По сути .223Rem и 5,56x45 это не просто одно наименования одного и того же патрона. Это, скорее, две группы патронов, которые как бы взаимозаменяемые.
Под патронам 5,56x45 как правило подразумевали патрон M193 (лёгкая пуля и ствол с шагом нарезов 1:12) и SS109 (похоже появились обозначения M885 и пр. Но они все как правило близки к SS109 и рассчитаны на шаг 1:7).
На самом деле список более полный-
F1 (Australia): 5.56×45mm FN SS109 equivalent produced by Thales Australia, formerly Australian Defence Industries (ADI).
F3 (Australia): 5.56×45mm Blank cartridge produced by Thales Australia, formerly Australian Defence Industries (ADI).
SS109 (Belgium): 5.56x45mm 61-grain [3.95 g] Semi-Armor-Piercing cartridge w/. steel penetrator produced by Fabrique Nationale. It replaced the US M193 cartridge in 1979 as the NATO standard.
C77 (Canada): 5.56×45mm FN SS109 equivalent used in the C7, C8 and C9 type weapons. Made by General Dynamics Canada.
C79 (Canada): 5.56×45mm blank cartridge used in the C7, C8 and C9 type weapons. Also made by General Dynamics Canada.
DM11 (Germany): 5.56×45mm 4.1 g dual core ball cartridge w/steel core, produced by RUAG Ammotec. [green tip]
DM21 (Germany): 5.56×45mm tracer compliment to DM11, also produced by RUAG Ammotec. [orange tip]
L2A1 (United Kingdom): 5.56×45mm M193 equivalent produced by Radway Green.
L2A2 (United Kingdom): 5.56×45mm FN SS109 equivalent produced by Radway Green.
L1A1 (United Kingdom): 5.56×45mm tracer compliment to L2A1, produced by Radway Green. [red tip]
L1A2 (United Kingdom): 5.56×45mm tracer compliment to L2A2, produced by Radway Green. [red tip]
5.56 mm, Ball, M193 (United States): 5.56×45mm 55-grain [3.56 g] ball cartridge.
5.56 mm, Grenade, M195 (United States): 5.56×45mm grenade launching blank.
5.56 mm, Tracer, M196 (United States): 5.56×45mm 54-grain [3.43 g] tracer cartridge. [red or orange tip].
5.56 mm, Dummy, M199 (United States): 5.56x45mm dummy cartridge, non firing, indented case.
5.56 mm, Blank, M200 (United States): 5.56×45mm violet-tipped blank cartridge.
5.56 mm, Ball, M202 (United States): 5.56×45mm 58-grain FN SSX822 cartridge.
5.56 mm, Ball, XM287 (United States): 5.56×45mm 68-grain ball cartridge produced by Industries Valcartier, Inc. An Improved version was also produced designated XM779.
5.56 mm, Tracer, XM288 (United States): 5.56×45mm 68-grain tracer cartridge produced by Industries Valcartier, Inc. An Improved version was also produced designated XM780.
5.56 mm, Grenade, M755 (United States): 5.56×45mm grenade launching blank specifically for the M234 launcher.
5.56 mm, Ball, XM777 (United States): 5.56×45mm ball cartridge.
5.56 mm, Tracer, XM778 (United States): 5.56×45mm tracer cartridge.
5.56 mm, Ball, M855 (United States): 5.56×45mm 62-grain FN SS109-equivalent ball cartridge with a steel penetrator tip over a lead core in a partial copper jacket. [green tip]
5.56 mm, Ball, M855 Lead Free (United States): 62-grain bullet with a steel penetrator tip over a tungsten-composite core in a partial copper jacket. Primarily used during training in countries with strict lead disposal laws. [green tip]
5.56 mm, Ball, M855A1 (United States): 62-grain bullet w/ a 19-grain steel penetrator tip over a copper alloy core. [bronze tip]
5.56 mm, Tracer, M856 (United States): 5.56×45mm 64-grain FN L110 tracer cartridge. [orange tip]
5.56 mm, Tracer, M856A1 (United States): 5.56×45mm Lead Free Slug (LFS) Tracer with similar ballistic performance to the M855A1 and improved trace to range consistency.
5.56 mm, Plastic, Practice, M862 (United States): Short Range Training Ammo (SRTA) has a smaller charge than standard ball, reducing its aimed range to 250 meters, and fires a plastic bullet. The M2 training bolt must be used in the M16 Rifle / M4 Carbine when using SRTA for the weapon to cycle properly due to its lower power. It is used during training on shooting ranges near built-up or populated areas. [Brass primer, Aluminum case and Blue plastic projectile].
5.56 mm, Armor Piercing, M995 (United States): 5.56×45mm 52-grain AP cartridge with a tungsten core. [black tip].
5.56 mm, Tracer, XM996 (United States): 5.56×45mm so-called \"Dim Tracer\" with reduced effect primarily for use with night vision devices. [red tip?]
5.56 mm, Special Ball, Long Range, Mk 262 Mod 0/1 (United States): 5.56×45mm 77-grain Open-Tipped Match/Hollow-Point Boat-Tail cartridge. Mod 0 features Sierra Matchking bullet, while Mod 1 features either Nosler or Sierra bullet.
5.56 mm, MK318 MOD 0 enhanced 5.56 mm ammunition (United States): 5.56×45mm 62-grain Open-Tipped Match Boat-Tail cartridge.
5.56 mm, Ball, MLU-26/P (United States): Early USAF designation for 5.56×45mm ball cartridge produced by Remington.
5.56 mm, Ball, M1A3 (South Africa): 56-grain FMJ Ball round based on the M193 cartridge. It was used with the R4 assault rifle.
Причём это далеко не весь список... Но в целом их можно разделить на рассчитанные на ствол с шагом 1:12 и 1:7.
.223Rem может иметь какие угодно навески (в определённых рамках естественно), часто с расчётом на ствол с шагом 1:9, хотя не обязательно.
Так вот если стрелять патронами M193 из ствола с шагом 1:7 или наоборот использовать SS109 в винтовках со стволом с шагом 1:12 куча сильно падает. Некоторые этого не знают, потом удивляются, дескать ничего ваши Мки и не точны.
Кстати допуски в случае с .223Rem и 5,56x45 тоже отличаются.
Так что хотя современные AR\'ки довольно прожорливые и их недостатком является требовательность к магазинам, всё же неплохо думать, что именно ты заряжаешь в свою винтовку. Пихать в армейскую AR\'ку со стволом с шагом 1:7 патрон с лёгкой пулей и ослабленными спортивными навесками это не самая хорошая идея. И наоборот, пихать в спортивную AR\'ку M885 тоже далеко не самый умный шаг.
Как это ни парадоксально звучит, но большой сортимент патронов это, в некотором смысле, не всегда хорошо
