Have i missed it or is there a rule about minimum strength for using certain weapons? I mean we hear a human can't use a marine bolter but what would the minimum strength needed to use it be? I know standard astartes armor adds 20 to strength which apparently doesn't add to the unnatural strength bonus a marine gets but how strong would a human or other creature have to be to use marine weapons or is it a matter of having unnatural strength?

1. In 40k there are just generic "bolt pistol", "boltgun" and "heavy bolter". In 40k RP there are "mortal" and "astartes" versions, with Astartes bolt pistol / boltgun damage equal to that of normal human sized heavy bolter, implying there are 3 calibres.
2. Generally, Astartes weapons are simply marked "Astartes X". On account of being sized for Astartes with power armor, they are presumed to be not usable normally for baseline humans without.
3. There's no "minimum SB", but there are variations of "-10 (Scatheros "Blackhammer" Defence Shotgun, Sacristan Bolt Pistol) | -20 (Lathe-Ptn. Boarding Gun) to BS test unless the wielder braces or has SB 4" or even "-10 to BS per point of SB below 4" (Melta-Cutter) and "Counts as Basic for wielders with SB<4" (.54 Tranter Hand Cannon) or "Counts as Heavy for wielders with SB<6" (Relic Boltgun - with Astartes bolter damage).
4. Black Crusade also tweaks "handcannon" subclass so that the penalty only applies for wielders with SB <5.

6 hours ago, Techpriest support said:

I know standard astartes armor adds 20 to strength which apparently doesn't add to the unnatural strength bonus a marine gets

Armor add after UnnStr calculation, for example Astartes with Str 43 has StrBonus 8 without armor, in armor he has Str 63 and Str Bonus 10. With minimal Toughness Bonus 8 Astartes just don't need to calculate weight of any equipment with exception of power armor with damaged/destroyed power generator.