View Full Version : AR 70/90 Build Thread (sdk1968)

02-16-2021, 11:05 AM
gonna jump in with the finished (minus barrel) pictures...

FINALLY a decent finish on this!!

not sure how im gonna post the full tutorial here.. if the PDF will load, will do it that way. if not will try to rebuild it over here in a post friendly manner.

let me know if you can or cant see these pix. will try to C&P this over here.

Lets get this all laid out & out of the way first:

There are several BENDING tutorials out on the board already. This is not about that. This is how im going about building the SA version from the Century parts kit & 1 of Troy's Builder Kits.

What you see me do... may or may not work for you. There is no 1 way only to build weapons. This is just the way that worked for me. It WILL work for me.

The pictures that you see in this tutorial are from multiple people as i suck at taking pix when im working.

There are NO guarantee's on this method working for you. Also there is no responsibility on my end for your safety or legality.

These things are all up to YOU. So follow your local laws, perform the work with whatever variation works for you!

Enough "talk". Lets get to building this thing.

02-16-2021, 11:06 AM
Starting with a parts kit from Bullfrog. He was the cheapest person at the time that i heard of this gun. His kit showed up still in the Century box & in good shape with very little wear.


Yes the usual sorting of parts & seeing what would work or was missing is next. As far as i can tell? nothing is missing & its all here.

Next came the clean up of the front trunny. they were definitely weld happy on this thin metal on that trunny.
After that came getting the ejector off of the tiny piece of rail that it was still attached to.

This is when we came across a guy making flats. Lots of us ordered them from him. The flat was fine & i even bent one up in my box break. it didnt come out too bad & could be used with just a slight bit of tweaking on my end. This was my first clue that this gun COULD be done by a home builder & not just a machinist or super talented gunbuilder.

IF i can do it? You can do it too.

While all this playing around with flats was going on, several other talented WG'ers were working away on their own bending jigs. 21HK, Rowland, Bigllama, Katzma, MarkSerbu, Mykle & they all have helped me along with how this played out in the mind & where i would build from. If i missed one of you guys who helped me? Im very sorry & will ad you in !

This is right on par with how WG always operates, a bunch of like minded guys pounding out things until suddenly everybody can build the toy!

In these conversations, posts & pm's a new name was given to me by one of the guys (im keeping that private for a reason) his name was Troy & he was on GB advertising that he would soon have "flats". Many conversations with Troy later: he not only has flats but offers a "kit" that has the flat, rear trunny, rails & front repair tabs all for a nice price. we call it a "Builders Kit".

Troy is doing this first batch of kits to get all the wrinkles out & im doing one of the earliest prototype kits that he has.. its a version #2 & there will only be a few of these that test builders are using. after that Troy has a run of 500 of these kits coming for the masses. When those are available i'll make sure the link to them is posted.

This build is being done from how that kit is set up. IF you are using a different flat, kit or parts? You will still find some things here that will help you along.


Now that you have seen the parts kit & the builders kit... its time to get to work.

02-16-2021, 11:06 AM
The Denials is where i go next.

FCG denials:

There is already a thread posted about taking apart the lower by Collenthedestroyer right here: https://weaponsguild.com/forum/index.php?topic=83171.0

This makes doing the denial of the lower & removing the FA parts pretty easy. Several different ways to go about it.
I blocked the selector & removed/altered the parts. (The rails also act as a denial, which is detailed a little later!)

Think there is a thread on that as well, if not? Someone else will make it because i was totally focused on getting this done & didnt take any pix. Had build fever at this point & was having to get something accomplished. Then to top it off: I struggled getting this thing back together with my shaky hands for a bit & several bouts of cursing.
Unless something breaks? it will NEVER come apart again. That in itself is a denial. )*((

Upper to lower & lower to upper denials:

At the rear trunny block, i added a bead of weld down the block on the right hand side & on the rear towards the backplate. YES it is a big ugly weld on a small pretty piece. This can be done with a bar/pin or weld. I used weld because... well because thats what i wanted to do. :o Chose this side & rear because thats where the hollow spots are in the lower. This allows you to notch out the lower right there on the right and further fill the rear center as well. This is a VERY tight little space. i had to chuck this up in the mill & do a plunge cut to make the slot for the weld. There is not a lot of room here & i ruined the little retaining spring for that rear pin. totally my fault. AGAIN: this is VERY tight!! no room to spare & no room for a mistake. It took a long bit to reach down that far from mounted in my mill.

