Seating depth is often a critical ingredient to accuracy. But before you can control and tune seating depth, you must be able to accurately measure seating depth in relation to chamber throat length. Once you have obtained that measurement, you can place bullets exactly where you want them in relationship to the rifling that's about to take them from static to some 200,000 rpm in just milliseconds.
There are several different seating depth measurement tools on the market, and of course there's the old-fashioned "try" method (see accompanying sidebar). I've used tools by Hornady and Sinclair, and we'll go over the use of each as well as pros and cons.
Hornady Lock-N-Load O.A.L. Gauge
Priced at $43 (straight) and $49 (curved for use with limited-access chambers) these simple tools must be paired with a $6 to $8 "modified cartridge case" for use with your particular caliber. The modified case is simply a new, sized cartridge case with the primer pocket drilled out and threaded to accept the front of the gauge tool.
To use, thread the modified case's base onto the threaded end of the tool, insert the bullet you need to measure into the mouth of the modified case, making sure that the plunger running through the center of the gauge allows the bullet to seat deeply — well past practical depth — and insert the front of the gauge into your rifle's chamber. You'll need to first remove the bolt or open the action of a single-shot rifle when using the straight version of the gauge, or lock open the action of a semiauto, pump-action or the like if using the curved version.
While maintaining firm pressure on the gauge to keep the modified case well-seated in your rifle's chamber, gently slide the plunger that runs inside the body of the gauge forward, pushing the bullet inside the modified case forward until it contacts the rifling. Maintain pressure on both the gauge and the plunger and snug down the thumbscrew to lock the plunger in position.
With the plunger locked, draw the gauge out of the chamber and shake the bullet out. You may need to gently run a rod into the muzzle to bump the bullet rearward out of the chamber throat. Be sure to use a clean, coated one-piece rod that you can gingerly slide into the muzzle and down the bore to pop the bullet out. You don't want to damage your muzzle's crown. And it's critical to gently push the bullet out of the chamber, since any blunting or distortion to the nose of the bullet will invalidate overall length measurements.
Once dislodged, replace the bullet into the mouth of the modified case. Contact with the front of the gauge's plunger will position the bullet at precisely the same overall length as it was in the chamber. With a quality caliper, measure the overall length (there's a cut in the body of the gauge to allow your caliper to measure from the base of the modified cartridge), note it, and you've got your maximum cartridge length reference number. Paired with a benchrest-quality seating die stem marked in thousandths of an inch, you can now achieve and control very precise seating depth adjustments.
Only one Achilles' heel affects this tool: To accommodate a broad spectrum of firearms, the modified cases are made to minimum size, and sometimes they'll give a slightly shorter base-to-bullet-tip measurement than a case actually fired in your rifle's chamber. The solution is simple: neck size a case fired in your specific chamber and have a local gunsmith bore out and thread the base for your Lock-N-Load O.A.L gauge. The resulting modified case will provide perfect measurements in your chamber. Hornady will also perform the drilling/tapping service if you send in your neck-sized case. This is also a good solution for odd calibers not listed in the standard modified case lineup.
Pros: simple, fast, intuitive; curved version compatible with all action types
Cons: requires caliber-specific modified case
Sinclair Bullet Seating Depth Tool
While it is less intuitive to use, is arguably a bit more awkward and won't work in chambers without inline access through the rear of the action, Sinclair International's $35 tool takes a reading off a cartridge case previously fired in your chamber, making it very precise, and it doesn't require spending extra dollars on modified cartridge cases.
To use with a bolt-action, place the unit's Delrin polymer rod guide into the rear of your action, drop a bullet into the chamber, and run the stainless steel measuring rod up through the rod guide until it contacts the base of the bullet. Slide one of the two metal stops onto the measuring rod with the large-diameter end forward until it contacts the rear of the rod guide. Keeping firm forward pressure on both the measuring rod and metal stop, tighten the thumbscrew on the metal stop.
Remove the measuring rod and shake or bump the bullet out of the chamber in the same fashion as described above when using the Hornady tool. Slide the other metal stop onto the front of the measuring rod — this time with the small-diameter end forward — and re-insert the measuring rod into the Delrin polymer guide until it contacts the base of the empty, fired cartridge case in the chamber. Maintaining firm forward pressure on both the measuring rod and new metal stop, tighten the stops thumbscrew, locking it in place.
You now have the precise measurement from the base of the cartridge case to the base of the bullet when in contact with the rifling. Using your dial or digital caliper, measure between the shoulders of the two metal stops locked on the measuring rod and note the number, then measure the complete length of the bullet itself. Add the two numbers together and voila! you've got your maximum overall cartridge length with that particular bullet.
Pros: takes precise measurements, doesn't require a modified case, inexpensive, adaptable to all bolt-action calibers
Cons: limited to straight-access chambers, somewhat complex to use
For both tools you have to repeat the process every time you try a new bullet manufacturer, weight or type. The OAL number obtained is valid only with the specific bullet type measured. Determining the right bullet jump/seating depth is a separate topic, but note some bullets — namely those of homogenous construction or with thick, dense jackets — must be jumped at least 0.020 to the rifling to prevent pressure spikes.