Ceratac AR308

[Lane]

So I think I gather; none of these prints can help me directly since nobody know what they are. What I can do is measure the lower I receive in the Ceratac kit and post yet another blueprint then if there any differences in the location of the selector detent which sets the FCG location. Otherwise it will then be verified which one of the many layouts this particular kit matches.

There is an 80% lower with the Ceratac kit, though some of those blueprints would indeed be sufficient to mill from a solid block.

[98Z5V]

This is what you're buying, right?

https://www.ceratac.com/Rifle-Classic-Kit-80-AR308-p/kit-308-r-ck.htm

[Lane]

That is the one; and they already took my money so I'm committed.

I didn't buy their jig conversion kit because I don't have a 5d tactical jig. The cost of both of those would simply make this way too expensive to justify.

That's why I'm doing my best to get a handle on all the different variations. I suspect it really isn't totally proprietary, but that they simply don't want to give away their secret. On the other hand; I don't know how 5d tactical has a jig that covers all other 308s except this one. I guess that I why I had been spitballing earlier that they might have copied something they don't have permission for and it actually does match something else out there.

 

[98Z5V]

Get it on your hands, and figure it out, man.

There's ONE thing, really, that's gonna determine what pattern it really is - and that won't concern you, with the kit you get... upper receiver threading.  Armalite and DPMS upper receiver threading are different, but you're getting the handguard and its parts with your kit.

Whatever you receive, your upper parts are included, so it kinda doesn't matter.  What you do, for information here, on that, is invaluable.  That upper is either Armalite or DPMS threaded. 

No matter what, with the pictures they show, with MagPul PMags in that thing - it's NOT the Armalite B-Model.  If it's truly an Armalite model, it's the A-Model.  Becauseof the PMags they show...

Upper receiver threading determines this... Armalite or DPMS threading is your answer...

[Lane]

Yes sir, still waiting to lay hands on it, but at least it shipped today. With any luck I will have it Saturday... I will keep collecting information in the mean time. I have a number of hand drawn measurement documents that I will sort through and post anything that is unique.

I will measure the threads for the barrel nut and report back when it arrives (and take pictures for that matter). Hopefully it is either 16 or 18 TPI and not something totally off the wall.

I'm pretty clear on the AR-10A / AR-10B differences, it was jtallen1983 that mentioned trying his 10B upper on something else so I simply wanted to verify that for myself. The other mention was in regards to the blueprint I had posted with the 81 date. Clearly not related to this project, but posted in case anyone else finds it useful. Ceratac does indeed specify the PMag specifically, but that already seemed like a given with it being called an SR-762 style lower. I can't imagine anyone going back to the Armalite AR-10B style magazines in a new product.

I think my plan at this point is to quiz 5d tactical on the difference with the Ceratac lower first. Some other vendors noted getting their lowers checked for width since that jig isn't supposed to touch the sides of the lower receiver at all. I think the most important question I hope they can answer is if that width was the difference, or if the FCG was in a non-standard location. That way; if I still don't have all the answers I can still contact Ceratac without wearing out their good will before I even get started.

[Lane]

My plan worked. I got a reply from 5d Tactical's sales department at midnight here, and I believe they are in Massachusetts. Somebody really loves their job as that is pure dedication.

The reply was very thorough, stating that the FCG holes do NOT match DPMS Gen 1 style lowers. They do in fact match the location of the Ruger SR-762, and other compatible lowers. No issues with the width of the receiver fitting in their jig; so presumably a conversion kit is just a set of side plates with holes in a different location for the FCG. 

They also mentioned that there are other companies that use the same layout FCG layout, but the only jig available is the 5d tactical at the moment. They describe it as relatively new, having been developed in only the last six months.

I think that's all I can add to this until I see it in person on Saturday.

[98Z5V]

22 hours ago, Lane said:

They do in fact match the location of the Ruger SR-762, and other compatible lowers.

That means it IS the older Matrix Aerospace receiver dimensions - I can get a few critical measurements from the older Matrix, if you want.  Once you know where to start, it's on.

