Cartridge Headspace — Understanding the Basics
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.]
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.
Similar Posts:
- Cartridge Headspace 101 — Understanding the Basics
- Headspace — How to Measure This and Set Dies Accordingly
- Headspace 101: What You Need to Know
- Headspace Basics — What You Need to Know
- How to Understand Headspace — What You Need to Know
Tags: chamber Brownells, Headspace, SAAMI, Tech Tip
I recommend adding “Neck” to the otherwise-excellent photo illustration at the top of the page. It is one of the most often-quoted and discussed portions of the chamber.
I realize that Brownells simply borrowed the illustration with the nomenclature of the chamber but they should have gotten it right before they published it the first time in their GunTopics:
Gauging Success – Minimum Headspace and Maximum COL
Here is the correct description and location of the leade and throat:
About Free bore
From a machinists perspective (including reamer manufacturers), free bore starts at the case mouth and is measured forwards to the rifling.
From a shooters perspective, free bore refers to bullet jump, measured from the ogive of the projectile forwards.
Same term, two completely different uses of the term.
Going into a bit more depth (for those who want a headache):
Reamer manufacturers use the terms free bore and lead (or leade) interchangeably. In other words, free bore and lead both refer to the area from the case mouth to the beginning of the rifling taper.
Reamer manufacturers use the term throat to describe the ogive shaped part of the reamer. This describes the length of the taper before it comes into contact with the projectile. The throat of a chamber has two important measurements (to the reamer maker), length and angle.
Try not to dwell on this as it can be confusing for industry outsiders. Reamer makers such as PTG are very smart in that if you as a consumer say that you want a long throated cartridge, they translate this into meaning a long free bore cartridge and will work towards this goal accordingly.
A throating reamer cuts both lead and throat angle.
http://www.ballisticstudies.com/Knowledgebase/Determining+COAL.html
kurz, free bore and leade aren’t interchangable are they? free bore I have been taught is the neck to the start of the LEADE, the leade is the beginning of the rifling which is cut at an angle/taper.
free bore = throat.
bullet jump = proj. distance to contact w/ leade
http://illirianengineering.com/uploads/3/0/2/6/3026173/9323416.jpg?585