What I learned as a noob about building a "Frankenstein" LR-308

[Homebuilt]

My initial project began with a James Madison Tactical 80% polymer lower, with most of the remaining parts from Delta Team Tactical. Specifically, the "LE" 6-position stock with (mistake #1) AR-15 7.25" tube, 3.25" buffer and 11.25" spring, a generic LPK, "Goblin" upper, recoil technology BCG, a 15" free float hand guard, 18" Bear Creek (BCA) parkerized barrel, and an adjustable gas block with (mistake #2) mid-length tube. I also invested in a bunch of different brand mags, 20-round steel ASC, AR-Stoner (basically the same thing), 20-round poly Pro Mag, and 25-round Pmags.

Milling out the lower was fairly easy. For reference, A few years back, I had milled out a 7.62x39 AR-15 Polymer 80 lower. That went together well for a first-time build. Not so much this time. At about the same time here, i also bought a couple of BCA side-charge AR-15 uppers, one in .223 Wylde and one in 6.5 Grendel. Both functioned great on a Polymer 80 lower and a Rise Armaments drop-in trigger.

The first problem was the bolt would not lock back on an empty mag. The second one was unreliable feeding. I could get it to hold manually, but as it turns out the problems here were entirely related to the wrong buffer parts. I didn't find this out until I had tried several mags and bolt catches.

Along the way, I became interested in the side-charging 308 uppers sold by Bear Creek. I waited (not long enough, the same upper was $40 less about 2 weeks later) for one I liked to go on a 10% off sale and bought it. This one has their 20" parkerized barrel, and is very similar to the other upper aside from side charging. I found this upper would also not lock back on an empty mag. I had reached out to both James Madison and Delta for some help. While both were helpful, neither found the root problem.

I also picked up an 80 Percent Arms Gen 3 jig and their aluminum lower. I managed to punch a hole in the floor of it and break off 2 of the 3 tangs on the end mill, but the lower came out OK otherwise.

Since I'm mechanically-oriented (I mod cars and bikes), I looked at the parts and marked my BCG in the buffer tube while it was all apart. I found the buffer and the spring were both too long. I had a heavy (5.x ounce) Armalite buffer that was 3.25" long, so I cut it down to 2.5" and dropped one of the weights out of it. It ended up around 4 ounces (less than ideal). Trying this as well, I found the BCG still would not come back quite far enough, so I cut down a spring from about 11.75" to about 9". Success! The bolt catch now worked as it should. I knew a cut-down spring and light buffer was not the permanent solution, though, so I did more reading and learned the LR-308/AR-10 uses a longer (7.75") buffer tube than the 7.25" AR-15 tube I had.

I did some test firing with the above combo and the guns ran well, I had a 3-4 o'clock ejection using steel case ammo.

I picked up a couple of 2.5" buffers, one 5.x ounces the other 3.x (don't recall exact numbers).  See attached pic for reference. One has a "nub" on the end that prevents the upper from opening for servicing! The other has a high shoulder and won't work even with the cut down spring (it will likely work with the longer buffer tube).

I did some more reading, some at this site, and also at others. Here's what I learned to solve my problems:

1. The mid-length gas tube was slightly too short for my upper per a very helpful post by member 98Z5V. The tube should terminate halfway in the notch you see with the bolt open. Mine (Goblin upper) was about 1/4" too short. I found the actual Armalite AR-10 tube was the correct length (12.x), and bought it.

2. There IS a difference between AR-10 and AR-15 carbine buffer tubes! My tube, AR-15, was 7.25" long. The AR-10 tube is 7.75" long. Just like with the gas tube and buffer, a fraction of an inch makes a difference. I have ordered 2x Leapers # TLU002 tubes for my guns (not depicted).

So my plan at this point is to get another proper 5.x ounce 2.5" buffer and another of the Armalite springs, then use them with the Leapers tubes.

 

[392heminut]

1 hour ago, Homebuilt said:

 

So my plan at this point is to get another proper 5.x ounce 2.5" buffer and another of the Armalite springs, then use them with the Leapers tubes.

 

 

You DO NOT want to run a 2.5" buffer in the "7.75" buffer tube! The longer tube uses the standard length 3.25" buffer. If you use the shorter buffer your BCG will slam into the lower receiver where the buffer tube screws in and beat the hell out of it!

[Homebuilt]

Thanks for the info. That makes sense, so I can use my current 7.25" tubes then. I installed an AR-10 specific spring I got from Midway with my cut down 2.5" buffer after I had posted the above and it seemed to work well on a manual check. The high shoulder 5 oz buffer will not work in my setup, though, need to see about returning both of those. I found a 2.5" 5.3 oz buffer on sale at Primary Arms and ordered 2 of them along with a pair of Sprinco AR-10 orange springs.

I plan on building a couple more LR-308s, and have 2x 80% lowers ready to mill once the replacement mill gets here (the place I ordered one from sent me the wrong part). I can use the 7.75" tubes with those, as I think the 3.25" 5.x oz buffers are less costly.

 

EDIT: Forgot to say great user name! The best new car I ever owned was a 2009 Hemi Orange R/T Challenger.