Tell me what you meed from the Matrix, and I'll get it up here with pics. 

[Lane]

I've been going over all of my blueprints with a fine tooth comb to see what measurements I should be comparing to begin with (and what differs between them). Some drawings appear to be missing vertical placement for the selector hole (or any part of the FCG). 

If anyone can measure from the top of the selector hole to the top of the lower, and bottom of the hole to the lip over the pistol grip of the lower I think that is a good start. Might even be nice to measure both sides and average the results. There aren't a lot of great reference points in the horizontal axis which is typically measured from the front (pivot) takedown pin on centers which is just plain tough to do by hand. The mag release button might be at least acceptable for that measurement though. It would really just be used as a sanity check. 

I have a pretty high resolution photo of the SR-762 shot from dead flat (no perceivable distortion). I think I can print the FCG at 1:1 scale and overlay it to double check everything as well. Not a perfect solution either, but for all I know at least one of the prints I have does match. My first order of business will be measuring everything relevant and comparing to each of my prints to start ruling things out. I'll also post the pictures of the significant parts, barrel thread measurements, etc. as soon as I get it unwrapped.

My current plan is to do a one day AR-15 80% build first, just to clear my head. Then get started on the 308; if I start roughing in the 308 pocket first, it gives me just a bit more time shore up all the measurements to be confident I know where the FCG should be before I drill those final holes. 

Much appreciated if you can grab those measurements off the matrix. Also; if you can try to measure from takedown (on center), to the selector hole (on center), but parallel with the flat top of the lower that would be a huge help too. The numbers I have for that are all over the map. So far the variations I've seen are: 6.077", 6.115", 6.136", 6.16". I won't mention which one I'm hoping for...

Thanks

[98Z5V]

I can get that for you on Sunday. 

[Lane]

Sorry to report that the postman rolled up with only one small box today. I got out the camera to document the contents only to find the packing slip had the AR308 kit crossed off. It may ship out of a different warehouse or something, but of course they are closed today so I can't even ask until Monday. That was a real buzz kill so I started grinding the threads off some barrels to make them legal while I wait. I will check back in the moment I have something.

[Lane]

The Ceratac AR308 kit has finally arrived here. It was not actually late as they claimed 3-5 business days for shipping when I ordered it. I had simply assumed it would ship with some other things I bought at the same time and I was clearly wrong. 

I have taken some preliminary measurements though I haven't had time to sort through what they all mean yet. The barrel threads measured 0.485" in the groove with 8 threads between the calipers. I measure 6.2785" between the center of the front take down and the center of the selector detent hole. That measurement matches none of the prints I have seen so far. From the rear of the selector detent hole to the flat at the back of the trigger well (where the pistol grip would attach) is .415", minus half the hole size would be .337" on center. Selector detent hole is .156" diameter and the front pivot takedown hole is .266". The gas tube is 11-11/16 long. 

[98Z5V]

It looks like you'll have to base everything off that rear takedown pin hole.  The way that thing is setup, I'd just run Armalite AR-10 prints, based on the takedown, in order to cut the fire control group pocket.

Grab the info from CNC Guns:

https://www.cncguns.com/downloads.html

Edited September 14, 2018 by 98Z5V

[Lane]

98Z5V: I saw your reply last night and have been mulling it over. Measuring from the rear takedown seems like it would give me the same errors as the front if I used that blueprint. 

I measured the distance between takedown pins to be 6.880" just like the AR-10 print. Where that falls flat is the relation to location of the selector and FCG. Closer inspection of the prints appears to indicate that my measurements are different for those locations in a few ways. The AR-10 print indicates a measurement of 0.102" from the back of the bolt catch slot to the front of the FCG pocket. My rough estimate for that on this lower is more like 0.249" based on the location of the detent hole. The images I've seen of Matrix lowers mirrors that estimate. Another way to look at that comparison is the bolt catch slot is wider than the distance from the slot to the front of the pocket on the AR-10 print; on the Matrix lowers there is a lot more material between the slot and FCG.

