Ceratac AR-308 Build

[Lane]

It has begun.

This is to be my first .308 build, and I must admit the larger lower feels great in my hand; I think I'm already hooked... This topic continues the Ceratac discussion located in the Building a .308 section. I still do not have the measurements for the front of the pocket to the back of the bolt catch slot. It is said to be a Ruger SR-762 compatible lower, but in 80% form. Older Matrix Aerospace owners are welcome to chime in with that measurement if they are able. It also should match the Larue, KAC, LMT, Genesis CNC, and Armalite AR10A; but I have had not been able to verify that. Ideally I would like to get a few measurements to be sure there aren't small variations among them.

I decided to get started on the 80% lower without any measurements in hand. I put in the front takedown pin and marked off the area for the rear to mate up. This is the first time I've had to mill that part on a lower but it really wasn't that hard. I had to make a bit of extra room for the corners since I don't have the tools to cut them square. I still have a tiny bit to cut out of the bottom of that well, and a hair off one side for it to mate perfectly. It's a bit tight still but that interference shows me where I need to mill or not mill. I certainly don't want to take too much off now since I can still spend hours in the FCG pocket too without doing any harm.

The holding blocks I use for AR-15s were very close to functional. I only needed to cut out a bit for the rib on the left side of the magazine well, and tiny bit further for the nub next to the selector on the drivers side as well. I will go back and modify my templates later on though since there are some other areas that could be made tighter. I had to use two 1/8" shims on each side of the trigger well, where I only used one on the AR-15 builds. 

The next thing I want to do is mark off the min/max positions for the trigger well so I can avoid it for the time being, then I'll get started milling out the rest of pocket around that area. I'm already getting excited about this project, but I have at least a week to go. I don't want to rush any of this as the lowers aren't nearly as cheap as those for the AR-15 (which are almost disposable these days).

[Lane]

I spent some more time measuring things out and scribed off the basic outline of the trigger hole and the side walls, as well as the assume front of the pocket based on the selector detent pin hole. I thought I was ready to have a go at the main pocket until I measured the height; it turns out to be a bit shorter than I expected when measuring into the trigger well.

Attached is a picture of the side profile with the caliper adjusted to the standard FCG pocket depth on the AR308 and an AR-15 lower for comparison. EDIT: The images appear to be posted out of order, Top is AR-15, middle is scribed off AR308, bottom is AR308 side profile. I'm a bit concerned that there may actually be a difference in that area which would mean adjusting the height of the entire fire control group up higher than usual, and a pocket that isn't quite 1.249" deep.

It is not a show stopper yet, but it certainly gives me pause. I honestly don't think that vertical difference would change the functionality with the upper, but having a pocket that is too thin on the bottom might be an issue. I would also like to ensure some level of useful information is available for anyone else to attempts this in the future.

My current list of desirable comparison measurements is now: rear of the bolt catch slot to front of fire control group pocket, top of the deck to top of the trigger well, and any pin hole (trigger, hammer, or selector) to top of the deck and/or bottom of the trigger well. No need to try and measure a hole on center; top or bottom is fine as I can calculate the rest. 

Edited September 21, 2018 by Lane
Image order changed

[98Z5V]

Shiit.  I'm digging out my Matrix right now, and getting you some measurements and pics...

[Lane]

Take your time sir, I had read some of your other posts lately and can only imagine what you are up to. Working on some one mile loads?!

The pocket depth can also be measured pretty easily with a set of calipers; tail dropped down into the FCG pocket without taking anything out. I just noticed how thin the bottom would be at 1.249" when I was checking things over today and began to think twice. I already triple checked my templates against prints and everything else is looking pretty straight forward. As you can see I could easily gouge out the pocket to 1.200" deep and keep myself busy for a while. 

In my searching around today I also found that SMF Tactical appears to be selling an 80% lower (SMF Tac-10) which they say is an AR-10a cut for $249; again no published measurements for that. 

