Full metal jacket (FMJ)
Practice and plinking
Lead base exposed on many; the nose does not expand. Good cheap practice, and not for hunting.
What's in a loaded round
The finished round is a cartridge: case, primer, powder and bullet. The bullet is only the part that leaves the barrel. Here's what every part does and how a finished round is measured.
Anatomy of a cartridge
A cutaway of a loaded round. Pick a cartridge, and hover or tap a row to find it on the drawing. Bullet shape and the inside of the case are representative.
A rimless bottleneck rifle round. It stops in the chamber on its shoulder. Drawn with a 168-grain-style match boat-tail bullet.
| # | Part | What it does |
|---|---|---|
| Bullet | ||
| 1 | Meplat | The tip of the bullet. Its size and how uniform it is affect drag. |
| 2 | Ogive | The curved nose, from where the bullet starts to narrow up to the tip. |
| 3 | Bearing surface | The full-diameter section that touches the rifling. |
| 4 | Jacket | The copper-alloy skin over the core. It holds the bullet together in the bore and controls expansion. |
| 5 | Core | The heavy centre, usually lead. Monolithic copper bullets have no separate core. |
| 6 | Boat tail and base | The rear of the bullet. A taper like this one cuts drag at long range; a flat base is common at moderate range. |
| Case | ||
| 7 | Mouth | The open end of the case. It is chamfered and deburred after trimming so it does not shave the bullet. |
| 8 | Neck | Grips the bullet. Sizing dies set how tightly. |
| 9 | Shoulder | The slope from body to neck. This rimless round stops against it in the chamber. |
| 10 | Body | The main length of the case, tapering slightly so it comes out of the chamber easily. |
| 11 | Web | The thick wall between the primer pocket and the powder space. It holds the case together under pressure. |
| 12 | Extractor groove | The channel in front of the rim. The shell holder on your press grips the case here. |
| 13 | Rim | The lip the extractor grips to pull the fired case out of the chamber. |
| 14 | Head and headstamp | The solid back of the case, stamped with the maker and cartridge. Read it before you load: it is how you know what the brass is. |
| 15 | Primer pocket | The recess the primer is pressed into. A pocket gone loose after many firings means the case is finished. |
| 16 | Flash hole | The small hole that carries the primer's flame into the powder. Boxer cases have one in the centre; that is what your decapping pin pushes the old primer out through. |
| Primer and powder | ||
| 17 | Primer | A small cup of impact-sensitive compound. The firing pin crushes it against the anvil and it lights the powder. |
| 18 | Powder | Burns fast inside the case, making the gas that drives the bullet. Charge weight comes from published load data, weighed in grains. |
A rimless, slightly tapered pistol round. It stops in the chamber on its case mouth. Drawn with a 115-grain-style round-nose full metal jacket bullet.
| # | Part | What it does |
|---|---|---|
| Bullet | ||
| 1 | Meplat | The tip of the bullet. Its size and how uniform it is affect drag. |
| 2 | Ogive | The curved nose, from where the bullet starts to narrow up to the tip. |
| 3 | Bearing surface | The full-diameter section that touches the rifling. |
| 4 | Jacket | The copper-alloy skin over the core. It holds the bullet together in the bore and controls expansion. |
| 5 | Core | The heavy centre, usually lead. Monolithic copper bullets have no separate core. |
| 6 | Base | The flat rear of the bullet. Some jacketed handgun bullets leave lead showing here. |
| Case | ||
| 7 | Mouth | The open end of the case. It is chamfered and deburred after trimming so it does not shave the bullet. |
| 8 | Body | The main length of the case, tapering slightly so it comes out of the chamber easily. |
| 9 | Web | The thick wall between the primer pocket and the powder space. It holds the case together under pressure. |
| 10 | Extractor groove | The channel in front of the rim. The shell holder on your press grips the case here. |
| 11 | Rim | The lip the extractor grips to pull the fired case out of the chamber. |
| 12 | Head and headstamp | The solid back of the case, stamped with the maker and cartridge. Read it before you load: it is how you know what the brass is. |
| 13 | Primer pocket | The recess the primer is pressed into. A pocket gone loose after many firings means the case is finished. |
| 14 | Flash hole | The small hole that carries the primer's flame into the powder. Boxer cases have one in the centre; that is what your decapping pin pushes the old primer out through. |
| Primer and powder | ||
| 15 | Primer | A small cup of impact-sensitive compound. The firing pin crushes it against the anvil and it lights the powder. |
| 16 | Powder | Burns fast inside the case, making the gas that drives the bullet. Charge weight comes from published load data, weighed in grains. |
Dimensions of a finished round
The numbers below are the cartridge limits from the SAAMI standards, which US makers of ammunition and firearms work to. Where SAAMI gives a limit with a tolerance, both ends are shown. Your manual has the numbers for your exact load, and those come first.
