Weapons: custom firearm build ideas; the ultimate AR-15, ARAK-21, ARAK 31, & Glock 20

For those of you who have seen my Pinterest board, "Custom weapon ideas" (https://www.pinterest.com/StopDemocide/custom-weapon-ideas/), I had ideas for custom weapons for awhile. Now, they're going to be shown on my blog, particulary by some of the basic parts they're going to be made out of. You can find these parts with a simple internet search (preferably with DuckDuckGo or Ixquick/Startpage to protect your privacy).
Now, while I have an at least somewhat deep interest in weapons, please bear with me that I'm getting almost of my information from the internet, and I'm not yet physically experienced with firearms.

This article does not cover the accessories for these firearms. 

Custom AR-15 with Shrike upper receiver; polymer lower receiver, Fostech Echo trigger, stainless steel threaded bull barrel (preferably fluted), free float quad rail, "bad lever", magazine storing stock, storage pistol grip, and mil-spec buffer tube. For the best upper receiver, it would have an ambidextrous charging handle, titanium bolt carrier (or if not available, stainless steel), Head Down upper receiver, and short stroke gas piston. Send to to ARES to convert it to the shrike, attach the lower and upper receivers, and you have quite a bit of firepower!

ARAK-21; the regular ARAK-21 right-side eject only upper receiver with heavy stainless steel barrel on a Windham Weaponry RMCS-4 lower receiver. Use with Magpul 40 round PMAG's with 5-round extensions. 

ARAK-31: ARAK-31 upper receiver with heavy stainless steel barrel and ambidextrous ejection put on an Head Down AR-10 lower receiver, folding stock, and perhaps a 3-pound trigger.

Glock 20 Gen 4: the original Glock 20 Gen 4 but with as many parts replaced by titanium or stainless steel parts as possible (so it will last a LONG time), TRUGLO night sights, and caliber conversions, particularly 9x25mm Dillon and .460 Rowland with 30+ round magazines.

Juggernaut M1A: Fulton Armory M25 Peerless Sniper Rifle Barreled Action, heavy contour stainless steel barrel, stabilizing receiver bolt, and 30 round magazine or 50 round drum.

So what would you build, assuming that you had the time, money, resources, and skill to do it? Feel free to say in the comments below.