This has been a lot of fun for me and I'm still hoping to grasp some of the nuances between AR308s and AR-10s as I continue to work through this.

[98Z5V]

25 minutes ago, Lane said:

Take your time sir, I had read some of your other posts lately and can only imagine what you are up to. Working on some one mile loads?!

 

I'm not a Sir, I'm just Tom-Ass...  I work for a living...     i've got the mile figured out on the .338 Lapua. I think I can get 1600 supersonic from the .260 Rem, and I've only got one month to figure that out - less than that, actually. I'm in a crunch - in order to give the .338 LMs a headache...  

[98Z5V]

Pocket depth on the Matrix M-762 is 1.240".  Receiver thichkness is 1.310".  That should give you the bottom internal thickness, if these things are really the same.  0.070" should be plenty, on the bottom thickness.

Edited September 22, 2018 by 98Z5V

[Lane]

That fits almost exactly what I measured for receiver thickness, and what I was expecting for a pocket depth. I assume that means I need to shift my FCG up by the same difference to make the trigger work right. 

Only other thing I care to double check is that bolt catch slot to pocket measurement. It looks like there is enough space to get an accurate measurement without removing the catch itself. I've got mine marked off at 0.250" but it might be a little less (pocket further forward) which is easy enough to fix later. Seems like I can get started anytime now.

[98Z5V]

17 minutes ago, Lane said:

Only other thing I care to double check is that bolt catch slot to pocket measurement. It looks like there is enough space to get an accurate measurement without removing the catch itself. I've got mine marked off at 0.250" but it might be a little less (pocket further forward) which is easy enough to fix later. Seems like I can get started anytime now.

I'm getting a direct, consistent depth of that bolt catch pocket at 0.208", on my receiver.  Measuring on the outside of the catch, into the pocket.

[Lane]

I meant the measuring from the back of that slot into the FCG pocket, but it looks like the radius of that FCG pocket corner might make it tough to be accurate. You might have to eyeball it with the calipers open and sight across the top looking from the right side of the receiver. Appears that it might be less than the 0.250" that I marked on mine; my minimum estimate there was 0.230".

[98Z5V]

31 minutes ago, Lane said:

I meant the measuring from the back of that slot into the FCG pocket, but it looks like the radius of that FCG pocket corner might make it tough to be accurate. You might have to eyeball it with the calipers open and sight across the top looking from the right side of the receiver. Appears that it might be less than the 0.250" that I marked on mine; my minimum estimate there was 0.230".

I'm getting 0.215" just off the radius, with the bolt catch up as far as I can get it.

Edited September 22, 2018 by 98Z5V

[98Z5V]

Yep, 0.215"...  Off the radius.

I don't know if this will help - this is gonna be a giant picture...

Edited September 22, 2018 by 98Z5V

[98Z5V]

^^^  That was a 5.8 meg upload.  Lemme know if you want it emailed to you, so you can blow it up.  I think the board software shrunk it...

[Lane]

The 0.215" measurement looks like as perfect a match as I can get based on the selector detent hole. That measurement is a mere 0.102" on the AR-10 print. I purposely estimated long because I knew I could take off more later without doing any irreparable harm.

I'm going to start on this as soon as I clamp it up level and straight again. I really appreciate you taking the time to measure that out for me. I feel a lot more comfortable that I can begin cutting with purpose now. I'll double check everything again; but I think I'm in the clear.

 

[98Z5V]

40 minutes ago, Lane said:

The 0.215" measurement looks like as perfect a match as I can get based on the selector detent hole. That measurement is a mere 0.102" on the AR-10 print. I purposely estimated long because I knew I could take off more later without doing any irreparable harm.

I'm going to start on this as soon as I clamp it up level and straight again. I really appreciate you taking the time to measure that out for me. I feel a lot more comfortable that I can begin cutting with purpose now. I'll double check everything again; but I think I'm in the clear.

