I've got to ask:  why do they have you use a 21/64 drill bit for the starter hole?  Why pick such an odd size that's not readily available at a hardware store?

While I ordered one online via Amazon, can I just use a 5/16 (equivalent of a 20/64)?

Trust the science Attached File

While I can't answer to as why they made it that size, I think it best that you use that size.

Your mill bit is 5/16.

21/64 will make it slightly oversized which is what you want so the mill bit does not catch when starting the router.

That makes sense.  Wish they had just gone with something more common though, like 11/32.

I'll wait until Amazon delivers it then.  Ugh...  patience.

Maybe trying to keep the trigger slot from getting even wider than it is.

Post number 3 answers the question! If your router bit catches when you first start to mill out the lower, you may break the bit or screw up the lower to where you have to throw it away & start over. None of us want that situation.

Several weeks ago, I ordered my jig & tools, but haven hear a peep from 5D as to a delivery date. They did send out an informative blurb on their website that they are super busy & will get all orders filled by 24 August of this year. By then, I will have forgotten where I put my lowers.

At some point, just setting up the mill & milling it out myself without the jig is probably going to happen as I hate long waits for projects. I milled out the parts for several 1919a4's during builds with no jigs when I built them, & aluminum is much softer than hardened steel.

Sounds like a problem caused by using wood working tools to do machine work.

I have found that if you first drill with the 21/64 bit completely thru the receiver & then drill 95% of the way thru with a 3/8 drill bit, it is easier to  align the router in the hole to stop the initial start chatter.

Here is another little tidbit that has come to light in my learning curve. The 5D drill bits that come with the kit send a 5/32" bit for the trigger & hammer. When I finally got my FCG pins, they are the standard 0.153 pins & the hole that a 5/32" bit drills is about 0.004 too big for the pins.

I have a number & letter drill bit assortment, so figured that using a number 23 bit for the hammer/trigger holes gives me a 0.154 hole for the 0.153 pin to go in & with a little polishing in the bore, the pins fit perfectly. The receivers that have been drilled with a 5/32" bit will work, just not like I like to complete work.

Since the number 23 bit is smaller than the 5/32", a 5/32" transfer punch was used to make sure the bit started in the right place & didn't get a pin hole drilled wrong.

I used my 5D kit with the drill bits that came with it and after sending my completed 80% receivers to get type 3 hard coat anodized, the trigger pin holes were the PERFECT size, if not a tad bit too tight.

That coating made up the difference for the 5/32" being a few thousandths oversize, just like when the manufacturer chrome lines a barrel. My blanks will be coated in flat black Moly Kote & baked per the instructions as my AK are done. If it turns out badly, it is my fault. I am willing to try it!

Maybe they picked that because nobody uses it for anything so they knew it'd be sharp?

Same.  I use the 21/64th bit that came with the 5D kit to bore all the way through, then oversize the hole to eliminate chatter.  I have a 3/8th bit marked in my routing kit with the right depth not to widen the trigger channel.