Seating Depth and Accuracy
Date: Apr 06 2009
Every handloader believes that by tinkering a few hundreds of an inch with the distance of the bullet to the beginning of the rifling his rifle will magically start shooting the nubbins off a button. After experimenting with various seating depths of bullets for a .223 and a .22-250 Remington, I can confirm there is a sweet spot of bullet seating depth where accuracy is the best in those rifles. However, the only way to determine that agreeable spot for your rifle is to test it. All rifles have different amounts of wear in the leade, or beginning of the rifling, and their chambers are cut with reamers of slightly different dimensions. Cartridges that fit and shoot accurately in my .223 Remington might well shoot poorly or even jam the bullet into the rifling of your .223.
Determining Bullet Seating Depth
Before experimenting with bullet seating depth, you need to determine the cartridge overall length (OAL), where a bullet’s ogive contacts the rifling. Several methods can be used to establish this OAL.
One method is to make up a cartridge with a bullet seated obviously long in a sized case with no primer or powder. Fully chamber the dummy cartridge into your rifle, then extract it. Marks should be visible on the bullet where the rifling engraved it. If these marks are difficult to see, the bullet can be first coated with soot from the flame of a stick match or candle. Carefully insert the cartridge into the chamber if the bullet has a coat of soot, because any contact with the feeding ramp can leave false marks in the soot.
With definite rifling marks on the bullet, turn in the seating stem on your seating die a quarter- to a half-turn to seat the bullet deeper. Keep chambering (and if need be, applying a new coat of soot) until there are ever so slight marks of the rifling on the bullet. Measure the cartridge length, and that OAL is where that bullet contacts the beginning of the rifling of that rifle.
Another technique uses a fired case, cleaning rod with a flat tip and a fine tip marking pen. Take a fired case and ever so slightly bend the case mouth rim, just enough so a bullet slips in with a bit of a push. Make sure the fired primer is flush or below flush with the case head face, or an incorrect reading will occur when measuring final OAL. Slip
a bullet in the case mouth and leave it protruding obviously long out of the case. Gently insert the cartridge into the chamber and close the bolt. The bullet will be resting up against the rifling. Slowly insert the cleaning rod into the muzzle until it contacts the bullet tip. Mark the rod at the muzzle. Remove the cartridge and then close the action. Push the rod into the bore until it contacts the bolt face and then mark the rod again. The distance between the two marks is the OAL where that bullet contacts the beginning of the rifling of that rifle.
The easiest way to determine where a bullet’s ogive contacts the rifling is to use the Hornady L-N-L OAL Gauge, formerly made by Stoney Point. A modified case is screwed onto the front of the gauge, a bullet put in the case mouth and the case inserted snugly into the chamber. A rod inside the gauge is slid forward to push the bullet into contact with the rifling. The rod is locked into place and the gauge removed from the chamber. The bullet will most likely remain stuck in the rifling. A cleaning rod inserted from the muzzle will knock it out. Put the bullet back into the mouth of the modified case and measure the cartridge. That length is where that bullet contacts the beginning of the rifling of that rifle.
How much, if any, the OAL should be reduced for the best accuracy depends on the rifle’s use. Some target shooters (whose guns have near-zero tolerance chambers) seat their bullets contacting the rifling, because they think the bullet will enter the rifling straightly when it is in firm contact with the rifling at the start. Often, though, a bullet will stick in the rifling when the unfired cartridge is removed from the chamber. That’s not good for hunting rifles. Plus, at that long length, a cartridge may not fit into a rifle’s magazine. Many bullet manufacturers recommend seating bullets .03 to .05 inch from contacting the rifling for the best accuracy. A maximum OAL has also been established for each commercial cartridge. At that length cartridges will definitely fit in a magazine and chamber and extract without a hitch.
I shot two rifles with cartridges with bullets seated at various depths to determine what OAL shot the best in the rifles. The first rifle was a Cooper Firearms Model 52 .22-250 Remington. This single-shot bolt action has an extremely long leade and some wear in the bore, so OAL is 2.576 inches for Sierra 55-grain BlitzKings to touch the rifling, more than .2 inch longer than the 2.350- inch maximum OAL established for the .22-250. That long cartridge length, though, makes no difference, because the Cooper has a loading pan length that will accept cartridges nearly 3 inches long.
As the load table shows, the Cooper .22-250 certainly had an OAL sweet spot of 2.546 inches. The deeper bullets were seated from that spot, the worse the rifle shot. I had wanted to seat the Sierra 55-grain BlitzKings in the .22-250 case even deeper at an OAL of 2.200 inches. However, that would have required sizing the full length of the necks. With only half of the neck sized the full diameter of the bullets was seated too far into the cases for the necks to hold the bullets.
Note in the load table that average velocity decreased as bullet seating depth increased. That’s because a .22-250 Remington case has more than enough internal capacity to hold 37.5 grains of Big Game powder. With some room left over, deeply seated 55-grain bullets do not encroach on the powder space. The deeper bullets are seated, the more pressure and velocity go down because the bullets have an increasingly unhindered running start (freebore) before they contact the rifling.
The Sisk Rifles .223 Remington is based on a short-action Remington Model 700. This handy little rifle wears a thin 20-inch barrel and is intended mainly for hunting coyotes. This rifle is continually loaded and unloaded. So an OAL that seats the bullets into contact with the rifling is impractical, because a bullet might remain stuck in the bore when the rifle is unloaded.
An OAL of 2.300 inches set Berger 50-grain Match bullets up against the rifling in the Sisk .223. As the load table shows, accuracy at that length was so-so. The rifle shot much tighter five-shot groups when the bullets were backed off the rifling .03 to .10 inch. Accuracy went downhill significantly with the bullets backed off the rifling .125 inch. No doubt that long jump allowed the bullets to enter the rifling somewhat crookedly.
The load table shows the velocity of the 50-grain bullets remained fairly constant with bullet OALs of 2.300 to 2.260 inches. Velocity dropped a bit with an OAL of 2.200 inches. However, velocity spikedwith an OAL of 2.175 inches. The 27.0 grains of W-748 pretty well filled the .223 case, and bullets seated that deeply intruded on the .223’s powder space. That was enough to raise pressure sufficiently to jump bullet velocity a good 100 fps.
A bullet seated crookedly in the case neck will enter the rifling unevenly and may ruin an otherwise tight group. So unless bullets are seated straightly in the case to begin with, it is a waste of time going to the trouble of seating bullets an exact amount from engaging the rifling. I ran all the .22-250 and .223 cartridges over an RCBS Case Master Gauging Tool. In general, the deeper bullets were seated into the necks of .223 cases, the less bullet runout occurred. The opposite occurred with the .22-250. That might be just coincidence. I do keep my reloading dies clean, as a buildup of crud in the seating die can cause a bullet to enter a case crookedly.
This experiment shows each rifle has its accuracy sweet spot of bullet seating depth. Some rifles are very particular about this seating depth, like the Cooper Model 52 .22-250 Remington. Others are more forgiving, like the Sisk Rifles .223. The only way to determine that agreeable spot for your rifle is to test it.