.30-caliber cartridges share a bullet diameter, a pile of acronyms and a habit of sounding alike. That makes this shelf the easiest place in the store to order a box that will not chamber.

Eight recurring errors, each with a fix.

Assuming every listing is loaded ammunition

A search for 300 prc ammunition lands mostly on loaded rounds, but the same cartridge's results are salted with bare bullets and brass, because a precision cartridge attracts reloaders. Read the description before adding to cart.

Confusing the two Ultra Mags

300 sa ultra mag is the short-action version; the full-length .300 RUM is a different, longer case. The shared “Ultra Mag” is the trap, and a listing that just says “ultra mag” needs its second look.

Assuming all Weatherby .30s are one round

30-378 weatherby ammo and the .300 Weatherby share a founder and a bullet, not a case; the .30-378 is far larger and belongs only in a rifle stamped for it.

Treating the H&H .300 and .375 as siblings that swap

h&h 300 magnum shares a belt and a maker with its .375 relative, and nothing else. Same story for any two belted magnums of different bore.

Reading ‘Norma’ as one cartridge

300 norma is the short form of the .300 Norma Magnum — but Norma also made a .308 Norma Magnum, a 1960 belted round that is not the same cartridge and is still occasionally listed. Cross-reference against the rifle's stamp.

Assuming a compact magnum is a short magnum

300rcm — run together as shoppers type it — is a Ruger Compact Magnum, a shortened .375 Ruger case; it is not a .300 WSM and not a .300 RSAUM, though all three fit short actions. Only the one on the stamp fits the rifle.

Assuming an AR .30 fits every AR

30 rem ar needs a rifle built or converted for it — barrel and bolt, because its case head is larger than the .223's — not just any AR-15. 300 whisper needs its own barrel too: it is a .30 built to run subsonic through a suppressor, and it will not chamber in a .223 or, safely, in a .300 Blackout — the two are close cousins, not twins.

Reading a rifle-maker's name as a cartridge maker's

30 thompson center ammo carries Thompson/Center's name because the rifle came first; the cartridge is Hornady's. And is 300 savage ammo still made — the question buyers keep asking about it — is still made, in batches, for the Savage 99s that never stopped hunting.

When in doubt about a match between a cartridge and a specific rifle, consult a qualified gunsmith before firing. Confirm current stock and your state's shipping requirements before ordering.