Edited October 2, 2021 by Homebuilt
Added info

[edgecrusher]

Just to be clear, there has never been an internal depth receiver extension that measured 7.25”….. or at least not one that worked. This chart is from Clint @heavybuffers.com, THE #1 guy in the industry on buffers

[Armed Eye Doc]

19 minutes ago, edgecrusher said:

Just to be clear, there has never been an internal depth receiver extension that measured 7.25”….. or at least not one that worked. This chart is from Clint @heavybuffers.com, THE #1 guy in the industry on buffers

I think this may be internal vs. external dimensions.  @Homebuilt, the accepted way to measure receiver extensions is the internal depth.   There can be too much variation with external length.  1/8" difference of internal depth or less can be the reason for a rifle to function correctly or not.  We are not picky or bitchy but we may be anal about some terms, descriptions or measurements.  It is because of the lack of standards across different manufacturers.  As you have already discovered, what sound like small details can make a big difference in a rifle's performance. 

Feel free to ask any questions about compatibility.  It is cheaper to ask before buying, unless you say money is not a concern.   We can certainly spend your money for you like a soon-to-be ex-wife. 

[98Z5V]

OP, read this.  This is a decade of experimentation that came to writing this, in the first place.  That goes back to 2010  - before shiit got crazy with the Large Frame ARs. 

The craziness that developed, about the 2010 timeframe, is why this board was even developed in the first place - too much bad information out there, and nothing making any sense.  This is a decade of research, and gun builds, to both prove points, and prove other points WRONG...  It stands on it's own, and can't be refuted.

In my own life, and own world, I'm still waiting for someone to counter this information, and prove any of it wrong.  It's been a long process, and no takers on that.

 

[Homebuilt]

Thanks! I used that thread to help me get the proper length gas tube on my gun. I clearly did not read it far enough to learn about the buffer/spring system. As it stands now, my final setup here will be a 5.4 oz 2.5" buffer w/ Sprinco orange spring in the 7.25" +/- internal depth tube. I forgot that I had bought a couple of 5.4 oz longer buffers (3.25"), so if for some reason the above setup does not work, I can use them in the longer tubes I have coming via slow boat from California. 🙁

I got my replacement end mill and went ahead and milled out 2 more 308 lowers, one aluminum and the other polymer. Definitely went better the 2nd time around, the 3rd gen jig really makes it easy- so long as the router doesn't change depths. Too much downward pressure is what I think caused my initial problem. I did catch the router slipping on the initial cut depth setup but caught it before I did any milling.

Thanks again for all of the advice.

[98Z5V]

15 hours ago, Homebuilt said:

Thanks! I used that thread to help me get the proper length gas tube on my gun. I clearly did not read it far enough to learn about the buffer/spring system. As it stands now, my final setup here will be a 5.4 oz 2.5" buffer w/ Sprinco orange spring in the 7.25" +/- internal depth tube.

You'll need a 7.000" internal depth carbine receiver extension to run a 2.500" buffer in a .308AR platform (not counting RRA). 

Again, there is no 7.250" internal depth carbine receiver extension - if you have one, it's out of spec, and it'll tear up the ears on your lower receiver, when the BCG smashes into it, because it's too long, internally...  

[Homebuilt]

Thanks, my error, I meant to type OD since that was what I had measured, but the 7.0 ID is the better reference.

[Homebuilt]

Update- I got the 2.5" heavy buffers installed in my short tubes along with the Sprinco "orange" carbine springs. Shot some steel case ammo this afternoon & got a perfect 3-4 o'clock ejection. No fails to feed or any other problems. On the side charge receiver, I can see the bolt is being stopped about 1/4" or so before the slot in the receiver. On the rear charge gun, it feels/sounds like the cushioned end of the buffer is hitting the end of the tube.

Thanks again for the advice here.

[98Z5V]

Great news, and great follow-up, man.  Glad that gun is running. 

[Homebuilt]

UTG tubes #TLU002 arrived today and I can confirm they are indeed 8" long.

[98Z5V]

3 hours ago, Homebuilt said:

UTG tubes #TLU002 arrived today and I can confirm they are indeed 8" long.

Internal depth is what matters.  OAL doesn't mean a thing. 

[Homebuilt]

My tape wouldn't fit inside the tube, but I found a long handle ratchet and marked it. Internal depth is as expected at 7.75".

[98Z5V]

2 hours ago, Homebuilt said:

Internal depth is as expected at 7.75".

There are two internal depths for carbine extensions.  7.75" isn't one of them.

[Homebuilt]

Interesting. I know a ratchet is not the ideal measuring tool (rounded end), so I used a plastic rod with a flat end. Accounting for the kerf for the marker I used, I used a digital caliper and got 7.749" inside depth.

I then assembled the Leapers tube onto my side charge rifle with the Sprinco orange spring and a 5.4 oz 3.25" buffer. It's too early in the day to shoot it, but the function check looks to be the same as before. The bolt catch works properly, and the charge lever stops maybe 1/4" before the end of the slot in the upper. I can feel the buffer hitting the tube end at full travel. I'll fire some rounds later today and update then.

[jtallen83]

5 hours ago, Homebuilt said:

I used a digital caliper and got 7.749" inside depth.

Picture of where you are measuring to? If it is that deep you may need to add a quarter or two so the carrier doesn't hit the ears on the lower. 

[Homebuilt]

I'd need 3 hands to take that pic and measurement LOL! I put the rod into the tube and marked it with a sharpie at the end of the threads. The caliper measurement was to the inside of the mark, the shortest length. EDIT, I measured the shorter tube while it was off and got 7.035" inside depth to the edge of the threads. That's a difference of about .714".

Update, field tested the longer tube, 3.25" 5.4 oz buffer, and Sprinco orange spring. Consistent fail to feed. Using the great idea of if it ain't broke don't fix it, I'm going to put the shorter tube and buffer back into place. 😀

Edited October 9, 2021 by Homebuilt
Added info