Gunsmith Achievement in Mass Effect 3
Upgrade any weapon to level 10.
How to unlock Gunsmith
SP: In the shuttle bay of the Normandy, there is a weapon upgrades station. Here, you can upgrade any of your weapons for a price. As you purchase upgrades, the cost starts getting higher. The first upgrade choice will be level 2 for each weapon you own and you can get up to level 5 in a single playthrough. To get it to 10, you have to start a New Game+ and find the weapon a second time. This will unlock the final upgrades, allowing you to purchase up to level 10.
MP: Buy the recruit packs consistently and you will have a level 10 weapon fairly quickly. This is probably the quicker/easier of the two options of unlocking this achievement. I got three upgrades for the Mantis sniper rifle in my first five purchases.