XL70E3s are definitely craftable with stats. As far as the avengers, are you upgrading normal miniguns into avengers? Because if you use a normal non-statted minigun to make the avenger, it will retain its stats. For example,
You have
Advanced Minigun
+1 AP
+25% Min Damage
+25% Max Damage
+2 Range
If you upgrade this to Avenger, it will keep these stats.