 

I owed it to you, what now...three weeks ago?  I should have gotten it to you WAY sooner. 

Measuring long is the smart way to do it -you can take that out later...

[Lane]

I ripped on my .308 lower for a while after getting the measurements but couldn't clear the chips with the vacuum at 3:30 in the morning. I broke a few bits when I started again in the morning; in a bad mood. I'm still saving my favorite end mills for her and the finishing process. Progress at least, and the few marks on the top don't upset me at all. I like to have characteristic markings on all my lowers as a substitute for a serial number. I love NY.

The lady of the house asked to chuck up for her lower so I've taken a break. She did great work by herself while I cooked her dinner. I think she is further along than I am in terms of pocket depth, removal, and cleanliness now. I'm going to give her a chance to finish up tomorrow before I take it out of the vise and keep ripping mine. I have plenty of other things to do and I know better than to attack a lower without a clear head. 

I still haven't milled outside the lines on my .308, so I'm not at all worried about the finished product. And so it goes.

[Lane]

This is by far my favorite part of any build. The pocket is nearly to depth and the edges are cleaning up nicely. I've punctured through the trigger hole but might finish this one with a file in the end. It is far too easy to make it lopsided or too wide when milling that deep in the pocket. 

I had a momentary freakout when the end mill slipped out considerably while I was hogging out the floor of the pocket. It did it to me twice actually before I searched around for a marker to darken up my depth marking. Can't say I've ever had that happen before but after measuring everything no harm was done. I was definitely getting a little bit excited about how far along I was and taking off about 1/4" at a time while side cutting with a 1/4" bit. 

From here on I measure everything over and over with the calipers as I work slowly. I also switch over to carbide end mills as opposed to the much cheaper high speed steel. I do like the HSS mill bits for roughing though since they can take serious abuse without breaking. They do get dull pretty quickly, even cutting aluminum; and have  tough time punching through the hard anodizing on the top.

I'll likely be done with the milling later today but want to take extra care laying out the FCG pin holes. The hardest part is going to be adjusting them up 0.010" and I haven't yet settled on a method for that. 

[98Z5V]

Damn, Lane - that is awesome!

[Lane]

I am roughly finished with the milling now. I left a bit extra on the bottom and front of the pocket to trim out at the very end once I have the fire control group installed and checked for clearances. The trigger fits nice and snug so the side walls are just right. I simply like to take my time on those last bits so that I don't have to make modifications if I were to take too much off now. 

I left a little extra in the selector area walls because I don't think it matters. I will likely clean that up during any finishing work on the mill. I might even simply trim the front only for hammer fitment since there isn't anything else that matters in that location. 

Once I lay out the fire control group holes I'm ready to blast them out and finish this up. It's been a lot of fun so far and I can't wait to get it assembled. Unfortunately I don't have headspace gauges for the .308 so I'm going to have to figure out what to do in that respect. At least I can lube it up and break in the mechanics in the short term.

Thanks a lot 98Z5V; I wouldn't have jumped in this deep without your help.

[Lane]

I laid out the fire control group and checked all the vertical measurements. I think I'm going to go grind the threads off my barrel (thanks NY), and work on the upper build for a while while I think everything over. I think it's worth installing the rear takedown pin and double checking all my horizontal measurements from that before I go ahead and drill. I don't have anything else in mind at the moment but that's why I'm going to take a break to make sure I'm totally confident. Moving right along now...

[Lane]

I got the threads ground off the barrel and blued it. Assembled the upper receiver, attached the barrel, and torqued down the nut.

Installing the gas block roll pin was a huge pain; I'm glad Ceratac keeps shipping two with their kits as I ended up punching it out and installing the second one. I had difficulty getting the gas tube to clear the barrel nut and feed into the upper receiver. That as the reason I pulled the pin before determining I could tweak the gas tube to help it clear. I ended up tweaking it a bit more to get the gas block to mount correctly in the dimple as well. It all looks good now and the bolt carrier group feels nice and smooth all the way home.

