I regularly get questions about info in this post.  But I've been hit up a few times recently with access requests.  If you can't see the files/docs/pics, let me know here.  Thanks and build on!


ETA:  whoops, that was an unintentional bump.  Oh well I guess, like I said, I get a lot of comments from this thread, so obviously people are still using the info.

People might have mentioned this already but Lakeshore Carbide make some ridiculously good endmills for aluminum.

The Chinese TiN-coated HSS endmills that I got with my mini-mill couldn't handle 0.010" DOC when side-milling. Now, using Lakeshore's "Aluminum Shredder" variable flute rougher, I can do 0.020-0.030" no problem. Maybe even more, I haven't found its limit yet. This is probably a normal DOC for a Bridgeport or similar industrial machine, but being able to do it on an HF mini-mill makes me giddy.

@mb44kar

I've used some some quality US and some cheap Chinese 3 flute roughing end mills.  They both make a HUGE difference in what you can cut on these little mills.  I added a line loc misting systems, but only use the air 99% of the time.  Keep those end mills cool and you can really push them.  Careful they don't gall though, the chips will weld themselves to the end mill if the Temps get too high.  Lubrication will help, but constant air is easier and cleaner.

I'm about to start work on my first large frame 308 lower.  I'll be doing it on a milling machine.  Just wondering if there is anything new I need to know, or any new and improved drawings for the 308's?  This is a DPMS pattern lower.

@OP Did you ever get info on that holding jig up anywhere?

I think the pics were victim of the photo bucket issues of the past.  I can get some pics and post them.  


Crazy to see the views this thread still gets.  

Happy building gents!

Thanks! I have been mainly just using the vice and rods/parallels but it would be nice to have something repeatable. And the thread is epic a last bastion of freedom lol.

The only issue I found with the 308 sized receivers is a lack of consistency in the distance from the front pin to the safety detent hole.  You want your safety centered on the detent hole regardless of anything else.  Otherwise, the process is the same.  Same holes, same sizes same same.

Thanks. The safety selector hole I indicate off the detent hole in the bottom. I use a drill bit and indicate to find center.  But the fire control pocket is a question, I realize the size is basically the same as the ar15 receivers but where to start it?  How forgiving is it?

I checked the distance from the rear pin to the safety hole.   If it's off, I then adjusted the pocket by that same difference.  If I remember correctly, the worst I saw was only about .025 off.  Not enough to cause any issues with the FGC interface with the upper (firing pin) but possibly enough to cause an issue with the safety if the hammer, trigger and safety relationship is changed.

Using a mill with a DRO the process is easy.  I just double and triple check my math.  


ETA, rear pin, not front.

In this diagram, I use the rear pin as a reference as it's easy to indicate in either orientation.  For drilling the pins or milling the pocket.  Once you locate the safety hole, use that as the reference for all other operations.  

Attached File

I stumbled on to this thread from a video you posted.

Here to learn and this has put a lot of info at my finger tips.

I have a new LMS 3990 mini-mill and already you have me wanting to order a DRO ...... Thanks for helping me spend my money!  LOL

Youtube has taken down all my tutorial type videos.  Even got a hard strike.  I started posting to Rumble but I don't put enough time and effort I to the videos as I should.  I have far too many projects to spend 3x as much time videoing at the same time.  But I still field a lot of direct message and email questions.  It's a rabbit hole for sure.

I hope you enjoy the mill.  I just used mine again tonight.  Pretty much every day I'm in the shop, it sees some use.  Honestly, I'm surprised the motor and control have lasted this long.  I wish I had an hour meter on it.  FWIW, once you have a DRO on a kill, you’ll ask how anything was ever made with out one.

TONS of info to digest here, got me thinking even I could finish out an 80% lower. Odd question though, if I had a scrap lower, with nothing wrong with it whatsoever except it has a high shelf, would it be easier or more difficult to basically finish out the high shelf lower to the dimensions of the low shelf m-16 TDP, or would it be simpler/easier to just start with a fresh 80% lower?

A little insight to this newfound inspiration, after tossing the idea around a bit a few years ago and deciding easier to buy what I want specifically, but with the govt's attack dogs getting way outta control, coupled with the climate of ever-increasing political tension between the left and right, having something that may or may not exist doesn't sound like a terrible idea. Besides that, I've always pursued interests I'm passionate about to their fullest extent, leaving no stone unturned, so a project like this only seems natural. There are no boundaries or limits to scientific exploration, which is all my tinkering amounts to. No ill intent or desire to break federal laws, just a student of the gun who has a never-ending thirst for more knowledge and a more complete understanding of the weapon systems we've all grown to love.