I printed out a 1:1 scale template I've used for the AR-15 FCG and fit it up to match the selector detent. I then aligned the hammer and used a pencil to mark the upper. I get right around the same measurement of .250" from that line to the back of the bolt catch slot so I think I'm on track here. As always; correct me if I'm making some glaring error in judgement. I know pretty much everyone else here has had more exposure to the different styles than I have.

A few other things to note now that I've had time to look this over a bit more closely. My takedown pin holes are actually .277 which appears to be standard. There seems to be a bit of extra coating in at least one of the holes that I'm sure will work out once I install pins and work them in. The magazine well on this appears to be straight up and down, where the AR-10 print shows it to be at a 3 degree angle. That may be part of the accounting for differences in those measurements. I also did a bit of trig to see what would happen if I did cut the pocket to the AR-10 print and use the FCG location based on the selector detent in this lower. The hammer would hang forward at a 5 degree angle; presumably that would keep the upper from being attached though I'm under the assumption it would still function if I could hold it back upon closure; still a bad idea in my mind.

I'm not quite ready to start milling the pocket so there is still time to work out these details. I can also mark off both ends of the pocket and only cut out the center portion which is most certainly the same. Once I have absolute confidence in the final location I can go back and open up one end or the other as needed. The rear slot can also be cut to fit the upper in the mean time without any impact on FCG location. 

I have included pictures of my estimate marked with a pencil line and one I pulled out of the Matrix section for comparison (thanks Blue109, very few people had pictures of the inside of the lower). The AR-10 print shows that pocket to be further forward by 0.147" which would make it very thin and presumably cause issues with the hammer.

[survivalshop]

 AllI have to say is good luck , just one measurement a little off & you will most likely have issues . it would be nice if you could find someone with the Jig you could barrow or get the measurements from . 

[98Z5V]

8 hours ago, Lane said:

I printed out a 1:1 scale template I've used for the AR-15 FCG and fit it up to match the selector detent. I then aligned the hammer and used a pencil to mark the upper. I get right around the same measurement of .250" from that line to the back of the bolt catch slot so I think I'm on track here. As always; correct me if I'm making some glaring error in judgement.

There is nothing wrong with using AR15 prints for the FCG pocket - both these rifles use the exact same triggers (in geometry) and the exact same safety selectors - the FCG pocket is pretty much standard.  Once you determine the location of it, you're golden. 

I didn't get you that Matrix measurement last Sunday, like I said I would.  I was too excited to go break in a new .260 Rem build last Sunday, and completely forgot about it. I will get you that measurement this weekend. 

[Lane]

Not a problem that you didn't get to the measurements last weekend, my kit only showed up a few days ago. This weekend will still be just fine for me.

It looks like I can get away with just the measurement between the bolt catch slot and the FCG pocket now that I've reviewed everything. Front pivot takedown pin to pocket would be a good sanity check (and/or rear if that's easier since the takedown pins are 6.880" apart). The vertical measurement for the selector seems like it should be already set in stone given the pocket depths are the same between AR-10/AR-15. 

I've had good results with the paper templates on previous 80% builds. Print quality and alignment are critical; as well as verifying the 1:1 scale. It is also imperative to check every measurement repeatedly before gluing it on. I appreciate how the rest of the paper stays in place to show how well centered an undersized drill bit was so that any mistakes can be fixed before drilling full size holes. You can also verify centering as the bit begins to cut through the paper before plunging through the lower. I use another paper template for the top of the pocket to keep me from getting too close to the edges. I remove that one part way through the process, then simply measure pocket width and wall thickness as I get closer to finished. Marked bits keep me from plunging too deep.

[Lane]

I thought I would take a few minutes to update this original Ceratac thread now that I have one up and running. I did not use the jig conversion kit that Ceratac offers because I don't have a 5d tactical jig to attach it to. I can now say with reasonable confidence that the lower is a copy of the early Matrix Aerospace (and compatible; SR-762, KAC, LMT, Mega, etc). All that is thanks to 98Z5V who got me some critical measurements from his Matrix to get me started milling the FCG pocket. 

