Determining Bullet Jump
March 24, 2011
Optimum bullet freetravel (for best accuracy) prior to rifling engagement in the barrel of my rail gun varied from bullet to bullet.
An RCBS caliper, seating-depth tool, and hex-style comparator were used to establish the correct amount of bullet jump for the chamber of the rail gun.
Optimum bullet freetravel (for best accuracy) prior to rifling engagement in the barrel of my rail gun varied from bullet to bullet. In determining this, I used a couple of items from Sinclair International. One called the #59-4000 Bullet Seating Depth Tool consists of a stainless-steel rod with two adjustable stops and a Delrin guide for the rod. After the overall length of a particular bullet is measured, it is pushed nose first into the chamber of the barrel until its ogive makes contact with the rifling lands. I inserted a full-length cleaning rod guide into the receiver of my rail gun and used a cleaning rod to push a bullet through it until it rested firmly against the lands. The cleaning rod guide was then removed and replaced by the shorter guide that came with the rod of the seating-depth tool. After the tool rod is pushed through its guide until its end makes contact with the base of the bullet, one of the stops is pushed against the rear of the guide and its retention screw tightened. Then everything is removed, including the bullet, which is pushed out by inserting a cleaning rod through the muzzle of the barrel. The next step is to insert a fired case fully into the chamber, insert the rod through its guide until its end is resting against the head of the chambered case, and then lock the second stop in place with its retention screw. A caliper is used to determine the distance between the two stops, and that measurement is added to the length of the bullet to come up with overall cartridge length with the bullet resting against the lands.
For example: A Swift 165-grain Scirocco II is 1.350 inches long, and adding the stop-to-stop measurement of 1.440 inches to that gave me an overall cartridge length of 2.790 inches. When loading ammo for a hunt, I prefer to seat bullets no closer to the lands than .010 inch, and as I determined through trial and error accuracy testing, the Swift bullet shot best when it was seated .020 inch shy of rifling contact. That put cartridge overall length (from head of case to tip of bullet) at 2.770 inches.
This system alone is accurate only with a bullet that does not vary in length, which is usually the case for those with plastic nose inserts like the Swift Scirocco II.
Bullets of softnose and hollowpoint design often vary slightly in length, and for those I use, along with the Sinclair seating-depth tool, another handy gadget from the same company called a Hex-style Bullet Comparator. After using the seating-depth tool to determine overall cartridge length, I use the comparator--along with a caliper--to measure from the head of the case to the point on the ogive of the bullet (loaded in a dummy cartridge) that contacts the rifling and start from there for adjusting bullet jump. For the Swift bullet, that measurement was 2.104 inches or 2.084 inches for a jump of .020 inch. It all sounds a bit complicated, but the learning curve is actually quite short.