Its not real complicated & someone may say its "too simple".... but a denial is a denial & until someone tells me its not enough? Thats what im going with. It does the job of denying a FA lower from being put on my SA upper & at the same time it denies a FA upper(if anyone has one) from being put on my converted SA lower without intentional alteration. Thats always been the key from the ATF & this is no different. If someone does that? They are showing intent.


Bolt/carrier denials:

Now we move on to the denials built into the rails of this kit by Troy. The rails are thicker/wider & when mounted its an automatic denial for the bolt/carrier. They absolutely do not fit into the space. This means that you have to cut/grind/file/mill off the trip on the back of the bolt & massage the slots in the bolt/carrier and/or rail so that they will fit.


Using the bolt setting on top of the rails after the nub is cut off to show you that the rails are too wide for the carrier to fit between them in its stock form. This was prototype #2 style of rails, cant promise that everyone else's will be this thick.

Then after the adjustments, it was still a bit too tight in this pic for operation & needed some hand work with a diamond file & a little massage. Then it went into battery under hand operation with the flick of your wrist.

That wraps up the denials & now we can finally get on to building this friggin thing.

02-16-2021, 11:07 AM
Before we go any further a fellow builder reminded me of something that for some of us is automatic, but for others just starting? might be an easy thing to over look.
This is where you set that front trunny on the lower to get your spacing & reweld your lugs on. Everybody says the straight part of the lug goes towards the rear!

To double check your spacing for this:
Put a magazine in the lower. Put the trunny on the lower. slide it back till its snug. Now look at where the ears were cleaned off. this should give you perfect spacing! If you are too tight the mag wont fit or will hit the bottom of the feed lips. all bad mojo.

i already had my ears welded on using numbers from Mark Serbu (which were spot on, naturally) but this is still something to check.


Setting this up Troys way is what im going to do next. In the testing stages of this (& im doing a prototype version BTW) Troy discovered & shared several things.

For the homebuilder: Getting it all straight & keeping it there was gonna be a challenge. This is one of the few times that ive got to step away from my MIG & go with TIG. You just cant get the rails fused to the front & rear with a MIG without getting a build up & too much material in the way. So TIG it is.
NO i dont have a TIG. YES i have friends who do & they like all the rest of you say that i need one for this very type of thing.

In the end Troy decided (& i agree) That its better to set the rear trunny & the front trunny on the lower, fit the rails up, tack the rails to it, get it nice and smooth with the carrier running along it under hand operation. It should look like this:

Now if you got smooth operation here? GREAT!! Remember the shell on this is actually just that. Its a shell. Think AK top cover.

IF you dont? then this is the time for you to adjust. LOTS of things could be out here. Were you a touch out of square or level from the front trunny?

EXTRA INFO: some people were commenting about the rails not being square/equal: THEY ARENT. The drivers side is taller than the ejection side. This is why actually lining the rails up to the trunnys is how im going to test on operation. THEN you fiddle with the shell a bit later.

Also want to ad that i went back & TIG/fused the front & rear trunny seams completely after was sure about the operation being good.

almost forgot to ad this part in here! im mounting the ejector with a fine thread screw to check its clearance to the bolt. It needs a little bit of surface off of the rail in this kit as Troys rail is thicker than the factory one.

i have Version #2 of Troys rails. in this version you need to remove .048 from the side of the rail to get enough room for the bolt to slide past. You do NOT need to make it any deeper. Just off the side. also am going to leave the screw & ad a rivet to this ejector in case it ever needs replaced. its a very tight spot to try & weld or remove. Troy is going to correct this milled out part for version 3.


No amount of pix on this can give you the true feel of how much time this takes to get this part done & right. But, if you dont have this part right? There is no reason to go any further. You will just ruin your shell.

02-16-2021, 11:07 AM
Shell work!

If you are following along with this method, you still have a shell laying on the bench doing nothing at this point. Now its time to fix that. Cut the front & rear locating tabs off where they need to be & now move to the steps below.