First and foremost I would stress that you get what you pay for. The Ceratac kits contain roughly the cheapest parts money can buy. That said; I would still check over every part just as thoroughly if I had paid twice as much and haven't had many problems to speak of over a number of Ceratac builds. I will outline in detail what I ran into with this particular AR-308 kit build (my first 308) and let you be the judge. None of these matters bothered me, as I knew roughly what I was getting myself into before I made the purchase. So in no particular order, these were the issues I noticed with my kit:

The original hammer that came with the kit appeared to have a hole coming out at a crooked angle. I asked for a replacement and they shipped one along with a pre-paid return envelope. The replacement looks and works ok, but has a very tight J spring which holds a death grip on the pivot pin.

All but one of the  hand guard attachment screws were missing from the kit. Had I asked for the missing hand guard screws they would have sent them, but I didn't notice until a later date and found a solution at the local hardware store (had to cut off longer M5 0.8 screws). 

The bolt catch was floppy in the slot (which comes already milled in the 80% lower) and wouldn't catch the bolt without milling down the rear surface slightly. I would personally replace this part with another one that fits better in the future, and adjust that one's catch face accordingly as well. It appears that the charging handle stops don't allow the bolt to be pulled back far enough by hand, but it does seem to work fine during firing operation to hold the bolt back on an empty magazine.

The McFarland gas ring installed on my bolt was too small by 0.006" which doesn't sound like much, but it didn't contact the bolt carrier without adjustment. I stretched mine oversize on a ring mandrel until it relaxed to the correct size which matches the AR-10 spec (.656"-.659").

I had some trouble pounding in the gas tube roll pin, in hindsight I probably should have cleaned out the hole in the gas block first. The gas tube itself really didn't want to feed into the upper receiver without considerable tweaking once the barrel nut was installed. I would guess the upper also had some extra coating in that slot, but in the end I was able to flex the tube enough to clear. It was the mouth of the gas tube that didn't want to go straight in; the rest of the gas tube fit just fine.

The cam pin on the bolt carrier group rotated counter-clockwise during early live fire testing and took a nick out of the upper receiver's cut away near the gas tube. This may or may not have caused other problems for me, but I can say for sure it only showed up after firing the first few live rounds. I polished the firing pin, the corners of the cam pin, and added lubrication before more live fire testing; it seems to be resolved now. 

The paw on the forward assist appears to have been too long (or the bolt carrier group too floppy in the upper), and took some early damage. When function testing I could make it contact the bolt in a number of places without pressing the button at all. I added oil all over that area and worked the bolt to wear the paw down so it is no longer a problem.

The upper and lower mating lines (especially near the buffer tube attachment) aren't perfect. This really doesn't bother me, especially given the price; but some other people might not appreciate it. 

There were a few extra parts in my kit. An extra gas tube roll pin (which I ended up using), an extra buffer retainer, and a roll pin of unknown use. They also upgraded my hand guard to a 15" M-LOK style for free, and I paid extra for an 18" barrel. With an 80% lower (which isn't an option NOT to buy on the website), it cost me $444.97 (with a $20 off sale over Labor Day weekend), plus shipping. 

For the price, I would absolutely do it again and may in the future. I also enjoy the process of building and troubleshooting any problems that come up. I personally can't speak to how it compares to the problems with other AR style 308 kits on the market.

[98Z5V]

You did a fantastic job on this beast, man - very well done. 

[Lane]

Thanks 98Z5V! I very much appreciate all of your help and support along the way. I doubt I would have completed it already without you. 

Here is a link to my build log with pictures over in the Member's section to make this thread more complete: https://forum.308ar.com/topic/16723-ceratac-ar-308-build/

 

[98Z5V]

Meh...  I showed up, threw out some numbers - and finally showed up 3 weeks (or later) with the details I promised you on the first weekend.  If anything, I should be chastised for it...