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From the book "Gunsmithing: Shotguns by Sweeney"
1. Many people think the Mossberg rib is loose. The Mossberg rib is attached to the barrel at the rear with a cross pin. Along the length of the rib it is attached to each post by fitting into grooves cut into the tops of the posts. The rib has a channel underneath, and rides on top of the posts, secured by its two flanges riding in the grooves of the posts.

When a Mossberg barrel expands or contracts from the heat or cold, the rib is free to slide along the posts. Rather than being slightly bent from the rib expanding or contracting at different rate, the Mossberg barrel is free. It's a neat idea.

2. "How can the aluminum receiver of the Mossberg stand the shock of firing? By not taking it. Unlike many of its contemporary competive designs, the Mossberg bolt did not lock into the receiver to fire. Rather, it locked into an extension of the barrel. Much of the cost of the Winchester Model 12 and the Ithaca M-37 came from the need to precisely machine the receiver from a block of steel. The receiver has to be steel, because the bolt locks to the receiver when closed. The bolt had to be machined to a precise length. The locking recess in the receiver has to be machined to an exact location. The receiver walls have to be thick enough to take the load. On the Winchester 97, the carrier is also the locking block. The carrier, its pivot pin, and the receiver where the pin rests have to be strong enough to take the load. On the Mossberg 500, the bolt locks into an extension fastened to the barrel. The critical dimensions are the distance from the locking recess to the chamber, and the distance from the locking block in the bolt and the bolt face. Both are easily measured (in the factory) and kept to a precise figure.

The aluminum receiver only acts as a cover and guide to the moving parts, and does not have to take any of the forces of the firing except to transmit the force of recoil to the shooter.

3. If this was such a great idea, why did it wait until 1961 (year Mossberg 500 came out; August) to come about? It didn't. The locking mechanism is the same method used by John Moses Browning in the A-5. Browning also used it in a number of different pump actions he designed. So why didn't other manufacturers use it? Browning had the patents. By the time the patents had expired, many manufacturers had been making their models for years or decades. The customers expected a particular design, the tooling was paid for, and the cost and risk of switching was too high. Remington switched to the barrel extension in 1950 with the 870, and pulled it off. Winchester switched in 1964 with the 1200 and almost foundered. Their customer were not ready to have an icon of beauty and shotgun shooting like the model 12 replaced with an industrial tool like the Winchester model 1200.

Mossberg didn't have to worry about the patents, they had long expired."

4. Another interesting thing about the Mossberg 500 is that if the trigger housing broke or went defective the factory will fix it if you send in the shotgun. Sometimes without charge, as of 2000.

Hope you found it interesting.

regards,
Jay Gentry
Shotgunworld.com
 
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