A last minute check again & a mark on the front trunny for reference. Now im gonna check & see if that looks like its in the right area. It does.

ok i did this before the welding... just got ahead of myself with "build fever" and started posting this back before the welding & out of order.
You can do this either way... first or last. its all about if you trust the way you hold it in the vice on the mill. Or if you have steady hands on your rotzip/dremel.

Check it & adjust if needed. Of course i needed a little filing & sanding to get this bind free.

Time to put the shell on, line it up & weld her into place. This is a really short sentence for what is actually a lot of work & double checking.
With the question about people not getting the rails in right or them not being square, CHECK IT AGAIN!! Now try to slide that shell on.

YES it slid right on. NO it wasnt perfectly aligned.

i put the front where it needed to be: checked operation. Tacked front bottom 2 holes. then went to the rear, pulled it down until it bottomed on the shell, clamped it into place & tacked the rear 2 corner slots. checked operation. went back to the front, moved up to the next set of holes, tacked em... checked operation. went to the back to the round holes, clamped, tacked.... checked operation. worked my way all around... tack, tack, tack... operation check.

DISCLOSURE TIME: i fit it & tacked it & did all the checking... THEN== had my friend Jamie Connelly (who is a professional TIG welder) cpme over & go over all of these welds with his TIG to give them a good look & finish them off with a complete fusion/weld.

My tacks had everything in the right spot & having a pro like Jamie finish them off took a lot of the worry out of this about my finish weld "look". Jamie did not make the big ugly blobs that you see on the factory guns. He did more of his everyday welding style... so this baby will have more of a slick side look when the finish is on it.

I want to thank Jamie for bringing his rig over & sharing a skill set that was obviously out of my range. YES he has picked out a TIG set up for me & it will be in the works for the near future.

Is this the factory way? NOPE. Lets just say its my version of this gun. some will like it, some wont.

Check it & adjust if needed. Of course i needed a little filing & sanding to get this bind free. THIS is an understatement. This is also why i try to wait until im finished before doing the build thread..... so all that operation checking & success? soon as the last weld was capped & put this back together?

Wouldnt hand function. :o what happened? well folks this is how building goes. i searched this thing up & down & couldnt find a thing that was different. something obviously moved/pulled or warped. more searching, polishing, moving.. NADA.

Then in a fit of frustration had took off my gloves & was holding the receiver in my hand upside down, working the bolt to the stop point & i felt it!! was watching the rails/bolt/head, but felt it in the top of the cover! stuck him in the vice by the rear trunny & looked inside the receiver & tried hand operation.

BINGO!! evidently the top of the receiver pulled just a hair when TIG'ing and now the cocking handle was draggin on the LEFT side of the bolt where it sticks thru. for those who dont know about this cocking handle? It sticks all the way thru the bolt & out the other side "X" amount. This is to help keep it centered in that big hollow space of shell. with just that little bit of change? it was too much drag! This is so slight that you cant even see it to the naked eye unless you are running the bolt/carrier back & forth WITH the oprod & spring on it. THEN you can see this with just a few K's of clearance coming back at you & ... stick point! Now the problem is found. Time to fix it.

More searching shows that this is also at the exact point the ejector has maxumum push over on the bolt to the side. This could be a combo of needing a couple K's more out for the ejector pocket along with a K or 2 off of the end of the bolt handle. Will try to get to that tonite. Both are easy fixes. And it turned out to need the ejector clearanced just a hair more.

extra info: JTMs flats were running around .046 to .048 thick depending on where & which one i check. They bent really easy, even for me in a box break. (this is NOT a bad thing)

Troy's "shells" are .055 There is no variation on either of the shells i have. anywhere you check its .055

That doesnt sound like much, but in terms of holding them in your hand? You can instantly tell the heavier one. Same for bending it. There is NO WAY i could bend one of Troy's shells if he offered them as a flat. You would HAVE to have a jig.

02-16-2021, 11:16 AM
cocking slot cover:

Now install the cover with the little PITA spring. This takes a good bit of checking too! lots of checking, tiny bit of spot/touch welding to not warp or mess it up. im using some very small screws in 4 places to locate & secure this so i can then try to weld the tiny holes up. (might have to have Jamie do this so i dont burn thru it.)

Now we move on to hand operation double checking & seeing if im gonna get my hand in the ejection port or anything else that can make me bleed.

