Sure, some amount of crafting skill is easily justified. Like I said, Salamanders traditionally make their own weapons rather than rely on a forgeworld for supply & they are naturally gifted. Making your own combat knife, making a bolt pistol, repairing your armour are all things everyone will likely due during downtime bits. But you're also in your early 20s, even being a space marine (especially being a space marine) you simply don't have enough time to be a master at anything while also undergoing a lethal & exhausting training regime. 

Don't start trying to make plasma bombs before you learn to make a frag grenade.

And while I know many 40k video games have master crafted grenades as a blanket upgrade for game mechanic purposes. They don't actually exist in lore. In fact lore specifically states that consumables are virtually never made to a master crafted or artificer quality because it's seen as a waste of time and resources even by the admech. 

That's why for example master crafted bolters exist, but not master crafted bolt shells.