Basics: Seating Depth

Checking Seating Depth
by: Germán A. Salazar

Our club has many members who are relatively new to the area of precision reloading for rifles; accordingly, I will ocassionally present a small feature on some aspects of reloading that will benefit those members.  The previous topic on headspace was one such article, here is another.

Determining exactly how long a specific bullet may be seated in the case before it touches the rifling is a simple, though important, process calling for a few tools and a delicate touch.  Once you have determined this dimension, you can then vary it in an effort to extract the maximum accuracy from your rifle.  While that tuning process will be a topic for another day, a few recommendations about seating depth will be at the end of this article.  First, let's see how we determine the basic dimension.


As with any mechanical process, you'll need some tools.  For this job, I use the Hornady OAL gauge (formerly Stoney Point), the Sinclair hex nut comparator, a good dial caliper and a 1/4" x 36" wood dowel (not shown).  In addition to the basic tools, you'll need bullets from the specific lot to be loaded and the appropriate Hornady modified case for the cartridge you are working with.  The rifle's barrel should be clean for this process as any fouling tends to cause erratic readings.


Begin by threading the case onto the tool, it should be snug, but just by hand.  Retract the inner rod by loosening the lock screw and insert a bullet into the case.


Remove the bolt and insert the tool with the bullet from the rear.


Now comes the step where you'll need a delicate touch.  Push forward on the rod with the heel of your hand while pushing forward on the tool with your thumb and forefinger.  You should feel the case seating firmly in the chamber.  Then, as you push the rod forward, you will feel some resistance to the bullet's movement as it enters the throat and then you will feel it come to a stop as it contacts the rifling.  Try this a few times when you are first using the tool; depending on the dimensions of your chamber, the resistance in the throat may be somewhat high and can fool you. 

Assuming that you've now developed a good touch for when the bullet hits the rifling, bring the bullet to the touching point, then lock the thumbscrew while maintaining forward pressure on the rod and the tool as described above.  Pull the tool back slightly, insert the wood dowel into the muzzle and tap the bullet free from the rifling (it tends to stick slightly).  You may remove the bullet and tool together or separately, it makes no difference.

Now, making sure not to allow the thumbscrew to loosen, insert the bullet back into the case. 


If you don't yet have the Sincalir hex nut comparator (or something similar) you can measure the OAL (bullet tip to cartridge base) and you will have an approximate dimension at which your bullet will touch the rifling.  I say approximate, because with modern match bullets, which are almost all hollow-point types, the tips vary somewhat due to the jagged nature of the jacket when it is drawn to a point.  The bullets I used for this illustration are full metal jacket (closed point) and are a bit more consistent in their dimensions.  If you aren't using a comparator, you should take the reading with several bullets and determine an average value.


If you have the comparator (highly recommended) then slip the hex nut comparator onto the bullet (in the appropriately marked hole for your caliber) and measure the resulting overall length.  The comparator rests on the point at which the bullet reaches full diameter and thus where it contacts the rifling.  This allows for a very precise reading and one that is not affected by irregularities of the tip.

I usually denote this dimension as COAL (comparator overall length) in my reloading notes.  Obviously this is a longer dimension that a plain OAL (bullet tip to cartridge base) but the actual value of the number is meaningless except for your reference.  If the comparator were larger, for instance, the dimension would be larger, but you would still know what's right with that bullet for that rifle.

Now that you have the dimension at which the bullet will touch the rifling, what do you do next?  You can now experiment with seating depth based on whether the bullet is jumping to the rifling or is jammed into the rifling.  Adjust your seating die and check finished rounds with the comparator until you reach the desired COAL.

A good place to begin (and frankly to stay unless you're already at the Master level in NRA Highpower) is to jump 0.020" for conventional tangent ogive type bullets such as the Sierra MatchKing line or Berger's new designs and to jam 0.015" for VLD type bullets such as Berger VLD, Hornady AMAX, JLK VLD, and others of the secant ogive design. 

There are slight variations in any lot of bullets, so when you check your finished rounds you will see some variance in the COAL - don't worry about it too much.  The reason we selected the jump and jam figures given is because they allow for some slight variance and still maintain the basic concept of jump or jam.  If you were to set the jump for 0.003" for instance, then a slight variance might have some bullets jumping, some right on the lands, and some actually jammed a few thousandths.  By sticking to 0.020" jump or 0.015" jam, those same slight variances will have minimal effect - imperceptible for the Highpower shooter anyway.

There you have it - quick, simple and better scores are sure to follow.  This is an important part of handloading for your rifle, rather than simply producing generic reloads.
 

All contents Copyright 2012 The Rifleman's Journal