I will get back to the lower again in a bit and should have that together fairly soon. I just want to double check all my measurements and layout before I go ahead with that. So far it all looks good though.

[98Z5V]

11 hours ago, Lane said:

Thanks a lot 98Z5V; I wouldn't have jumped in this deep without your help.

No thanks necessary, at all - glad I could help.  This is pretty damn cool, right here!... 

[Lane]

A couple of final questions on this build...

I looked at the sticky thread about checking headspace and the .pdf file that was linked from the Armalite website is not longer available. Does anyone have a copy of that? I asked Ceratac directly about the headspace today; and apparently they don't check if you buy the kit unassembled. Could have paid $15 more for all that, but I didn't know it or think about it at the time.

I went to my local gun shop this evening and asked about getting a headspace check; their gunsmith quit around the time the NY SAFE act went into effect (what a shock). Apparently there are very few left around. They only had one recommendation and it's very much out of the way for me. It looks like I'm going to have to invest in a set of gauges myself; but if I do that I might as well keep building more .308s for that kind of investment.

Does anyone trust the hack method with a 3x5 card; using a round, and then padding the bolt face with a small piece of a 3x5 card as the extra length of a No Go gauge? What about test firing it from a distance and checking the brass? I'm not saying I want to cheap out on this (surgery is far more expensive), but I figured I would ask anyway.

I think I'm going to drill out the FCG holes on an AR-15 lower I milled out a few weeks back just so I get my bearings. I would much rather make a mistake on that at a mere $30 than the .308 lower which is close to $100. Once I feel like I've got the confidence; I'm going to go for it (but without a headspace check it's not a rush tonight). I have a vise bolted to the table that is perfectly centered right now; I'm going to have to use a different method than I have in the past since I don't have any stubby 3/8" bits. I would have to remove the vise to use a full length bit and I'm just not ready to do that yet.

[98Z5V]

8 minutes ago, Lane said:

A couple of final questions on this build...

I looked at the sticky thread about checking headspace and the .pdf file that was linked from the Armalite website is not longer available. Does anyone have a copy of that?

Link that thread you're referring to right here - if it was one of the Armalite Tech Notes, then we have them.

[98Z5V]

13 minutes ago, Lane said:

Does anyone trust the hack method with a 3x5 card; using a round, and then padding the bolt face with a small piece of a 3x5 card as the extra length of a No Go gauge?

No, but there's another way, but it's very general it won't give you specifics, but it'll let you know if you're safe to fire or not.

Let me dig it up - I just sent it to someone.

Quote

Unfired round, and masking tape. Note, that's not an empty casing, it's an actual live round.

One disk of masking tape in the end of the unfired round, check it - that's "GO" gauge.

Two disks of masking tape, check it (shouldn't need this one, on a new parts build) - that's "FIELD" gauge.

Three disks of masking tape on the end of the unfired round, shouldn't close - that's "NO GO" gauge.

Make sure you remove the firing pin, just for safety's sake.  You should removed the extractor and ejector from the bolt, but you can do it wihout removing those.

Edited September 29, 2018 by 98Z5V

[Lane]

Yes; it was the thread regarding the Armalite tech notes: https://forum.308ar.com/topic/2774-measuring-headspace-on-an-armalite-ar-10/

When I click the link to the Armalite website, I get a page not found error so I don't know what the .pdf file has to say about the process.

The masking tape method sounds ok enough, but I would certainly like to read the tech notes at the very least. I also don't want to be the guy that got some mismatched parts and pays for it dearly. Spending the $90 on gauges isn't the end of the world for me, it just means I have to wait a bit longer, and perhaps sacrifice on optics in the short term. Then I'll also have to start another .308 build to make me feel better about buying those gauges. Any suggestions?