Cheatsheet — Laminate-Ready

Print this one. Everything below is a condensation of Vols 1 through 13, arranged for the bench rather than for reading, with every number traced to its source in the earlier volumes.

14.1 The Definition

Headspace: “the distance from the face of the closed breech of a firearm to the surface in the chamber on which the cartridge case seats.” — SAAMI, quoted by Forster.

Measured from the bolt face. SAAMI’s own chamber drawings originate the dimension at a line labelled BREECH BOLT FACE.

14.2 The Four Breeching Types

Table 1 — The Four Breeching Types

Type	Stop	Headspace is	Examples
Shoulder	shoulder cone at the datum diameter	a length	223, 308, 30-06, 6.5 CM
Belt	front face of the belt	a belt thickness	300 Win Mag, 7mm Rem Mag
Rim	front face of the rim	a rim thickness	30-30, 303 British, 45-70
Mouth	case mouth	the case length	30 Carbine, 450 Bushmaster, all rimless pistol

14.3 SAAMI Dimensions — Shoulder-Breeching

Min = GO. Max = SAAMI maximum. Datum = the diameter the length is measured to. Band is 0.010 on every row.

Table 2 — SAAMI Dimensions — Shoulder-Breeching

Cartridge	Min (GO)	Max	Datum
222 Remington	1.2936	1.3036	0.3300
223 Remington	1.4636	1.4736	0.3300
22-250 Remington	1.5749	1.5849	0.3470
220 Swift	1.8060	1.8160	0.3350
243 Winchester	1.6300	1.6400	0.4000
308 Winchester	1.6300	1.6400	0.4000
6.5 Creedmoor	1.5410	1.5510	0.4000
6.5 x 55 Swedish	1.3460	1.3560	0.3600
7.62 x 39	1.2520	1.2620	0.3622
300 Savage	1.6017	1.6117	0.3968
257 Roberts / 7x57	1.7937	1.8037	0.3750
270 Winchester	2.0487	2.0587	0.3750
30-06 Springfield	2.0487	2.0587	0.3750
280 Remington	2.1000	2.1100	0.3750

14.4 SAAMI Dimensions — Other Types

Table 3 — SAAMI Dimensions — Other Types

Cartridge	Min (GO)	Max	Band
Belted — 7mm Rem Mag, 300 / 338 Win Mag, 8mm Rem Mag	0.220	0.227	.007
Belted — 257 / 7mm / 300 Weatherby	0.220	0.224	.004
Rim — 30-30 Win, 32 Win Spl, 25-35	0.0630	0.0700	.007
Rim — 303 British, 30-40 Krag	0.0640	0.0710	.007
Rim — 22 Hornet, 32-20	0.0650	0.0720	.007
Rim — 45-70 Govt, 348 Win	0.0700	0.0770	.007
Mouth — 30 Carbine	1.2900	1.3000	.010
Mouth — 450 Bushmaster	1.7000	1.7100	.010

14.5 NATO Chamberings — Forster’s Published Numbers

Table 4 — NATO Chamberings — Forster's Published Numbers

Chambering	Minimum	Maximum
5.56 NATO	1.4636”	1.4736”
7.62 NATO	1.6355”	1.6455”

7.62 NATO is 0.0055” longer than .308 Win at both ends and does not conform to SAAMI tolerances. Do not fire 5.56 NATO in a chamber marked .223 Remington. The reverse is fine.

14.6 What the Gauges Mean — Read Your Maker’s Definition

Table 5 — What the Gauges Mean — Read Your Maker's Definition

	Forster	Manson
GO	SAAMI min	SAAMI min
NO-GO	a build limit, not the SAAMI max	SAAMI min + 0.004”
FIELD	shorter than SAAMI max	SAAMI max exactly

They are not the same gauges. A rifle can pass one maker’s FIELD and fail the other’s. This is the reason to measure a number.

14.7 Gauging Procedure

1. Verify unloaded. Eye protection.
2. Bolt out. It must run freely — a sticky bolt has no feel.
3. Strip the extractor and the plunger ejector. Check the plunger can go below the bolt face.
4. Clean the chamber and the bolt face. Dry them.
5. Insert the gauge. Close on gravity on the bolt handle. Nothing more.

Should close on GO. Should not close on FIELD. Closes on NO-GO but not FIELD: still serviceable per Forster.

Never force a bolt closed on a gauge. Never force a break-open action closed on a gauge. Never tighten a barrel or barrel nut with the bolt closed on a gauge. Never fire a rifle that closes on a FIELD gauge.

14.8 Measuring the Number — The Feeler Stack

1. Micrometer every feeler blade you will use. Write the real values on them.
2. Strip the bolt. Clean chamber and bolt face.
3. Bare GO gauge — confirm the bolt closes.
4. Add shim behind the gauge head, against the bolt face.
5. Work up in 0.0005–0.001 steps until the bolt will not close.
6. Confirm the transition twice in each direction.

headspace = GO dimension + thickest shim that still closed

Record: number / over-minimum / date / round count / gauge used / step size.

