These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
The orange that Ichiroku wears in her hair is a reference to one of the things the author thinks about America, which is a large exportation amount of oranges. The M4A1 also has a hamburger hairclip as well for the same reason: being a well-known part of America.
The orange slice also bears a resemblance to the sight adjustment knob on the M16's carry handle.
The aforementioned rivalry between the Uzi and MP5 sisters.
The similar rivalry between MP7-tan and P90-tan in an omake.
L's Gag Boobs refer to the L85 being rather heavy and having a larger handguard, compared to other assault rifles; also, according to the Gun Girl anatomy book, the handguard is the bustline. (Now consider where a guy's hands usually end up...)
53 is considerably larger than the other elementary girls. She's based on what is, essentially, an MP5 that fires assault rifle bullets rather than pistol bullets.
This is also related to why MP5 is repeatedly called mini-G3 (chiisai Jisuri-sempai, shortened to "chii-3") and wears the same variant school uniform as G3; the MP5 is essentially a G3 in submachinegun size, chambered for 9mm Parabellum. (And also why she's apparently G3's favorite imouto.)
In real life, the G3 platform was so successful, nearly all of Heckler & Koch's product lines during the Cold War were derived from the G3; this is reflected in the show and manga by the significant resemblence the HK sisters have to each other.
Funco admitting that she's not popular like Ichiroku.
This is true since the M16 series has been adopted worldwide from the Americas all the way to Asia and Europe. This is unlike the FNC, which did terribly in NATO standardization trials and was only adopted by a small number of countries like Belgium and Venezuela, or Indonesia and Sweden under license.
Crossing into Incredibly Lame Pun territory; when Sensei is reading up on Gun Girl anatomy their back side is the equivalent to a rifle's stock or butt.
16 eats an impossibly-hot chicken nugget and then yells at Funco, saying she's delicate, and later jams; this is a reference to the M16's ill-advised change of ammunition propellant powder in Vietnam, which caused a lot of problems with jamming.
In Episode 3, 16 is shown meticulously cleaning herself, even though she didn't go out like the other gun girls; the M16 family of weapons requires a lot of cleaning, due to the direct impingement operating system, which has been described as similar to "eating where you shit at."
Also, the 18's mention of balls getting stuck to 16 - in Vietnam, the US Army switched to ball powder propellant for the M16's ammo (as opposed to stick powder for the M14). This, coupled with the direct impingement system, causes carbon deposits and powder residue to build up in the working parts.
In episode 9 when the girls bring out some modern rations, Ms. Thompson comments that, "Back in my day we used to eat canned food all the time," which is true of World War II-era rations.
Galil tries to open a bottle using her teeth, and 18 tells her not to do that because it'll damage her magazine lips. In Israel, newly-conscripted soldiers have been known to do exactly that, which is why Galil ARM bipods have a built-in bottle opener. But as 18 points out to AUG, Galil-chan is the AR model, which lacks the bipod.
When Funco, 16 and L skipped out on long-range shooting and tried to pass off Sig's shots as their own, it doesn't work because the SG 550 doesn't use 5.56x45mm NATO. She uses the dimensionally-identical but ballistically-different 5.6x45mm GP90. She has a different rifling twist rate than the NATO rifles, meaning that each would be less accurate using her ammunition.
16 moonlights as a fashion model because the M16/AR-15 series is one of the two most popular assault rifle types in the world, constantly appearing on gun magazine covers. Likewise, FAL is the class president of the battle rifles/high school girls, because the FAL was the most widely-used post-war battle rifle in the world (the "Right Arm of the Free World").
Guns that have integrated optics, such as the AUG and SAR-21, are shown wearing glasses.
Her big sister FAL gets mistaken for molesting G3 later on in Upotte Nano. It's actually a big misunderstanding, when she acts upon her suspicion about G3 (she was right; it was actually one of her little sisters HK33 filling in for her).
Memetic Mutation: Fans will declare a gun to be their "raifu"note contraction of "rifle" and "waifu" whether it's talking about the show itself or in firearm or video game discussion.
The Woobie: L. Her history makes FNC and the submachine gun lolis cry and give her hugs. When England finally created a new low-caliber bullpup rifle model in 1985, she was prone to jamming, breaking and would be thrown away after about 100 uses. The Special Air Service actually gave up and abandoned the L85 for the M4A1 (a compact version of 16's M16A4) and G36C carbines.
And then to add further insult to injury, in Episode 4 she breaks her firing pin, and is depressed for much of the episode, especially after 16 repeatedly calls her a dud. This became worse in the jungle wargame arc when Funco remembers the times when L can't attend a wargame ranging from tripping herself to stomach pains or trips to the nearby hospital.