A rimless bottleneck rifle round. It stops in the chamber on its shoulder.
| # | Measurement | SAAMI | mm | What it means |
|---|---|---|---|---|
| 1 | Cartridge overall length (COL or COAL) | 2.490 min to 2.810 max in | 63.25 to 71.37 | Head to bullet tip. Your load data gives the length to start from; the SAAMI maximum is the longest a standard round should be. |
| 2 | Cartridge base to ogive (CBTO) | No published figure | — | Head to a set point on the ogive, measured with a comparator. Bullet tips vary, so it repeats better than COAL. You record your own; it depends on the bullet. |
| 3 | Case length | 2.015 max, 1.995 min in | 51.18 max | Brass stretches when fired and resized. Trim it back once it passes the maximum; manuals commonly give a trim-to length of about 2.005 in. |
| 4 | Headspace (head to shoulder datum) | 1.634 max, 1.627 min in | 41.50 max | Measured to the point on the shoulder where the diameter is .400 in. This is what stops the round moving forward in the chamber, and what your sizing die setting controls. |
| 5 | Bullet diameter | .309 max, .306 min in | 7.85 max | Called .308 in on the box. A .30 caliber rifle has lands at .300 in and grooves at .308 in. |
| 6 | Neck diameter (loaded) | .3435 in | 8.725 | The bullet plus two neck walls, measured on the finished round. Tension here is what holds the bullet. |
| 7 | Shoulder diameter | .4540 in | 11.531 | Where the body ends and the shoulder begins. |
| 8 | Base diameter | .4703 in | 11.946 | The body just ahead of the extractor groove, measured .200 in from the head. |
| 9 | Extractor groove diameter | .409 max, .389 min in | 10.39 max | The shell holder and the extractor both grip here. |
| 10 | Rim diameter | .473 max, .463 min in | 12.01 max | About the same as the base on a rimless case, so it feeds from a magazine. |
| 11 | Rim thickness | .054 max, .044 min in | 1.37 max | The rim's thickness front to back. |
| 12 | Shoulder angle | 20° from the centre line | — | How steeply the case narrows to the neck. |
| 13 | Neck length | About .303 in | Reference | Case length less the 1.7116 in reference distance to the neck. It is how much bullet the case can grip. |
A rimless, slightly tapered pistol round. It stops in the chamber on its case mouth.
| # | Measurement | SAAMI | mm | What it means |
|---|---|---|---|---|
| 1 | Cartridge overall length (COL or COAL) | 1.000 min to 1.169 max in | 25.40 to 29.69 | Head to bullet tip. The right length depends on the bullet and the barrel, so start with your load data and check that the round feeds and chambers in your pistol. |
| 2 | Case length | .754 max, .744 min in | 19.15 max | On this round case length is also the headspace, so it matters. Handgun brass grows slowly and rarely needs trimming, but check it. |
| 3 | Bullet diameter | .3555 max, .3525 min in | 9.030 max | Called .355 in on the box. Grooves are .355 in and lands .346 in. |
| 4 | Case mouth diameter (loaded) | .3800 in | 9.653 | Headspace surface: the mouth stops the round against a ledge in the chamber, so it takes a taper crimp, never a roll crimp. |
| 5 | Base diameter | .3910 in | 9.931 | The body just ahead of the extractor groove, measured .200 in from the head. |
| 6 | Extractor groove diameter | .347 max, .327 min in | 8.81 max | The shell holder and the extractor both grip here. |
| 7 | Rim diameter | .394 max, .384 min in | 10.01 max | About the same as the base on a rimless case, so it feeds from a magazine. |
| 8 | Rim thickness | .050 max, .040 min in | 1.27 max | The rim's thickness front to back. |
| 9 | Headspace (chamber) | .754 min to .776 max in | 19.15 to 19.71 | From the breech face to the ledge the case mouth stops against. There is no shoulder or rim to do the job. |
Source: SAAMI Z299.4-2025 (centerfire rifle) and Z299.3-2022 (centerfire pistol and revolver). Minimums are the drawing's maximum less its tolerance, and neck length is worked out from its reference distances. For gauging or tooling, use the drawings themselves.
