Sako
Everyone seems to say ˈseıkoʊ (Say-Co), but i say it like the Finnish word it is, ˈsɑko (Sah-Co, short Ah, short O).
Sig Sauer
SIG is an acronym, so say it as it's spelled for all i care. I like to think it was formed to resemble the word Sieg, German for victory. Sauer is like Sour, but with a Z if you like.
Oehler
If it's German, then ˈeːlɚ (Ay-Ler) or ˈøːlɚ if you can manage.
Heckler & Koch
I sometimes avoid the whole thing by saying Hecker and Cough, or Heckler and Cock or Cohh when i'm serious. If it were Cotch, it would be spelled Kotsch. Coke is spoken with an oʊ diphthong, whence an older oː long vowel, whereas Koch is spoken with a short o. Cook is an interesting way, since that's what Koch means. I like it.
Lapua
ˈlɑpwə (Lahp-wuh), i think.
vihtavuori
I'll go with ˈvıhtəvwɔrı (Vihhta-Vworry)
Geschossgeschwindigkeit-messgera[e]t
Ga-Shoss Ga-shvindizh-kite Mesga-Ret
Heh, it's 'shot-swindiness mete-rede', if i anglish it.
ETA: Now i prefer Ga-Shoss Ga-shvindi
g-kite Messga-Ret
Baikal
BY-Cull? By-CALL?
Póg Mo Thóin
poːg mə hoːnʲ
According to SAKO, it is pronounce "socko". A rep for the company said (para-phrased)" Seiko (Say-ko)is a watch,we make rifles"
Right on, lol.
Und Koch (sounds like clearing your throat of nasty mucus)