Benchmark: SAAMI’s own test barrels are held to no more than +0.003 over minimum.

14.9 Barrel Rotation to Headspace

Table 6 — Barrel Rotation to Headspace

Thread	Per full turn	Per 1/60 turn
16 TPI	0.0625”	0.00104”
18 TPI	0.0556”	0.00093”
20 TPI	0.0500”	0.00083”
24 TPI	0.0417”	0.00069”

1/60 of a turn is the distance a minute hand moves in one minute.

14.10 Reloading Numbers

Table 7 — Reloading Numbers

Shoulder bump, bolt action	0.001–0.002” (Hornady)
Shoulder bump, gas gun	0.003–0.006” (common practice, not a standard)
Body diameter under fired	0.003–0.005” (common practice)
Zero the comparator on	your longest fired case
Measure a sample of	ten cases, note the spread

Bottleneck: trim length is not headspace. Mouth-breeching: trim length is headspace. Do not over-trim. Taper crimp only. Rimmed: trim length is irrelevant; sort by rim thickness. Belted: gauge on the belt, reload to the shoulder.

Lube technique changes your bump. Set the die with the same lube you will run the batch with.

14.11 Hornady Bushings vs SAAMI Datum

Table 8 — Hornady Bushings vs SAAMI Datum

Cartridge	Bushing	SAAMI datum	Match?
223 Rem	A .330	0.3300	yes
308 Win / 243 Win	D .400	0.4000	yes
30-06 / 270 Win	C .375	0.3750	yes
6.5 Creedmoor	C .375	0.4000	no
7.62 x 39	C .375	0.3622	no
6.5 x 55	C .375	0.3600	no

A comparator reading is your brass compared to your brass. It is not a SAAMI chamber number unless the bushing sits at the SAAMI datum.

14.12 Cerrosafe

• Melts 158–190 °F. Indirect heat only — overheating separates the alloy.
• Patch dam ½”–1” into the bore ahead of the throat.
• Warm the barrel until “uncomfortable to hold with your bare hand.”
• Push the cast out within half an hour. After an hour it expands and must be remelted.
• Exactly chamber size about one hour after cooling to room temperature. After ~200 hours it grows about 0.0025”.
• Do not cast a chamber that is bulged, badly pitted, or has a throat eroded wider than the chamber mouth.
• A cast cannot give you headspace — on most bolt actions the bolt face is behind the barrel breech and is not in the casting.

14.13 Reading Fired Brass — In Order

1. Proud primer — earliest excess-headspace signal. Confirm with a full-pressure load.
2. Bright ring ¼–⅜” ahead of the extractor groove — thinned brass.
3. Paper-clip hook dragged up the inside — a catch at the ring means one firing from separation. Scrap the batch, gauge the rifle.
4. Head-to-datum, ten cases, note the spread. Wide spread points at lug bearing.
5. Case length growth — 0.001”/firing normal, 0.004–0.005” is a symptom.
6. Body diameter just ahead of the extractor groove.

When the gauge and the brass disagree, the brass is right. The gauge measured the rifle at rest.

14.14 Symptom to Cause

Table 9 — Symptom to Cause

Symptom	Look at
Number grew since last reading	lug abutment set-back, lug wear, throat erosion
Gauge is fine, brass says otherwise	lugs deflecting under load
Number changed after bolt work	bolt-face truing — one thousandth removed is one added
Number different every measurement	extractor / ejector left in, or a dirty chamber
New rifle, number is what it is	where the chamber was reamed
Light primer strikes	headspace, or firing pin protrusion — measure both
Cases need trimming constantly	excess headspace, or over-generous sizing
Case separated	stop. Extract the body. Gauge the rifle before shooting it again

14.15 The Kit

Minimum: GO gauge for the cartridge, feeler set, micrometer, chamber brush and solvent. Under $120 for most cartridges.

Reloading: add a comparator with the right bushing, and a caliper you trust.

Gunsmithing: add NO-GO and FIELD, action wrench and barrel vice, torque wrench.

Diagnostic: add Cerrosafe, a dental pick, and a notebook.

Buy by family — one gauge covers a group. 270 Win uses the 30-06 gauge; 260 Rem and 7mm-08 use the 243 Win gauge; most belted magnums share one standard .535” O.D. belted gauge.

14.16 The Two Things Worth Remembering

Check every thousand rounds. Forster’s own interval. Headspace changes, and the only way you find out is by looking.

Write the number down. The first measurement is worth very little. The second one, three years later, is worth everything — it is the only way to see movement, and movement is what actually hurts you.