Guncraft Blocked and Loaded

Think Prestige, Re-up, Passing into Legend etc...

** When you Re-Craft, you will lose all your unlocks, including game modes, and return to level 1 but you will be able to select a bonus to unlock permanently.**

To Re-Craft, you must reach level 51. I didn't track exactly but you will get this long before the 10,000 kills achievement. To give you an idea, after Re-Crafting I boosted a couple of games for 1000 kills and went from lvl 1 to lvl 18.

Once at Level 51, go to "Edit Characters and Classes" in the main menu. The first option here will be "Re-Craft". Select this, and you will get a warning screen which you will have to confirm. Do so, and the achievement will pop.

You will then be able to choose a bonus, which is one of the unlockables (eg. a weapon, kill-streak, etc...), which will be available permanently.

Progress towards other achievements will not be affected.

So, not a difficult achievement, just time-consuming.