^^^^^^^^^^^^This little detail is on hold. mine is bent all to mush, it was treated pretty rough on the demill.

On top of that? I dont like it, it doesnt really serve any purpose in my world & it looks like it has a great chance of causing some warpage in that bolt handle area. Maybe i'll come back & put it on later after the gun is running.


This is a tougher thing for me, cause im not a super technical "clean" welder. Dont want this to be all "sh*t up" at this point. im actually gonna cheat a bit here & leave the thin piece of steel under the site. gonna clean up the edges & dress it ... then see if i can tack it into place.

so i tacked it at the rear & ground that back off after setting it. did my best to clean up my ugly tiny beads down the sides of it.

02-16-2021, 11:17 AM

Well these so far have been the unobtanium of the build. There are guys who will be offering barrels at some point. Its just not "yet" or right now.

The barrel im using was made for me as a favor from a friend. He knew i was working on this build tutorial (before i even posted it)& he also knew i was a test builder for Troy on this set up. (they have talked) He very graciously took his time & skills & zillion dollars worth of equipment & made me a prototype barrel for this prototype build. :grin:

i have been very quiet about this: because the last thing i want is to cause him to have a million "what about me" requests, when this is something in the testing phase. The good news is this also lets him get the set up right, incase barrel blanks start to show up. Then he might be able to run them.

So again: Thank you very much to Mark Serbu for turning this barrel blank into what we hope is an actual useable barrel! & im definitely in the "OWE YOU FAVORS" column. Mark is not the type of guy who would ever say: "WELL YOU OWE ME".... but the truth is: i do.



02-16-2021, 11:17 AM
This post is for hand feed with dummies, test fire, finish pix & video

Good news is she cycles by hand, smooth as butter. Thats as far as i can get until the barrel gets here in the function area.

Will start on the finish now as its got some spots in/on it & some rough grinder marks to clean up. Looking at the work, im just not happy with the finish look of the welds right now. Im gonna go back & buff'em a little bit & then put the finish to it. im thinking about KG gunkote or some Cerakote.

AGAIN= while im happy it will "work/function" ..... im not real happy with my finish product here. Troys' kit is fine, but my finish work here isnt what it usually is. That means i'll just have to do another one! <:>

2-14-21 turns out the harsh light in the shop makes it look worse than it did. LMAO

FINALLY a decent finish on this!!

Function Test

well it feeds from mags & throws them out the barrel hole when it slams into battery. Tried right & left side feed from the magazine. Thats all i can do until theres a barrel in there.

insert pic of complete gun with barrel at test fire

02-17-2021, 07:04 AM
Scott says you guys cant see the pix... i can see them... but for some reason they aint showing..

will re insert them here in a bit.


02-17-2021, 08:20 AM
Very nice...as usual.

02-17-2021, 09:08 AM
Awesome!!! Can't wait until you get the barrel in! :)

Are these kits Safe/Semi or do they come with the fun trigger?

02-17-2021, 10:01 AM
thanks guys!

these kits are 1, 3 & FA.... you have to do the denials for the FCG. Colleen did a nice job over at WG of pointing out the blocking for that. i just used a pin so that you can turn it past 1 shot & then cut the FA parts off & re used them to hold the spaces & made some tacks inside so that you cant remove the parts after it was together. which means ...... if the FCG parts go bad.... its gonna take a F-ton of work to get it all apart again.

Pretty sure that Pryotex is gonna also do a postie for the shop though. No point having all those license's if you dont have toys to use them. :-)

02-17-2021, 03:44 PM
Nice. Sounds like a lot of work to get these going. Beyond my skills. Great work!

When do you expect to get the barrel in?

02-17-2021, 08:04 PM
Nice. Sounds like a lot of work to get these going. Beyond my skills. Great work!

When do you expect to get the barrel in?

Mr Serbu just sent me a message today & its on its way!

02-17-2021, 08:11 PM
Mr Serbu just sent me a message today & its on its way!


02-23-2021, 03:57 PM
pix added & here da video...



02-23-2021, 05:14 PM
This is really great to see! Mark posted in the 70/90 FB group that someone he owed a favor was getting one of his couple of test barrels, and I'm glad to see it's being put to use.

