduction of the bolt weight required in a pure inertia bolt to a value which results from the formula

Bolt weight

R2

In order to drive the bolt carrier with the increased velocity during the unlocking period, the G3 bolt system does not have a lever, but an angular transmission, installed symetrically to the axis of the bore, with two rollers as transmission elements.

The ratio of the bolt head carrier with the locking piece travels backward four times faster than the bolt head, and only as long as the rollers move on the inclined surfaces of the barrel extension and locking piece.

[ Image ]
Figure 4. Ratio between bolt head and bolt head carrier with locking piece

During the last phase of the bolt's rearward travel, i.e. locking, the bolt head carrier forces the bolt head forward via the locking piece. This causes the laterally projecting locking rollers to engage again. the bolt head comes to rest at the cartridge base and the rollers against the cam surfaces of the locking piece.

I will note and concede that Heckler & Koch describe this process as "locking," but as their explanation makes clear, nothing is really "locked," although the process accomplishes the same purpose. Since the cartridge case begins to move under the pressure of the expanding gasses as soon as the gun is fired, rather than the breech being unlocked later by the delay of a recoil system (requiring the barrel to move) or the delay of a gas system, which doesn't unlock and begin to cycle until the bullet passes a gas port, the whole business meets the definition of a blowback system, albeit one that is modified by a mechanical linkage that alters the effects of the bolt's inertia and the recoil spring's pressure, rather than truly locking the breech closed.