Measuring a bullet
A box of bullets gives you some of these, and the maker's website gives you the rest.
| Attribute | What it tells you | How you find it |
|---|---|---|
| Diameter | In thousandths of an inch: .308, .355. Must match the bore. | Caliper or micrometer |
| Weight | In grains. 7,000 grains make a pound. Your load data is specific to weight and to bullet. | On the box, or your scale |
| Length | Base to tip. Heavier or copper bullets run longer, which affects seating depth and the twist rate your barrel needs. | Caliper |
| Ogive | The nose curve. Tangent and secant shapes trade easy seating against low drag. | On the maker's page, or by comparator |
| Ballistic coefficient (BC) | A number for how well the bullet holds its speed through the air. Higher is better. Published as G1 or G7. | From the maker |
| Sectional density (SD) | Weight divided by diameter squared. A rough guide to penetration. | Weight in grains ÷ 7,000 ÷ diameter². A 168 gr .308 bullet is about .253; a 115 gr .355 bullet is about .130. |
Bullet types compared
Drawn at the same size to compare shape. Load data is specific to the bullet, so a change of type means looking up new data, not reusing what you have.
Jacketed hollow point (JHP)
Defense and hunting in handguns
The open nose expands on impact. Bullet length and shape change the seating depth, so use data for that bullet.
Soft point (SP)
Medium-game hunting
Exposed lead at the nose starts expansion. Often has a cannelure for crimping in lever and semi-auto guns.
Boat-tail hollow point match
Target and long range
The tiny hollow is from manufacturing, not for expansion. Check the maker's guidance before hunting with one.
Polymer tip
Hunting and long range
The tip keeps the shape consistent and starts expansion on hunting versions. Purpose varies by line.
Cast or coated lead
Budget handgun and low-velocity rifle
Cheap and soft on barrels within its speed limits. Lubricated or coated; needs load data for lead or coated bullets.
Plated
Handgun practice
A thin copper plating over lead. The bullet maker usually gives a velocity limit; the plating can fail if you go past it.
Solid copper (monolithic)
Lead-free hunting
No separate core. For the same weight it is longer than a lead-core bullet, so it needs data for that bullet.
Case types compared
Shape decides how a round stops in the chamber, called headspace. That in turn decides how you size and crimp it. Drawn to compare shapes, not sizes.
| Type | Shape | Stops on | Examples | For the reloader |
|---|---|---|---|---|
| Rimless bottleneck | Shoulder | .223 Remington, .308 Winchester, 6.5 Creedmoor | Most modern rifle rounds. Sizing die height sets headspace, so too much sizing shortens case life. | |
| Rimless straight wall | Case mouth | 9mm Luger, .40 S&W, .45 ACP | Most semi-auto pistol rounds. Case length is the headspace, so use a taper crimp, not a roll crimp. | |
| Rimmed | Rim | .38 Special, .357 Magnum, .30-30 Winchester | Revolvers and lever guns. Case length is less critical for headspace but still affects the crimp. | |
| Belted | Belt (commonly) | 7mm Remington Magnum, .300 Winchester Magnum | A band ahead of the extractor groove. Ask your manual how to size these: pushing the shoulder too far back shortens case life. | |
| Rebated rim | Shoulder or mouth | .50 Beowulf, .284 Winchester | A rim smaller than the body, so a fat case can use a common bolt face. Needs its own shell holder. |
Primers: nearly all US-made brass uses Boxer primers, with one central flash hole your die can push through. Berdan brass, common on some imported ammunition, has two offset flash holes and can't be decapped with a standard die, so most reloaders set it aside.