January 12th, 2020

Cartridge Headspace 101 — Understanding the Basicss

Brownells Headspace Gauge cutaway chamber drawing SAAMI ANSI

Do you know what the inside of a rifle chamber (and throat zone) really looks like? Do you understand the concept of headspace and why it’s important? If not, you should read the Brownells GunTech article Gauging Success – Minimum Headspace and Maximum COL. This article explains the basics of headspace and shows how to measure headspace (and length to lands) in your barrels with precision. The article also explains how to adjust your full-length sizing dies to “bump the shoulder” as needed.

Why is headspace important? The article explains: “Controlling headspace and setting proper C.O.L. also represent improved safety and reduced cost of handloading. Excessive headspace can cause case head separation and gases in excess of 60,000 PSI escaping from a rifle’s chamber. Too little headspace can result in a chamber forced bullet crimp and a bullet that becomes an obstruction more than a properly secured projectile. Excessive C.O.L. can result in a rifling-bound bullet, a condition that could result in spikes of excessive pressure.” [Editor’s NOTE: It is common for competitive benchrest shooters to seat bullets into the rifling. This can be done safely if you reduce your loads accordingly. With some bullets we often see best accuracy .010″ (or more) into the lands. However, this can generate more pressure than the same bullet seated .010″ away from initial lands contact. As with all reloading, start low and work up gradually.]

Brownells Headspace Gauge cutaway chamber drawing SAAMI ANSI

How is headspace specified? Most cartridges used within the United States are defined within ANSI/SAAMI Z299.3-4. Brownells explains: “In the case of the .243 Winchester, as an example, there are pressure specifications, cartridge drawings and, as pictured above, chamber drawings. Armed with a chamber drawing, each manufacturer producing a firearm for the .243 Winchester knows the proper standard dimension to cut chambers and set headspace. Notice there are two headspace reference dimensions for the chamber. The upper is a place in the chamber where the shoulder is .400″ in diameter; the “datum” or “basic” line. The lower is the 1.630″~1.640″ minimum – maximum dimension from the breech face (bolt face) to that point in the chamber that measures .400″.”

The actual headspace of any firearm is the distance from the breech face to the point in the chamber that is intended to prevent forward motion of a cartridge.

Finding Cartridge Length to Lands with OAL Gauge
Using a comparator on a set of calipers, you can quickly determine catridge base-to-ogive length. This is the measurement from the base of the case to the forward-most full diameter section of the bullet, typically called the ogive. Shown here, that ogive is 0.243″ diameter.

The next step is using a modified (threaded) case with a Hornady OAL tool to determine Length-to-Lands (LTL) in your rifle’s chamber. During this measurement process, the modified case, with a bullet in its neck, is inserted in the chamber. Go slow, take your time. Here are 5 tips that will help you get repeatable and reliable LTL measurements:

1. Start with a clean chamber and clean barrel throat.
2. Make sure the modified case is fully screwed down and seated on the OAL Gauge. It can sometimes unscrew a bit during repeated measurements.
3. Insert the modified case slowly and gently, but ensure the shoulder of the modified case is fully seated on the end of the chamber.
4. Push the gray plastic rod GENTLY. It is common for the bullet to be tilted a bit. You want to allow the bullet to self-center in the throat BEFORE you apply much pressure. Then tap a couple times and push until you feel resistance. Do NOT push too hard — that will jam the bullet in the lands.
5. Repeat the measurement at least 3 more times. If you follow our instructions, you should, typically, get a repeatable measurement, within 0.015″ or so, 3 out of 4 times.

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