So, another fairly successful day at the range. Wealther was overcast with an ambient temperature of +6C throughout. 340 rounds were fired through the primary Test Rifle, bringing the total round count to 1050. In addition I fired 100 rounds through the Alternate Test Rifle to confirm consistent accuracy between the two samples. Neither rifle demonstrated superior accuracy and results were identical between the two samples for all intents and purposes. Both rifles functioned with 100% reliability. Note that I cleaned the Barrel of the Primary Rifle in preparation for today's accuracy testing. Aside from that however, the rifle's internals and gas system have not been maintained at any time during the 1050 rounds of test-firing to date. Reliability testing (without further cleaning) will continue with the Primary Rifle until such time as I run out of ammo or reliability falters. I will of course, report either occurrence.
The results of accuracy testing were acceptable, but not as good as was hoped for especially when it came to Match Grade Ammunition. Average resuts for PMC XTac 55 grain 5.56mm Ball hovered between 2" and 2 3/8".
The R18's 1:8" Twist Pencil Barrel doesn't seem to particularly like the heavier 77 grain HPBT bullets found in the IVI LE load. Groups with this heavier load averaged 2 1/4" to 2 1/2".
Finally the Match Grade ammo. First up was Federal Premium .223 with a 55 gr Nosler Ballistic Tip. The result was a mediocre, but acceptable 2 1/4" .
Next up was 53 gr .223 Hornady V-Max Superformance Varmint with a very similar 2 1/4" group.
The final load of the day was Nosler 77 gr .223 Hollow Point Boat Tail Match with a group measuring 2.5". It is worth noting that all of the declared group sizes are inclusive of "fliers" which in many cases expand a 1.5" 4-round group to 2+".
The accuracy, at 2-2.5 MOA is acceptable for a pencil barrel, but is still not optimal. Properly free-floated, the Barrel ought to perform better than it has to date. The apparent problem is fliers - usually one or two per five-round group. Those fliers typically open up the groups from 1.5 MOA to 2 MOA+. So what's causing those fliers? After careful examination of the Test rifles I believe that there are two issues at play. The degree to which each of these functional issues is impacting accuracy I cannot say for certain. What I can say with some assuredness is that the issues are legitimate. The first is that the Gas Block is impacting the Handguard during firing. The clearance between the Gas Block and the Handguard is inadequate such that Barrrel Harmonics/Whip is causing the Cap Screws in the Gas Block to mar the inside of the Handguard through the anodizing down into the bare aluminum. This suggest significant force behind the Gas Block when it strikes the Handguard. The plan is to have SAI reduce the bottom profile of the Gas Block to create additional space between the Block and the Handguard such that contact during firing is eliminated. Note the proximity of the Gas Block to the inside of the Handguard in the following photos:
The second issue at play is the Barrel profile. I believe that the "pencil" profile lacks the necessary rigidity to withstand intereference from the operation of the Gas System. In their quest for balance and weight savings i believe that the SAI Engineers went too light in the Barrel profile between the Chamber and the Gas Block. I have recommended a beefed up circumference between the Chamber and the Gas Block and a new custom Barrel is being created specificallly for Accuracy Testing. For comparison sake, the original AR180 Barrel measures .665" in front of Chamber and .625" at the Gas Block. The R18 measures .565" at the front of the Chamber and .555" at the Gas Block, indicating that the current R18 Barrel is .1" thinner in profile than even the original lightweight AR180.
I will continue with Reliability testing on the Primary Rifle pending production of the Medium profile Barrel , the reduced-profile Gas Block and further accuracy testing. I would like to reach 1500 rounds without maintenance of the internals (less the Barrel which was cleaned at 710 rounds). I will advise when the new Barrrel and Gas Block are ready and installed for further Accuracy Testing.