Sharps Bros & Rainier Firearms Overthrow Lower Receiver ~ Lets Build It!

Mike gives us a run down on his build of the Sharps Bros & Rainier Firearms Overthrow Lower Receiver. Be sure and check out AmmoLand’s extensive article on tools you may need for your next complete AR rifle build. You can read Part Two of the Upper build for this rifle here.

UPDATE: Rainier has sent us a coupon code “AMMOLAND20” for the black colorway lower featured in the build so you can pick it up for $238.95.

Rainier Arms Overthrow Stripped Lower Receiver, Built by Sharps Bros
Rainier Arms Overthrow Stripped Lower Receiver, options Built by Sharps Bros

USA – -( The AR15, the black rifle, the Modern Sporting rifle: Call it what you want, it is probably the most user-friendly semiautomatic rifle in history. Not only is it easy to shoot and accessorize, but building one from the ground up is easier than working on a classic small block Chevy engine in the garage.

I have been shooting, modifying, building, and repairing ARs since I was handed an M-16A2 service rifle at the ripe old age of 17. I currently own close to 20 and about ¾ of them were built by us from virgin stripped receivers in the exact configurations which we wanted.

My latest assembly took on a life of its own during the process. I hesitate to call a typical AR assembly a “build” as most of the components are “plug and play.” There is very little if any, fitting. There is even less of a need for checking headspace and no heat treating involved. The only aspect of true gunsmithing may be the timing of specific components and of course applying the correct amount of torque in a few critical areas. I won’t get into the specific steps of an AR lower build as AmmoLand News has covered that in detail on related build articles found here.

Start with a lower receiver

As this is the part that is considered the rifle and needs to go through your local FFL for you to get it. It is typically my starting point. I happened into an Overthrow lowers by Sharps Bros and Rainier Firearms. Billeted lower receivers may be seeing substantial discounts now, but I was intrigued by the magazine well as it was milled to look like a Spartan helmet.

The helmet design bears battle worn dents, sword gashes and crack lines as if it was found at the Battle of Thermopylae Pass in 480 BC. It is machined top to bottom from 7075-T6 aluminum, a peak-strength corrosion-resistant aluminum alloy and finished in a hard coat anodize black finish per MIL-A-8625, the anodizing model for most military and aerospace products.

Rainier Arms Overthrow Stripped Lower Receiver in Black

Rainier Arms Overthrow Stripped Lower Receiver, Built by Sharps Bros

Rainier Arms Overthrow Stripped Lower Receiver is full compatibility with all mil-spec internal parts, aftermarket mil-spec upper receivers, GI standard issue magazines, and Magpul P-Mags. The caliber is marked “Multi” and unassembled it weighs 11.5 ounces.

Rainier Arms Overthrow Stripped Lower Receiver, Built by Sharps Bros

I installed a standard lower receiver parts kit and a match grade drop in trigger assembly from Timney Triggers to round out the basic lower build. This is a single-stage trigger with a 4-pound pull, little to no over-travel and breaks like glass. After installing it, you must tighten the two screws to lock it to the hammer and trigger pins.

AR-15 Competition 4-lb Trigger
AR-15 Competition 4-lb Trigger

The other part I splurged on was a Geissele Maritime Bolt Catch. A bit larger than a standard bolt catch Geissele’s version has increased surface area and aggressive checkering for ease of function when locking back the bolt or releasing the bolt after a magazine change, even while wearing gloves. The extended size adds an increased functionality for left-handed shooters who no longer have to break their grip to manipulate the bolt catch because it allows them to reach the controls using the index finger of their left hand.

Geiselle Maritime Bolt Catch.

I will forever applaud Sharps Bros for using the threaded bolt catch pin on their 5.56 pattern rifles. These were initially found on the LR-308 type of ARs. There is nothing wrong with the standard bolt catch pin when installed with the proper tools like a pair of padded-jaw pliers or an offset punch. The threaded version can shave several minutes off your assembly time.

Geiselle Maritime Bolt Catch. installed

On a side note, the magazine button installed properly in contrast with a build I did a few years ago using the Sharps Bros. Jack lower, where I had to use an extended magazine button due to the contours of the skull-shaped magazine well. The latest Sharps Overthrow shown here is contoured properly in this regard.

For a pistol grip, I chose a BCM GUNFIGHTER Grip Mod 0. It features a reduced angle for improved ergonomics and has a hinged trap door for storage inside the grip with a water-resistant rubber gasket. Additionally, you get both an extended modular insert to close the gap between the trigger guard and pistol grip and a smooth modular insert for use with some rifles with larger built-in trigger guards.

Rainier Arms sent me one of their Rainier Arms Ambidextrous Mod 2 Safety Selector Switches in red to try out. It installed perfectly and passed all of the function checks.

Rainier Arms Mod 2 Ambidextrous safety.

I was getting a little burned out on most collapsible stocks but did not want to go with a rifle-length buffer tube, so I went with a Mission First Tactical Battlelink Minimalist buttstock. This is a lightweight option that features an enhanced cheek weld and a non-slip rubberized butt pad, which is angled for faster presentation and a sure mount for the gun, even when wearing body armor or a tactical vest. The quick detach sling mounting point is placed under the buttstock to allow true ambidextrous use.

Mission First Tactical AR-15 Battlelink Minimalist Stock
Mission First Tactical AR-15 Battlelink Minimalist Stock

Rainier Arms Overthrow Complete Lower Receiver Money Saving Options

Rainier Arms Overthrow Complete Lower Receiver

So this Sharps Bros & Rainier Firearms Overthrow Lower Receiver build has a lot of super-nice higher-end parts, at last count, this lower build probably cost me around $755.00 give or take. Most of that extra cost being the match grade trigger. If you are on a tight budget or happy with basic stock parts for a truck gun but like the Overthrow lower look, Rainier has complete rifle and pistol builds of the Rainier Arms Overthrow Complete Lower Receiver in stock and at publish time they were running $499.99, just add an upper and your good to go.

With my lower fully assembled, I was ready to get to work on the upper receiver. And that will be my next article.

Overthrow complete lower assembled and ready to rock!

Read Part Two: Sharps Bros & Rainier Arms Rifle Build: Completing the Upper Receiver

Next Gun Button

About Mike Searson

Mike Searson
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, and the US Concealed Carry Association as well as AmmoLand Shooting Sports News.

  • Home page:
  • FB:


1 Comment
Inline Feedbacks
View all comments
Allen Steinbrink
Allen Steinbrink
2 years ago

Own 1 ruger 9mm love it