Thursday, July 29, 2010

Stocks: Bedding

Now that we've studied how stocks are made, it is time to study an operation called bedding. This is an operation that improves the accuracy and life of the weapon. So what is the need for proper bedding of the rifle?
  1. Eliminate stress to the barrel and action, for longer life
  2. Prevent flexing of the action when the weapon is fired for greater accuracy
  3. After the weapon is fired, the action and barrel should return back to the same position on the bedding, to ensure repeatability of shots. This is called the rifle's ability to maintain zero.
Poor fitting of the stock to the action causes many problems with the rifle. This problem is further compounded because of expansion of the metal parts, as well as warping of the stock (especially in the case of wooden stocks). By removing wood from the areas of contact between the stock and the barrel and providing a much more stable substance such as fiberglass composite instead, the fit between the barrel and the stock can be much more precise and eliminate lots of problems.

There are many forms of bedding, but we will look at two major types: Glass bedding and pillar bedding.

Generally speaking, injection molded stocks are not really suitable for glass bedding. It is mostly used for composite hand laid fiberglass stocks, wooden stocks and laminated stocks. There are several ways that people bed the rifles. Some only bed the barrel and leave the action (firing mechanism) floating in the air, some only bed the action and the parts behind it and leave the rest of the barrel not touching any part of the stock, some bed the action and the barrel completely. For a rifle to fire best, there must be constant pressure dynamic between the stock and barrel throughout the length of the stock.

Glass bedding is done by using a slow drying epoxy. The epoxy should have the properties of hardening when dry and should not shrink or expand with temperature differences. To make sure that the epoxy does not dry inside the action and jam it, the trigger assembly is first disassembled. Then modelling clay and masking tape or electrical tape are applied along the holes to make sure that the epoxy doesn't enter there. Release agents are sprayed on or manually applied with a brush to key areas, such as the action and underside of barrel, to make sure that the epoxy does not stick there. In the case of wooden stocks, the surface of the wood in the barrel channel is lightly scraped away with a chisel to remove any oils and greases before the epoxy is applied. This allows the epoxy to get a better grip on the stock. The gun is assembled and epoxy is poured into the barrel channel and allowed to harden. After one or two hours, the excess epoxy is scraped away and then the rest is allowed to harden for a few days. After the epoxy is cured, the parts can be disassembled as the epoxy does not stick to the areas where the release agent was previously applied. The masking tape is also removed and the rifle is now ready for use. The two videos below show how the glass bedding process is done.

Pillar bedding was first invented by the Germans in the late 1800s for the Mauser rifles, but did not really catch on in the United States until the 1980s or so. With pillar bedding, two precision metal pillars are affixed to the barrel using screws, and holes are drilled in the stock to accommodate these pillars. The pillars were originally made of steel, but these days aluminium is more often used. It can be used with injection molded stocks as well as wooden stocks, laminated stocks and hand laid fiberglass stocks, since the vibrations are absorbed by the metal pillars first. The screws and threads on the pillars are precisely machined to allow micro-adjustments as needed. Holes are drilled in the existing stock to allow the pillars to pass through them. The rest of the procedure is similar to glass epoxying in that masking tape and release agent are applied and then epoxy is poured into the channel to harden around the rest.

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