I couldn't find how to do it by searching online cause you need either lvl 2 mechanic for the gp coin barter or lvl 3 traders to barter for a 60 bullet mag straight up. Higher level friend told me it just wasnt doable without the 60 round mag, but I wanted to prove him wrong so I sat around tinkering on totov builder, flea market and the edit preset/modding screens trying to figure out how I could finish the quest using the 100 bullet mag from flea. This build will cost around 250-270k using only flea/skier2/pk2 (around 200k if you have the gun already), but this can be done with lvl 1 traders as well, you would just need to spend $2 more for the HK extended charging handle from peace keeper lvl 1, and around 4k more to get the moe handguard off the flea instead of skier lvl2.
Here's the build, just make sure you don't buy the upper receiver, it comes with the stock m4a1, unless you're using a stripped down m4a1 without attachments because the upper receiver is the only thing we will be reusing.
Feel free to share with others, I don't care about credit since anyone could have probably figured this out with a little time and effort.
what a legend