Another "Pointless" Project

AR Pushrod Conversion - October, 2009


Last year, the "Pointless Project" was to restore an M1897 shotgun. This year's chosen pointless project is a pushrod conversion on an AR-15 type rifle. Why is this pointless? The standard Direct Gas Impingement (DGI) system on the AR-15 (M16) family of firearms is a wonder of simplicity, light weight, and low cost. A tube runs from the gas block to the bolt carrier and uses propellent gases to cycle the action. This works really well with barrels 16" long or longer but not so well with barrels under 16" in length due to higher pressures, hotter gases, and less well-burned propellents. The drawback with DGI is that hot propellent gases are being dumped directly into the receiver. This causes carbon buildup (especially on the bolt), dumps heat onto lubricated moving parts, and increases wear. As the bolt and carrier heat up and the lubricant is blown away by gas and gums up with carbon, the mechanism can lock up until it cools down enough to be disassembled, cleaned, and lubricated. Annoying on the range and potentially dangerous to the operator in combat.

I've had enough stalls and overheating problems with the DGI system to start thinking about whether something a little better could be obtained. Looking at the available pushrod (PR) kits, I hit on a few selection criteria: it should be a mainstream product that will be supported for at least a few years, it should be user installable, it should be easily cleaned, and it should be affordable. One indication of the "mainstream" (since there is no military standard parts kit, as yet) would be use by more than one manufacturer. This winnowed the field down to Adams Arms (now also used by S&W) and CMMG, Inc (now also used by Stag Arms). The CMMG kit got the nod because the pushrod can be cleaned without removing the handguard (have since learned that's also true of the Adam Arms kit) and the price with a replacement carrier was greatly less than the Adams Arms kit with carrier since I didn't want to mess with replacing the carrier key.

The first problem to adapt a PR kit to my rifle had to do with the free float handguard (HG). At 1.75" internal diameter, there wasn't room for a PR kit. I would need an HG that had clearance equivalent to a round HG with a 2.0" internal diameter. Since my existing HG took the place of a barrel nut and delta ring, I would have to get an HG that did the same or get whatever parts were needed to mount the new HG. And I wanted to stay with a free float HG. Of the choices that CMMG would guarantee to work, the Daniel Defense Omega Rail looked like it offered everything I wanted so I ordered the CMMG package that included the Omega Rail. CMMG also gave me a choice of a bolt-on or pin on gas block. I didn't want to take the kit to a gunsmith to get the pin holes drilled, so I chose the bolt-on kit. Since the Omega Rail is designed to work with a barrel nut and delta ring, I ordered those parts from my rifle's maker.

I would also need to replace my sights because my front sight clamped to the DGI gas block. After many hours of Internet research, I selected the Troy backup iron sights (BUIS) and found the best price I could and ordered a set. This picture shows my rifle in its original configuration with the collection of replacement parts.

After removing the charge handle and carrier from the upper and seperating the upper from the lower, the first step was to remove the existing iron sights. This just required an allen wrench for the front and a straight screwdriver for the rear.

Step two was to remove the tapered pins that held the gas block. These are driven out from the left side of the rifle to the right side due to their taper. A cup point punch would have been a plus but a regular flat punch will work. Also a pin block is a good idea but I managed with a couple of lead ingots.

With the pins out, the gas block has to be driven off its seat. I put the barrel muzzle down with its flash hider on one lead ingot and drove the gas block off with a second lead ingot and a ball pein hammer

Next, I clamped the barrel in my vise and used a multi-tool to remove the flash hider.

This picture shows the stripped barrel with the removed parts.

Then the upper receiver gets clamped in the vise for removal of the free float HG. WIth the strap wrench tensioned in place, a rap on the wrench handle with my hammer got the HG broken loose to unscrew. The barrel can then be wiggled out of the receiver but that really isn't necessary. The barrel has a shank nut that sets headspace so there's no worry about changing headspace by dismounting the barrel.

Here's the old HG, the upper receiver, the barrel, and the strap wrench. The rubber strips are used to give the cloth strap enough grip to unscrew the free float HG. The pop with the hammer was still needed.

Mil-Spec Molybdenum grease is used to coat steel parts that are being mounted to aluminum parts so they don't become one piece of permanently bonded metal. I brushed grease onto the barrel shank and the threads of the barrel nut. This grease tube holds enough for thousands of barrels. Anyone want to buy a dab? Say $2 for a tablespoon (15 ml) in a baggie?

Next the barrel is put back in the receiver (it's got a key stud for alignment). The barrel-nut, delta-ring, weld-washer, and snap-ring assembly slides over the barrel and is wrenched into place on the receiver with the multi-tool (with a notch in the barrel nut, the hole in the delta ring, and the gaps in the weld spring and snap-ring lined up with the hole in the front of the receiver). Then the PR gas block slides down the barrel and is lined up and bolted against the shoulder on the barrel. Finally, the flash hider is threaded back on the muzzle.

This shot shows the main differences between the DGI carrier and the PR carrier. The same bolt is used with either.

This picture shows the removed DGI parts next to the converted rifle (sans HG and BUIS).

And here's the completed PR conversion rifle. Total elapsed time for the conversion about 90 minutes including photo journaling and searching for my snap-ring pliers.

On my first outing, I put about 400 rounds through my PR conversion rifle and demonstrated that old adage "A gun is only as good as its ammunition."

With Wolf "Military Classic" ammunition (55 gn bullet with lacquered steel case and Berdan primer) there were absolutely no failures of any kind. However, with Wolf "Performance Ammunition" (62 gn bullet with polymer-coated steel case and Boxer primer with primer sealant) there have been frequent cases stuck in the chamber that failed to extract. This ammunition has demonstrated this problem in two different rifles both before and after the PR conversion and with both dirty and freshly cleaned chambers. I won't be getting any more of it.

The PR system runs much cooler than the DGI system did. With my former unvented free float handguard, I couldn't touch the HG after 100 rounds. With the PR system, the rifle was comfortable to handle even after 400 rounds. The PR system was also much easier on receiver lubrication and I don't see any reason to do more than wipe fresh lube on the internal parts before the next shoot.

There's also an obvious improvement in extraction force. When the Wolf cases stuck in the chamber of a DGI rifle, the action wouldn't cycle and stayed closed on the stuck case. The action had to be pried or beaten open to get the stuck case out. With the PR system, the extractor tears right through the rim of the stuck case and hangs up in the open position when it tries to feed the next round. My friend, Dean, reported that he tried the Wolf "Performance Ammo" in his Mini-14 with the same result: the first round he fired stuck in the chamber but the PR action pulled the extractor right through the rim. To Wolf's credit, they are replacing (or refunding the purchase price of) the unused portion of the ammo for both of us.

Even ejection is improved with the empties landing neatly front and right in a small area. My friend, Joe, at the bench to my right was never struck by an ejected case. When I stood behind Joe with binoculars to call his shots, I was struck repeatedly by ejecting cases from his unconverted DGI rifle.

So the PR conversion gives me a cooler and cleaner running rifle that extracts and ejects better than before. Pretty worthwhile conversion overall for me.

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