I know you said you don't want to post anything publicly about who Troy is. But I bought a flat and rear from JTMS (I'm guessing the other guy you reference as selling flats) and have been looking all over for rails, off and on, for months. It's just rails and a barrel now, so any insight on those rails would be helpful.

02-23-2021, 05:35 PM
pix added & here da video...

Outstanding Sean! Looks great and functions great. Nice work!

02-23-2021, 07:22 PM
If ya watch my video there.... you'll hear everything. :-)

Troy has sold out his first batch of 100+... as soon as those are delivered he will take on another 100ish in the second batch.

he is going to try & keep his batches around 100 each. He will let me know when he's ready for people to get on the list... & i'll post it at that time.

then i can PM people his email/contact info.

he is offering complete builders kits= shell, rails, rear trunny, front repair section

or any of those parts individually... price varies by how much you want & engraving..... im not gonna put a price out here as that would not be cool for the forum.

a note on the build: i had to go back & make a barrel catch tonite to fit the barrel & front trunny under the muzzle nut, my kit did not have this. no big deal just a sliver of steel.


02-24-2021, 01:36 PM
If ya watch my video there.... you'll hear everything. :-)

Troy has sold out his first batch of 100+... as soon as those are delivered he will take on another 100ish in the second batch.

he is going to try & keep his batches around 100 each. He will let me know when he's ready for people to get on the list... & i'll post it at that time.

then i can PM people his email/contact info.

he is offering complete builders kits= shell, rails, rear trunny, front repair section

or any of those parts individually... price varies by how much you want & engraving..... im not gonna put a price out here as that would not be cool for the forum.

Much appreciated.

02-24-2021, 02:48 PM
update from Troy!!!


read it all before you ask any questions!!

02-24-2021, 04:10 PM
Although quality looks good, this is too bad. With the cost of the rifle kits now (~$500 on GB) and this kit ($375 to start) and eventually a barrel (Guess $250?) this is over $1100 before you can strike an arc, mill a slot or work on any modifications for semi. I guess it is a different beast though for someone's collection.

02-24-2021, 07:48 PM
for most of us who got kits they were $299 to $359.

i think after the dust clears barrels are gonna be about $350

same on these builders kits... they are not cheap. but what you get for the price is VERY complete & very doable for garage level skills.

granted this is not in everyones budget, but people can get whatever part they need & make the rest. some people just need the rails, others just the shell. not sure what the prices will be on that vs as the group.

we have several guys who are making their own flats & pressing them out at WG. some of them just want rails.


03-01-2021, 06:53 PM
didnt get anymore vids done.... crappy weather...

but Serbu posted this to his 70/90 group= its Troy stamping out some of the shells.


03-03-2021, 12:19 PM
eh erm....

centerfire has kits for $399 & Troy shipped out 30 more of these builder sets today. wont be many more shipped out for about a month, he has to do his real job.

but if you are on the fence about this gun? get a kit now while you can!!

as Blackwing said: it will cost you about $1,000 to build this gun. which is not a big deal if you have ever built a PSL, PKM, Gory, Maxim or any other high end kit.

the drawback most people THINK is that this is "another AR". eh well uh... not exactly.

think if an AK/AR/CETME all got together at a friends party.. then 9months later you have junior here & we will call him an AR70/90~!

AR mags, AR ammo.
AK reliability & operating style system.
Cetme style shell & rails

it really is a very solid platform. it doesnt shoot like an AR. has the best factory bipod ive seen for being out of the way & functional.

definitely gonna do another one of these & improve my workmanship, then go back & go over this one again too.

it really is time for me to get rid of all the AR's (again).

03-06-2021, 03:25 PM
so here is the 556 brass green tip high powered stuff.

have no clue what batch it is or any of that stuff, it just seems to be hotter than any other 556 ive got.

i purposely went slow & it did fling these about 35' away ... it actually threw them so far that we could only find 4 of them.

the rim, case edge & all of the "hangout" zone look perfect. no swell, no deforming or anything else that we were looking for.

the neck is good, but the rim up front got bent all to hell when it hit the side of the receiver & that was before it threw it the 35'

as soon as this was done? switched over to the standard gas port setting & put the rest of the mag down the pipe. It still ran just fine & that cut the ejection down to about 25'. will run on this setting for a while now.