BIGGEST OBSTACLE I face is lack of equipment, I don't own a router, mill, lathe, or a drill press any longer. I do have access to a router, but lack jigs and such, as well as any experience with said router. Though I've had my eye on the mini lathe at harbor freight for a while, since biden-flation has torn across our once thriving economy like a tornado, and my personal economy got hit like a trailer park in Oklahoma, SO, that mini lathe won't be happening for the foreseeable future, what are my options? Just the router and picking up a jig setup for said router, huh? Would I be better served actively seeking out a better option? I'm work in the trades, so I'm sure I've gotta know someone who knows a guy with a mill, or shop with CNC machines, LoL.

Damn I just wish I seem this thread when it opened up, things would be do different, but with my reverse time machine out of order, I'm stuck with big ideas, while being broke with my pecker in my hand hoping for the best 😉

It would certainly be easier to start with something already hogged out.  Not easier, as it's not difficult, just time consuming.  

As for equipment, I started this because I want to teach myself machining.  It'd be different if I only wanted to machine lowers.  If that were the case, a router and the latest versions of jigs would be much easier, cheaper and far more approachable learning curve.  That said, find a friend, make some.  I've walked a few through machining their own lowers.  

The more you learn, the more you'll increase your tooling and capabilities.  Then it's all exponential from there.  I highly encourage others to take on builds of their own.  But only if that's what your goal is, building and making stuff.  If your goal is saving money, gust buy parts and assemble them.  80%'ers are not part of the saving money game.  

If you didn't see it, check out My Machining Thread the rabbit hole is deep.

Stupid question of the day:

Why is the pivot pin detent hole cross-drilled, and is it necessary/advisable to cross drill it as the blueprint instructs?

Escape hole for water and debris.  Even excessive oil could cause hydraulic effects.  In reality, probably never be an issue for 99.99% of us.

I've got a dpms gen 1 80% ar10 lower and I've been trying to find blueprints for it. I've found alot of different information. Does anyone know of a thread? Or measurements. The ar15 measurements here are amazing.

I think I found it thanks.

AFCARBON15 Why are there two measurements on the FCG drawing when measuring from the back pin .740 and .756? And the height of the holes is measured from the centerline of the same take down pin?

They aren't all created the same.  The safety needs to be centered on the predrilled safety detent hole.  The FGC can move a few thousandths without affecting anything, but if the safety is not centered on the detent hole, it won't clock correctly and look stupid.  


"For AR308, I had to roll my own. I gathered specs until I got tired of the differences. I've only done two different brands, but they were different by a couple of thousandths. At least one spec sheet I found had a range instead of a solid number for the distance from the rear take-down pin to the safety. None of which were the same as the TM lower I had. So, to start, I locate off the safety detent for X zero, then locate the rear pin for Y zero, noting the X travel difference as I use that to drill the driver's side holes after locating the rear pin there. "

I found this out first hand... See the line

Attached File

I'm in the same boat right now with researching this. I'm going to mark my lower up with some sharpie soon. Can I post you guys some pics for advice?

Came here to ask about this.. for those of you with a fancy digital mill this sounds nice.  With a more basic mill that doesnt have that could the same procedure work just by offseting and keeping track of rotations of x and y?  Seems fraught with possible error.   Using a jig hasnt been perfect so far either.

Also drilling that hole - 5/32 drill bit?  End mill?  Special sligtly smaller bit?   I am using a drill bit presently and feel like im getting a pattern in which the bit bites to one side leading to a very small shift left to right.  

All of my machining has been self taught, so take my advice accordingly.  

Using dials is no less accurate, really, it just requires the user to focus.  For me, DRO's are cheap and make the process much more enjoyable.

For the FGC holes, I spot drill, then go right to a #25 drill followed by a .156 reamer.  If your drills are decent, following that process, you should have a right sized hole in the correct location.  Could it walk, sure, but if you follow the process, you will be within tolerance.

It is best practice to drill/ream the safety, trigger, and hammer holes before machining the pocket.

Yes, a deep hole can wander, however, if you peck drill, continually clearing chips, with as short a drill as possible, this should not be an issue.

Drilling across a large gap will tend to wallow out the top hole as the drill walks around to finds its center on the other side.  Ray-Vin's Machining an AR 0% Lower is full of god machining practices.

