COAL, bullet jump, throat geometry

[7000grain]

It’s Friday Night Reloading, and this week we’re covering one of the most talked-about but least understood subjects for new reloaders: COAL, bullet jump, throat geometry, and why reloading for your specific rifle almost always beats factory ammo.

Let’s start with the basics.

COAL (Cartridge Overall Length) is the distance from the base of the case to the tip of the bullet. COAL mainly answers two questions:
• Will this round fit in my magazine?
• Is it within published safe limits?

That’s it. COAL is important for safety and feeding, but it is not a precision measurement.

The reason is simple: bullet tips vary. Even bullets from the same box can be slightly different lengths. Measuring to the tip can give you different numbers even when the bullets are seated perfectly.

What actually matters for accuracy is where the bullet contacts the rifling.

That contact point is on the ogive, which is the curved section of the bullet just behind the tip. The ogive is what touches the rifling (the lands) when the round is fired.

Because of that, experienced reloaders measure base-to-ogive instead of base-to-tip. Base-to-ogive measurements are consistent and tell you what the bullet is really doing inside the barrel.

Now let’s talk about bullet jump.

Bullet jump is the distance the bullet travels after leaving the case before it touches the rifling. Every rifle has bullet jump. The goal isn’t to eliminate it, but to control it.

Different bullets behave differently. Some shoot best close to the lands. Some don’t care. Factory ammo usually has a lot of jump because it has to work safely in every rifle chambered for that cartridge.

To control jump, you first need to know where the lands are in your rifle. Load manuals can’t tell you that.

This is where the Hornady Lock-N-Load Overall Length Gauge comes in, made by Hornady.

There are two versions:
• Straight gauge – SKU 050033
• Curved gauge – SKU 050032 (for tight actions)

Typical price is about $35–$45.

You’ll also need a modified case. Hornady sells them, but you can make your own using a fired case from your rifle, which often gives better results.

To make your own modified case:
• Use a fired case from your rifle
• Lightly size the neck so it gently holds a bullet
• Drill out the primer pocket
• Tap the pocket to 5/16″-36 TPI (this thread pitch is critical)

Use cutting oil, go slow, and keep the tap straight.

Here’s the simple step-by-step to measure your lands:
1. Thread the modified case onto the gauge
2. Insert a bullet
3. Gently push the bullet forward until it just stops
4. Lock the gauge (do not force it)
5. Remove the assembly
6. Measure from the case head to the ogive using a bullet comparator

That measurement is your “touching the lands” length for that bullet in that rifle.

For base-to-ogive measurements, you’ll need a comparator. Without one, you’re guessing. Expect to spend around $35–$40 for a comparator set.

If you repeat the measurement and it changes slightly, that’s normal. Go slow and use light pressure.

Once you know where the lands are, you back the bullet off to create jump.

Common starting points:
• Bolt-action rifles: about .020″ off the lands
• Magazine-fed bolt guns: .040″–.060″
• Semi-autos: usually .060″ or more

Critical safety rule for new reloaders:
Any time you change seating depth, especially moving closer to the lands, reduce your powder charge and work back up. Seating depth changes pressure. This is not optional.

Now let’s explain throat geometry in plain terms.

Between the chamber and the rifling is the throat. It includes:
• Freebore (smooth section)
• Leade (taper into the rifling)
• Lands and grooves (the rifling)

Some rifles have long throats, some short. Military chambers are usually long. Match chambers are usually short. That’s why two rifles in the same caliber can want very different seating depths.

Bullet shape matters too. Some bullets are more sensitive to jump than others. There is no universal “best” number.

Another thing new reloaders often overlook is neck tension. If the bullet isn’t held consistently by the case neck, seating depth changes won’t behave predictably. Consistent sizing matters more than chasing tiny jump numbers.

Now the reality check: magazines set hard limits.

If the round doesn’t fit the magazine, it doesn’t matter how perfect the jump is. Seating bullets long often turns a magazine-fed rifle into a single-shot. That’s fine if it’s intentional, but many people discover it the hard way.

There’s also a safety issue. Seating bullets very close to or into the lands can cause the bullet to stick in the rifling when ejecting a live round, dumping powder into the action. This happens more often than people think.

Gas guns need extra caution. ARs and other semi-autos have violent feeding and floating firing pins. They need additional jump for reliability and safe pressure. In gas guns, accuracy is usually tuned with powder selection first and seating depth second.

This is where the value of reloading really shows.

Factory ammo is built to be safe in the worst-case rifle: short throats, rough chambers, weak actions. Handloading lets you tune ammo to your rifle, your chamber, and your magazine. That’s why tailored handloads often outperform even premium factory ammo.

Quick troubleshooting:
• Lands measurement varies → slow down, lighter pressure
• Bullet sticks in the bore → add more jump
• COAL doesn’t match book data → normal, books don’t know your throat
• No accuracy improvement → check neck tension and powder consistency

Bottom line:
COAL is for safety and feeding.
Base-to-ogive is for consistency.
Jump is rifle- and bullet-specific.
Throat geometry matters.
Magazines always get the final vote.
Reloading works because factory ammo has to compromise.