A common question asked by Mosin Nagant collectors is "How do I tell what size bore my rifle has?" In this context the term "bore" is referring to the inside of the barrel in general and is not interchangeable with bore diameter. This may sound confusing at first, but it must be understood that the inside of the barrel has two different dimensions measured at the lands and the grooves. The lands are the high parts that impart spin to the bullet and the distance between them is the bore diameter. The grooves are the low parts between the lands and the distance between them is groove diameter. All models of Mosin Nagant rifles have a nominal bore diameter of .300" or 7.62mm. Although bore diameter will vary somewhat, it is usually not a concern unless the bore is extremely worn. What is important in bullet selection is the groove diameter. A bullet larger than the groove diameter can cause dangerously high pressure when the rifle is fired. A bullet significantly smaller than the groove diameter might not stabilize properly and be inaccurate. Ideally the bullet should be slightly smaller than the groove diameter. Because of the wide variation in groove diameter between models and even within models the only way to be sure is to slug the "bore", or inside of the barrel. The result is a good first step in determining what ammunition or size of bullet for handloading will be most accurate in a specific rifle. The process is not difficult and requires a minimum of investment in tools, materials and time. See the picture below for an illustration of the realtionship between the lands, the grooves, and the bullet.

Tools and Materials While there are commercial bore slugging kits available from gunsmith suppliers, I have put together a kit with inexpensive and readily available materials. These include 1/8 size "egg sinker" fishing weights available at WalMart, a 1/4" diameter wooden dowel from Home Depot (or WalMart), a small piece of wire I had in the shop, some gun grease (any type of thick lubricant would work), a plastic mallet, a shop towel, and a caliper for measuring the slug. The dowel is cut up into approximately 6" sections. The number required depends on the length of the barrel. The caliper is by far the most expensive item at about $25-$30. I purchased mine from MidwayUSA, but have since seen them in the tool department of Home Depot. The total kit should cost no more than about $50. For rifles of larger or smaller caliber an adjustment would need to be made in the size of the lead slugs and possibly the dowel. It has been suggested that 00 buck shot will work as a slug, but I have not tried it and like the hole through the middle of the sinker for reasons that will be obvious.

The Process For larger views, click on the pictures.

The 1/8 size egg sinkers are very close to the approximately .310" size of a Mosin Nagant bore. It is best to "flatten" them with the mallet to slightly increase the diameter before using them. Place the butt of the rifle on the towel to protect it from scratches and lean the rifle against the workbench
Place the slug on the wire to make it easier to apply the grease.
Insert the greased slug into the muzzle of the rifle.
Gently tap the slug into the muzzle with the mallet taking care to keep it straight.
Using a section of the dowel rod and the mallet, continue to drive the slug into the bore. As each section of dowel is driven in, add another section on top of it and keep driving the slug. At some point the slug may become slightly loose and can be pushed by hand. Short sections of dowel are used so that they won't bend or break if the bore is tight. The wooden dowel will not damage the bore or crown of the rifle either. Hardwood dowels are recommended as they are much stronger and less likely to break than softer wood like poplar.
The slug will fall out of the chamber where it can be recovered. It might fall onto the floor so it is best to do this in a clean area over a surface that won't be stained by the grease. Recover the slug.
Wipe the grease off the slug with the towel. There will be 4 grooves cut into the slug by the lands of the rifle. Between the grooves will be 4 corresponding high points which are left by the grooves of the rifle. Measure across two opposite high points with the calipers following the manufacturer's instructions. In this example the groove diameter of our rifle, a 1938 Tula ex-PEM sniper, is .313 inches. .311 bullets (.303 British or 7.7 Japanese) should work well.
Before shooting the rifle clean the bore as you normally would to remove any grease or small lead shavings from the slug.