Just a small patch allows  Skibadaa's IF-54 to use 5.56 rounds. You can choose to have 5.56 as either the default ammunition (replacing .308) or as a conversion (replacing 5mm). The conversion adds a small damage buff to the relevant receivers to account for the larger caliber and the default add a damage reduction to account for a smaller caliber.

I'm not very familiar with modding and these have not been extensively tested, so if I screwed anything up, just let me know and I'll do my best to fix it.

All credit to Skibadaa for the amazing IF-54 : http://www.nexusmods.com/fallout4/mods/20336/?

.308 replacer currently lacks the proper names for the modifications in your inventory. I'm going to fix this soon.