One of the unique parts of the UZI design is its bolt. The wrap around design allowed the bolt to overlap the barrel 3.75Ē on the full sized UZI carbine. That resulted in several advantages:
The original UZI was an open bolt design, although closed bolt designs followed in the Mini and Micro as well as semi automatic versions of the UZI. One of the primary differences between the open and closed bolt design is the firing pin. The open bolt has a fixed firing pin that's an integral piece of the bolt face and causes the round to fire whenever the bolt travels forward to a closed position. The closed bolt design has a floating firing pin thatís attached to a striker assembly. That allows the firing pin to move forward under sear control, independent of the bolt itself. The closed bolt design, having an additional recoil spring on the striker assembly, will significantly increase the rate of fire over an open bolt. The closed bolt design is also considered to be more accurate, particularly in semi automatic fire, because the bolt does not move between the time the trigger is pulled and the round is fired. Another difference found in bolts is the bolt face. The bolt face on a full automatic bolt (either open or closed) is fully supported, meaning that the rim on the outside of the bolt face goes most of the way around. This rim helps the cartridge case pass over the firing pin as itís being stripped out of the magazine. The semi automatic bolt design uses a partially supported rim, where the bottom portion of the rim has been milled flat with the bolt face. Due to the unavailability of closed bolts for the full size UZI SMG, bolt conversions are sometimes done.
Another variable found in bolts is their weight, modified to control rate of fire. In the mid 1980's IMI experimented with a "heavy bolt" for the Mini UZI. It had tungsten inserts to increase the weight of the bolt and thus reduce the rate of fire. The IMI heavy bolt was never available for sale. However, UZI Talk member barrelxchange recently produced Mini UZI heavy bolts and are available in both 9mm/.40S&W and .45ACP. For more information, contact Troy at email@example.com.
Another modification is a light bolt for the full size UZI to increase rate of fire. While never a production item, individuals have sometimes modified bolts themselves by milling out section section to reduce the weight. This is a difficult process due to the hardness of the bolts. A much easier way to increase rate of fire is to add a rubber buffer behind the bolt, thus decreasing the bolt's range of motion.
Comparison of 9mm UZI Bolts
|The bolts for the UZI Carbine, Mini, Micro and Pistol maintain the same width and height but vary in length. Additionally, the bolt for the semi automatic version of the Carbine has a slot cut into the right side, allowing the bolt to pass over the blocking rail that is welded on the inside of the Model A and Model B receiver. Note that it is illegal to cut a slot in the side of a standard SMG bolt in an attempt to use it in a semi automatic UZI. Cutting the slot is considered manufacturing a machine gun and is illegal regardless of whether or not you put it in a gun or even own an UZI.|
Following is a comparison of several Uzi bolts. This information is provided for identification purposes only. No one should ever try to convert one bolt to a different type. Note that these measurements and weights are taken from individual specimens. They are not factory specs. Weights for the closed bolts are taken without the striker assembly.
With the gaining popularity of the .40 S&W for law enforcement use, IMI manufactured a few closed bolts for the Mini UZI in that caliber.
Recoil springs for the different model UZI's vary. Not only between full size, Mini and Micro but also between full automatic and semi automatic. The red pad on the SMG recoil spring holds the back end of the spring in the proper place. The semi automatic spring does not need one because the spring is held in place by the large recoil buffer that's inserted in the back of the semi receiver.
Mini UZI Bolts
The full auto version of the Mini UZI bolt has two little "feet" that stick out the front of the bolt while the semi auto version of the Mini UZI does not. Many people wonder just what those little feet do. On a full auto Mini UZI firing from an open bolt, the bolt must start from rest and gain enough momentum to strip the first round from the magazine. To gain that momentum the bolt must sit back from the magazine. Because the sear catches the bolt on the feet that project out the front of the bolt, it causes the bolt to sit farther back in the receiver and thus it can gain more momentum when it's fired. Two openings are made in the trunion that the feet slide into when the bolt closes, allowing it to close fully on the chamber. The same effect could be achieved with a longer bolt, but using the feet permits a shorter and lighter gun. The semi automatic Mini fires from a closed bolt and so it's not a problem. Registered bolts for the Mini UZI are typically full auto bolts with the feet cut off. (See "Legal Issues" below.) Because of that, the bolt must be pulled back far enough to make sure it engages the rim of the first round before firing. When the first round is fired, the bolt is already resting on the case instead of striking it with momentum. That can lead to light primer strikes or failure to feeds.
Any full auto bolt that can be inserted into an unmodified semi automatic receiver is considered a machine gun. Such a bolt has to be registered as a machine gun or it's considered an illegal machine gun. Therefore cutting a slot in the side of a full auto bolt is illegal (unless done by a class II dealer to produce a post 1986 machine gun) because it would allow the bolt to be used in a semi automatic UZI. In the same way, a semi automatic receiver cannot be modified to accept a full auto bolt. That would include removing the blocking bar from the inside of a full sized receiver or cutting holes in the trunion of a Mini UZI to accommodate the feet on a full auto Mini bolt.
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Last Modified: January 29, 2006