Wiat what?  Ive been drilling both sides separately using minimum insertion into the gap.  

Was i not supposed to do that?

I know the pecking thing helps some but deep drilling and pecking never worked for me when it came to popping a hole out the other side of a lower in spec. Maybe the lower speeds around 2000 or slower was part of the reason.

Thank you, Sir!  

prior 2 lowers, first I drilled the holes using a jig and a hand drill - came out functional but terrible.  2nd i used a 3d printed jig and mini mill (but for the 5/32 using the chuck), and it came out better but still not perfect.  

this time I took your advice, put a .250 exact (measured) pin through the rear takedown and built a spreadsheet of turns and offset for each hole from the takedown as X and the top of the lower as Y, adjusting for the actual width of the mills etc.. and even though some were ridiculous, and left and right were drilled separately everything came out absolutely dead on.  

I have to say it was terrifying to lose the jig and trust the math but the results are strunningly smack dab on.  

i wish i hadnt made a couple of minor garfs with the roughing mill, but overall i'm thrilled.

credit also to buying better bits and a 5/32 collet to ensure best hole quality.  

Attached File


One question - is it ok to put a #23 bit in a 5/32 collet?  seems like .002 wouldnt be a big deal but this is all new to me.

That looks pretty damn good to me.  Function is priority,  Once the upper is attached, all things cosmetic are no longer visible.  Practice and patience will have then looking better and better.  I know what you mean about trusting the math.  I do the same with the DRO.  I keep a machined lower in arms reach for a visual sanity check before plunging.  I've never machined a single lower that didn't function.


I'm probably not the one to ask about pushing limits of collets, if I can get it in there and the runout is acceptable, I run with it.  A #23 bit in a 5/32 collet is perfectly acceptable in my book.  

Thanks for this thread, so glad I found it.
Going to put the mini mill to work!  (after reading every post a few times)

For anyone reading this and thinking about milling a lower for the first or second time, a word about fire control groups -  make sure and use whatever FCG you plan to use as the test when you validate all is good.  milspec FCG seem to be pretty forgiving but aftermarket fcg are not.  for example i just spent a fun evening milling a few thou here and there because I tried to put a larue MBT in a place where a milspec type FCG went just fine.  examples - the tail on the trigger has a raised section on the underside to ensure no play when in safe - my pocket was at 1.245 there, and the safety selector just barely couldnt insert - had to mill the pocket down to 1.248 to get the safety selector in.  mark's trigger and hammer are also .691 wide - i was at .685.. yay.. the milspec type trigger from anderson was .67 . Also the milspec trigger was .285 wide and marks is .305..  my hole was .30 (admittedly a bit undersized) - easy enough to fix, but it becomes more of a pain after you think you're done, and build it vs fixing right there when its in the fixture and chips are flying.

another demanding one is the hoffman super safety.  requires very precise alignment on the FCG, where milspec has a lot of wiggle room.  

Good advice. I have gotten pretty good at finish, dropped in one I had laying around testing and it worked great. Cerakoted it then put the one I wanted to use in and it would not drop far enough, I didn't mill deep enough. I "adjusted" the drop in trigger housing lol.

Attached File

@AFCarbon15

Is the mag well centered on the receiver?
put another way, can I use the magwell interior, just behind the pivot lugs, to find Y center ?

On the drawing the magwell can be off centerline as long as it is in the allowed tolerances. Center line looks to me to be based off the tolerances and dimensions between the front lugs.

What he said.  It's called stacking tolerances.  If you use an existing feature to start from, you could be starting from an errored point.  Making your next feature even further out of tolerance.  

For an AR lower receiver, the plane of the centerline is defined by the mid point of the width of the receiver extension horn at its widest point, the mid-point of the width of the magazine well measured approximately 1 inch back from, and 9/16 below, the location of the center of the front pivot, and mid-point of the width of the rear trigger guard.  All measured off the forged surfaces.

From a practical point of view use the mid-point of the inside width of the pivot lugs and parallel to the forged sides of the fire control area, because then everything will be within 0.006" of the center-line of the upper

Use a quality center cut 7/16" long end mill , dimensions from the front push pin hole and have at it for the pocket.
For the holes used a sharp center drill , drill .010" undersize then ream to finish size.

This is a great thread.

As the worlds worst novice machinist, and on the occasion of nothing, here are some milling thoughts i have learned to share with anyone just starting out.

