I think that the Billet approach to AR180B Upper Receivers has reached its optimal form in terms of material thickness and consequent weight. Any further thinning out of the Receiver walls would (IMHO) be inadvisable and may introduce unwanted fragility or Receiver flex resulting in cracking. To further reduce the weight you would have to completely redesign the upper to utilize formed sheet-steel construction similar to that of the original AR180. Obviously not a cost-permissive option for SAI having already reached this point with Billet manufacture. Like JR, I happen to own and have extensively fired an original Sterling Ltd AR180. Trust me when I say that the original AR180 and the R18 may share the same operating system, but beyond that they are "apples and oranges" in terms of manufacturing and design refinement. I would personally take an R18 over the OG AR180 all day, every day (and twice on Sunday). They are worlds apart in terms of ergonomics and control suites, with the R18 being the far more refined of the two platforms.
"OG" Sterling Ltd AR180
I fired another 410 rounds through one of the test rifles today. Rather than split the session between both test rifles, I elected instead to run one rifle relatively "hard" and certainly hot. The rifle had not been cleaned since its earlier range session (160 rds thru each rifle) and I did not add any lube or otherwise "tweak" the rifle. I just dove right in, using the Swampfox 1-10x LVPO for function testing. Accuracy testing took a back seat to function testing today during the limited range time that I had. Still, looking through my Spotting Scope, 5-round groups @ 100m averaged between 2.5" (Winchester White Box 55gr M193 & IVI 77 gr HPBT) and 2" even (Federal American Eagle 55gr FMJ). I fired a total of 90 x IVI 77 gr, 100 x Federal 55 gr, and 120 x Winchester 55 gr.
R18 Test Rifle
There were 2 malfunctions attributable to the same magazine (2nd round nose-up fail to feed), and a total of 11 light primer strikes which are directly attributable to the Trigger Tech after-market Trigger that I have installed in the test R18. Of those 11 failures to fire, all but 3 of the rounds detonated on the 2nd attempt. Once nice feature of the R18 is that you can pull the action far enough rearwards to re-set the Trigger, without ejecting the round in the Chamber. This saves time re-setting for a second attempt on balky primers. There were no failures that I would attribute to the R18 iteself. Recoil was extremely mild, ejection was positive and consistent at 4-5 o'clock and downwards (due to the convex shape of the Brass Deflector).
In a rare flash of brilliance last weekend I decided to see if the Operating Springs out of my OG AR180 would fit and function in the R18, seeing as how the original Springs shipped with the pre-production Test Rifle were a bit on the soft/weak side. Well, as it turns out the OG Springs fit and function 100%, despite being somewhat "snappier" and 1/2" longer than the upgraded Operating Springs that JR Cox provided to me after my initial Range session. The Upgraded Springs that were provided to me functioned 100% as well, despite being a bit softer/weaker than the OG Operating Springs. My recommendation to JR is that SAI replicate the OG AR180 Operating Springs and adopt those for use in the R18. They fit and function perfectly, so no further modification is required. Simply "plug and play". The difference between the OG Operating Springs and the original Operating Springs provided with both Test Rifles is signficant. With the original springs the R18 had difficulty stripping the top round out of a 5-round PMag, let alone a higher-friction STANAG metal mag. With the OG Operating Springs installed, the R18 action strips the top round out of a 10-round metal pistol magazine with authority. The gas system, although fixed on the Test Rifles rather than the adjustable Gas Block to be featured in the Production R18, appears to be well-balanced with the OG Springs. The Bolt-Carrier throw resulted in full run-out, without excessive recoil, There is no perceptible gas blow-back to the user as is sometimes the case with the DI AR15. This is because all excess gas in the operating system is exhausted at the front of the Gas Piston System, rather than through the Bolt-Carrier near the firer's face.
R18 Operating System
Total round-count through the first Test Rifle is now 470 without cleaning, with no additional lube, and without any stoppages attributable to the rifle. The initial sluggishness chambering the first round of a full STANAG magazine was 100% resolved through use of the stiffer OG AR180 Operating Springs.
Comparison of Revised R18 Operating Spring (Top) versus longer & stiffer OG AR180 Operating Spring (Bottom)
Testing will continue ASAP, to include accuracy testing with 3 different brands and loads of 5.56mm Match ammo. Watch and shoot....