OAL and Free-Bore
Date: Jun 04 2010
When you are setting the overall loaded length (OAL) of rifle cartridges you handload, you have to make sure the loaded rounds are short enough to fit in the rifle’s magazine box or detachable magazine. If you build your ammunition to the specifications in the load manual, this is generally not a problem. With single shots this is not a concern either. Cartridges that fit in the magazine box should not be your only concern. Sometimes you can improve the accuracy of a certain load by controlling the distance from the point on the bullet where the bearing surface begins to where the lands start in the chamber.
With magazine-fed rifles you may not be able to shorten this distance because seating the bullet out farther would cause the cartridge to be too long to work in the magazine. Some cartridges, like the .257 Roberts, tend to be perfect candidates for this, because they are often housed in actions that are longer than necessary. (Many .257 Roberts rifles are built on long actions.) Also, with single shots, you may find the throat is cut long and you need to seat bullets out farther so the jump to the lands is not excessive.
There are a number of ways to do this. Last month we covered the RCBS Precision Mic and how you could use it to adjust your sizing die to match the headspace of your rifle. You can use that same tool to discover the distance from the bolt face to the lands in your rifle’s chamber just as easily.
Each cartridge-specific RCBS Precision Mic comes with a base that resembles a cartridge case. There is also a bullet shaped tip for the base that has a nylon connector. In the bottom of this connector is a screw that allows you to tighten the nylon connector in the base. This is called a free-bore tool. To determine the distance from the bolt face to the lands in the chamber, start by inserting the nylon connector into the base about ¼ inch. Then, place the free-bore tool into the base of the precision mic. Install the land nut and check the length. Adjust the free-bore tool until the reading is approximately .300 inch.
Remove the tool from the mic and while holding the tool by the nylon section, tighten the screw by inserting a hex wrench through the base. Now you will need to chamber the free-bore tool in the rifle. Be careful to ensure the rim of the base is behind the extractor and also be careful that while chambering the tool it does not hit against the edges of the chamber. When you close the action, the tip of the free-bore tool will engage the lands and the nylon connector will set back in the case. Be careful when removing the free-bore tool from the rifle so that you don’t further influence its setting.
Now you need to insert the free-bore tool back into the base of the precision mic and gently screw on the land nut so that it contacts the tip of the free-bore tool. Do not tighten excessively; this will further push the tip into the base of the free-bore tool and give you an incorrect reading. Write down this reading and assemble a dummy round, seating a bullet so that it gives the same reading when measured with the precision mic. When chambered, this dummy round should just contact the lands of the rifling in your rifle’s chamber.
After you have made sure a cartridge with this OAL will work in your rifles magazine, you can proceed to determine the OAL that will provide the best accuracy in your rifle. Start by loading three rounds with the bullet seated .010 inch back from the lands. Obtaining this .010 setback is easy now; just seat the bullet so the cartridge OAL is .010 shorter than the dummy round you loaded to mimic the free-bore tool. Load six additional rounds: three seated .020 inch back from the lands and three seated .030 back. (These cartridges would have an OAL of .020 and .030 inch shorter than the OAL of the dummy round that mimicked the free-bore tool when measured in the precision mic.)