1: geting a bunch of the same 80% lower is simpler.  having collected this and that over time- there are differences that have to be accommodated, and that means challenges you maybe dont need.  for example billet lowers with special shapes that dont fit the fixture or jig, some being high shelf vs others being low shelf.   Some have the rear lug area cut out some dont, some even have it cut out but not cut to width so you have to mill it to spec. Some have the rear lug extend area cut extend a bit forward so you mill the pocket and it meets the existing cut, others you have to mill that area youself - which is sad because you then end up taking off the annealing to get a smooth interface between the existing and fresh cut that extends forward.  some have special ridges that you can ram the bit into when navigating to the hammer hole .  

2: if the rear lug area is already cut, figure out if it was cut for high shelf or low before you get too far.  if something happens in that section above the selector detent, and its high shelf you can always just mill it off (m16 cut) but if its low shelf then m16 cut isn't a normal configuration.  if you have a high shelf lower and want it low shelf that's easy enough to fix, but the opposite not so much.

3: measure carefully.  i just had a wall get a little thinner than i wanted because i couldnt get the micrometer in there properly and i got a false reading.  take the time to get a good reliable reading.  a 4mm washer as a shim + anti-walk pins can be a godsend.  

4: as I noted in a prior post, understand your trigger group and its needs.  milspec triggers are generally pretty forgiving.  i grade my lowers (sadly) by whether they HAVE to have a milspec trigger or not.  perfect side to side alignment is required for some safety selectors, pocket width and depth is required for some trigger groups, but milspec accepts a lot of bullshit.  Quality milspec like CMMG at least arent awful like cheap LPK triggers.

5: Take the time to make sure your fixture of jig is perfectly flag so your cuts are going in 90degrees to the metal.  Especially true with billet lowers and their special non milspec shapes.  Make sure your workpiece is held down securely and fully.

6: Lock each possible axis when cutting.  i cant tell how many trigger holes in the bottom of the lower i have made too wide by forgetting to lock the y axis, so as the cut starts it can wander.  then the only recourse i have is to follow through and have the hole be oversized.  always be locking.  i am not sure if this is common but i tend to make the hold oversized anyway as a hedge against any kind of off-centerness or tolerance stacking in my milling - 9mm = .354 which is nice halfway house between 5/16 and 3/8.  

7: use the z stop always

8: Dont mill when tired or in a hurry.  its a fuck up waiting to happen.

9: use a roughing mill if you have a mini mill.  it makes milling out the pocket take about half as long.  but if you do, leave some metal for the end mill so you dont have to live with the rough walls.  

10: rough it out with some metal to spare and then go in easy on each wall with the widths and tolerances in mind.  dont be in a hurry to make each wall a certain way, but rather start with what matters and work in from there.  for example pocket width should be .690 +/- .005.  width of the forging is .87 +/- .010.  so if the forging is .86 then .690 pocket the walls would be .085, but if the width of the forging is .88 width the walls would be .095 for .690 inside width.  simplest way to manage would be to keep the walls even and go for pocket width vs wall width.  bonus, its easier to measure too.  

11: 80% lowers are cheap, consider the cost educational

Arf experts please feel free to contradict anything i stated that's wrong or bad practice

I need to get a jig & some plastic lowers to practice w/.

polymer p-80's I see cost more than metal.

Polymer AR 80%.  Not a P80.  Drop a polymer AR lower in a thumbhole stock & it should hold up.  I just need to learn how to do it instead of casting a lower from epoxy.

Have you cast a lower in epoxy?  I'd like to see that!

Just 3d print a Hoffman lower.

No, I prefer War Fairy's Charon w/ the rear magwell, but I need to learn how to mill 80% aluminum, so need to start on some EP Armory plastics.  They're only $25 a pop.

Yes, I bought a mold from the now defunct AR15Mold.com, it's a delrin mold you pour 2-part epoxy into and get a lower in about 30 minutes.  It's brittle & weak, but about the least expensive lower you can make until we start milling them from scrap wood.

Yeah, those have a certain appeal too.

As for the lowers, check RTB, and others for discounted and blem lowers.  They can be had regularly for $30/ea sometimes less.  You could also hit uo your local Scrap yard for aluminum drops.  Make your own trigger jigs, which could later be turned into modular lowers.

Trigger jigs can become firearms w/ an AR57 upper and a thumbhole stock.  

Just want to start on plastic, as it cuts easier than aluminum, and if I get it right, still a useful upper w/ a Vism thumbhole, cheap at Optics Planet.

I meant polymer, dont know why i wrote P-80.