This is part two of Mike's rundown on his build of the Sharps Bros & Rainier Firearms Overthrow Lower Receiver. Catch up on part one here.
USA – -(AmmoLand.com)- After assembling our Sharps Overthrow lower receiver, we turned our attention to the upper receiver. For some reason, this component of the rifle gives a lot of novice builders agita, because they hear terms that are unfamiliar such as barrel torque, headspace, timing and they worry about doing something incorrectly. Trust me; the upper receiver is almost as simple as assembling a lower receiver as long as you use the proper tools and components.
We went with the Rainier Arms UltraMatch Billet Upper (MOD 3).
In the past, we had a few other manufacturers, and builders steer us away from billetted uppers, even though they were using billetted lowers. The main reason they were cited was the looks of these type of uppers, and in the period they were correct. Well, that seems to have changed because I liked the looks of the Rainier Arms Ultramatch, This receiver is CNC machined from solid billet 7075-T6 aluminum which is 70% stronger than the commonly used 6061 aluminum and finished with a mil-spec hard coat anodized to a surface hardness of 60 Rockwell.
Most of the heavy lifting was done by Rainier as the upper came with the ejection port cover and forward assisted already installed. There are small parts and pins associated with installing these two components, and they can create frustration during what should be a quiet and calm act of assembling a critical part of your rifle.
I personally think the forward assist is a bad idea on any AR rifle. Think about it, if your high pressure cartridge will not chamber for any reason, do you really want to force it into a chamber? I don't, but since it was already installed, I am not going to have a stroke over it.
This receiver also has M4 feed ramps present. The majority of my rifles have these, but I have a few that do not and after 25 years or so since their introduction on the M4 platform, I am still not convinced if they make or break a rifle, except possibly with regard to resale. Lastly, the upper is T-marked in the slots so you can remove accessories and put them back in the same place.
The first step I took on assembling the upper receiver was preparing the gas tube and gas block. This is relatively simple. You insert the tube into the block ensuring that the pin holes line up with the hole in the gas block and the solitary hole is facing downward. Then you drive the pin through the hole and make sure it is even on both ends.
It is so simple, of course, it marked my one major screwup on this project.
I had always been under the impression that all 16″ barrels used a carbine length gas system. When procuring the parts, I did not order a gas tube as I had a perfectly good one from another project (which you will read about soon) standing by and proceeded to mount it to the LUTH-AR low profile gas block. When I went to install it on the 16″ Brownell's B-TAC barrel, I came up a few inches short.
The back end of the gas tube is supposed to go into the upper receiver, and when a round is fired, the gasses from the fired round are supposed to drive the bolt carrier group rearward.
This was a few inches short of the upper receiver. Apparently, I had ordered the Mid-Length Gas System Barrel, and this gas tube would not work. I checked my parts inventory, and while I had another carbine gas tube and a spare rifle one, I had no mid-lengths. A quick trip to Cabela's and $12 later I had an Aero Precision Mid Length Gas Tube ready to go. I drove out the old pin, pulled the carbine gas tube and swapped it with the mid-length tube.
I installed the barrel and the gas block along with the barrel nut for the rail. The rail in question was a fairly new one from Aero Precision called the Atlas S-One. This handguard allows you to free float the barrel and because I tend to prefer a full-length handguard, I opted for the 15″ black version with M-Lok compatibility.
One thing I liked was that the barrel nut had a low profile and I did not have to worry about timing the nut to allow the gas tube to pass through it. With the barrel and gas tube installed, I slid the handguard over it and inserted the dovetail nuts and screw to secure it.
This part can be a little tricky if you mix the nuts up as one has a left-hand thread. As you tighten one, it tightens the other, and the end-result is a unique method of securing the rail.
Turning back to the muzzle, I installed a neat flash hider that I have grown fond of called the Triumvir by Franklin Armory. It allows for a reduced dust signature when shooting prone and acts as a compensator to keep you on target. It propels the muzzle report forward and makes shooting unsuppressed rather enjoyable.
I next installed a set of MagPul Pro Iron sights. These were not the more common plastic MBUS sights, but real steel Melonite coated sights made by MagPul. They look great, adjust well, and I think they should hold zero much better than the polymer offerings. There are counterfeits out there, so don’t try to save a few bucks by picking them up on Amazon or one of the auction sites where fakes are rampant. Source yours from Brownells or MagPul, and you won’t end up with a pile of crap.
Brownells provided a bolt and carrier, but the charging handle came from Rainier in the form of theAvalanche MOD2 AR-15 Charging Handle. Made in the USA from aircraft grade 7075 aluminum, this charging handle is completely ambidextrous and may solve some problems with gas leakage for those of us who like to shoot suppressed.
With one last check, everything looked good to go.
Attached to the Overthrow lower, we were finally finished with our assembly. Time to hit the range for some target time. All in all, this was an easy build, and I ended up with a very unique rifle with a lot of better/custom parts than if I bought one pre-built. You can do it.
About Mike Searson
Mike Searson's career as a shooter began as a Marine Rifleman at age 17. He has worked in the firearms industry his entire adult life as a Gunsmith, Ballistician, Consultant, Salesman, Author and was first certified to teach firearms safety in 1989.
Mike has written over 2000 articles for a number of magazines, websites and newsletters including Blade, RECOIL, OFF-GRID, Tactical Officer, SWAT, Tactical World, Gun Digest, Examiner.com and the US Concealed Carry Association as well as AmmoLand Shooting Sports News.
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