will fl atten in response to any given internal pressurization. The former is a bit more complicated: Regarding effec-
tive, working headspace (which is how far the chambered round can move endwise in the locked chamber), striker impact and primer explosion drive the case forward, rather violently. A small rifl e primer will drive the case forward with about 500 pounds force; a large-rifl e primer will drive the case forward with about 1,000 pounds force (wild approximations but suffi cient to demonstrate that the primer explosion will almost certainly drive the case as far forward in the chamber as it will go in response to applied stress). Equally, the primer will be driven as far out of the case as it will go — until it hits the bolt face and drives that rearward as far as it will go in response to applied stress. The important point here is this: The greater the effective headspace, the more the primer will, upon fi ring, initially protrude from the case. Then, while chamber pressure is inflating the thin
primer cup (much as a balloon), chamber pressure drives the case head back until it touches the bolt face, thereby reseat- ing the primer. In this manner, that portion of the primer that once protruded from the case head swells and therefore fl attens, just as if pressure had been higher and headspace had been less. As a result of these variables, of itself, primer appearance is completely useless to a novice. With suffi cient, observant experience, seasoned hand-
loaders can use primer appearance as a decent proxy for actual chamber pressure: If we know the gun, the load, how the cartridge fi ts the chamber, and the characteristics of that particular primer (keeping in mind that primer cups tend to age harden and that no two production lots are identical!). Nothing to it!
OBSERVING CASE HEAD MARKING Next, consider case-head marking that occurs when a
round is fi red in a gun that uses a plunger ejector. This one is a bit more reliable. Generally, if the round does not create a bright circle on the case head, chamber pressure probably is not excessive. Conversely, if such a ring appears, pressure probably is excessive. This bright ring forms when chamber pressure exceeds
case head strength and therefore drives head material back into the ejector hole — slightly. When this happens, as we turn the bolt, the bolt face deforms (thereby polishing) the surface of this raised area, as the bolt face necessarily forces the case deeper into the chamber — so the bolt can turn. This is one reason hot loads generate diffi cult bolt lift. (The same basic thing happens in a rifl e with a fi xed ejector when head material fl ows into the ejector slot in the bolt face.) One problem with this method is obvious. How much
pressure the case head can tolerate before extruding signifi - cantly into the ejector plunger hole (or slot) depends on how hard and thick the case web is and on how large the ejector plunger hole is (the smaller the hole, the more effective the brass of the case head will be at bridging this gap). A second problem with this method is that case shoulder
design and thickness and hardness of the brass in the case shoulder affect what we see and feel. A narrow, gentle shoul- der (such as a 358 Winchester case has) and a thin, soft case shoulder allow the bolt to easily drive the case forward by moving the shoulder back. Conversely, a wide, sharp shoul-
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• Action forged from steel billets • Premium hammer forged barrel
• Trigger adjustable for weight of pull
• Integral scope dovetail • Detachable magazine • Walnut stock
• Rifles and additional barrel kits availabe with or without sights.
• .22 LR, .22 WMR and .17 HMR PROTECT • HUNT • COMPETE
CZ's new bolt action rimfire allows the shooter to convert their rifle to a new caliber in just minutes.
The CZ 455 retains the accuracy and quality that have made CZ rimfire models favorites with shooters, both in the woods and in competition.
Also available in a .22 LR/.17 HMR 2 barrel combo package.
CZ 455
AMERICAN
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