+P ammo: pressure standards, gun ratings, and when it is worth it

+P means the ammunition is loaded to a pressure band published by SAAMI above the standard maximum for that caliber. Same cartridge, same chamber, higher ceiling — and because the ceiling is standardised, it is a label you can verify rather than a claim you have to trust.

Two questions decide whether +P belongs in your gun: is the firearm rated for it, and does the extra velocity change anything you can actually feel or measure. For most shooters the honest answers are “check the barrel” and “less than you would think.”

The numbers

SAAMI (the Sporting Arms and Ammunition Manufacturers’ Institute) sets maximum average pressures for each cartridge, then publishes a second, higher band where a +P designation exists. The gap is real but modest — roughly 9 to 17 per cent, not a different cartridge.

That last row is the first surprise for shoppers. +P is only meaningful where a standard defines it, and the .45 ACP and 9mm Luger shelves are full of the label while the .380 buying guide has nothing to point at: .380 has no published +P band, so a box claiming the designation is a manufacturer’s own idea, not a standard.

Is your gun rated for it

Modern service pistols are generally built and tested for +P, and many stamp “+P” on the barrel or mark it in the manual. That stamp is the cheapest answer you will get: look for it before you buy. Where the rating is absent, firing +P is not a detonation waiting to happen — the standards include a proof margin — but it does mean accelerated wear on locking surfaces, more battering of the slide, and a warranty conversation you may lose.

Three cases deserve individual care. Older pistols, especially pre-war designs and hand-fitted guns, were proofed against their era’s ammunition. Lightweight alloy-frame revolvers and tiny concealed-carry pistols often publish their own limits, which can be “standard pressure only.” And revolvers wear differently: forcing cones and cylinder throat erosion respond to pressure over time even when the gun functions fine on the range.

What you actually gain

Muzzle energy follows velocity squared, so even a small speed increase shows up. A 124 grain bullet at 1,150 fps carries about 364 foot-pounds; the same bullet at 1,200 fps carries about 397 — roughly nine per cent for fifty feet per second. Typical +P gains are in that neighbourhood: 50 to 100 feet per second over the standard load of the same weight, varying with barrel length.

Whether that changes anything depends on what the load is for:

• Defensive hollow points. Many duty loads are designed to expand at the velocity their rated pressure provides, and short barrels are the constraint. If your pistol is on the +P list and the load was developed for it, the gain is real. If the load already expands well at standard pressure, +P buys speed you cannot use.
• Practice. Almost none. A standard-pressure load you shoot flat beats a +P load you flinch through, and the per-round cost difference compounds over a case. Practise with standard, then run a box of the carry load to confirm zero and function.
• Penetration barriers. Velocity helps defeat glass and sheet metal, which is why law-enforcement +P loads exist. For ordinary use the bullet’s construction matters more than the pressure stamp — see TMJ vs FMJ vs JHP for what is actually doing the work.

The grain weight you shoot stays the same choice it always was: a heavier bullet leaves the barrel slower, and +P does not overturn that arithmetic — it shifts it by a tenth.

+P+: the standard nobody set

+P+ is where the labelling stops meaning anything. No SAAMI band, no proof standard, no shared definition: it is whatever a manufacturer decides above +P, sometimes well above. Loads sold as 9mm +P+ have turned up near 40,000 psi and beyond, which is pressure the factory chose, not pressure the industry certified.

Police agencies have run +P+ in service pistols for decades, usually with heavier recoil springs and a short replacement interval. Without that context, +P+ is a round that eats a gun faster than any standard load and returns a velocity gain most shooters cannot exploit. Treat it as specialist ammunition: if you have a specific reason and a gun set up for it, fine; otherwise +P is already the top of the standardised ladder.

A decision rule

1. Read the barrel stamp or manual. No rating, no +P — buy standard pressure from the 9mm shelf at $0.27 a round and stop thinking about it.
2. Rated and carrying? Buy one box of +P defensive ammunition, run 50 rounds for function and point of impact, then load it.
3. Practising? Standard pressure, always, in the same grain weight as your carry load.
4. +P+ or unlabelled “extra power”? Skip it unless something specific demands it.

Pressure is one variable among several, and it is the one most often mistaken for a free upgrade. It is not. It is a rated capability of a specific gun, used on purpose, for a specific job.