American Built Arms Mod*X Chassis Review

We have used and reviewed a variety of chassis systems for bolt action rifles. At this point you would think that we would be tired of breaking out our test-action and bolting it into another. However each time we try a new chassis, we find things we like, dislike and may have never considered before. When American Built Arms contacted us to take a look at their Mod*X chassis, we humbly accepted.

The AB Arms, Mod*X Chassis is an “AR Type” chassis system. It utilizes an aluminum center section that accepts AR type buttstocks and pistol grips. The forend tube is similar to the railed hand guards used on AR rifles, but uses a proprietary attachment, unique to the Mod*X. Standard AR rail systems will not work on the Mod*X.

IMG_8142When the Mod*X Arrived we were anxious to get a barreled action into it. With no “.308 Limited” matches on the schedule, I grabbed our Hawk Hill barrel equipped Remington 700. This rifle was built for us by Southern Indiana Precision and is extremely consistent. I pulled it from the stock that it was in and noticed our first issue. The recoil lug on the action would not fit the recess in the Mod*X inlet. When SIP built the rifle, they used an oversized PTG recoil lug. The Mod*X inlet was sized to only accept factory dimension lugs. This is actually a fairly common issue on stocks and chassis systems. The PTG lug is truly massive, so this was not a complete surprise. What was interesting is that the Mod*X inlet is only 0.240″. That is not a lot of room for any aftermarket lug. This can be easily remedied by milling out the chassis, but it should have more clearance from the factory.

Clockwise from 12 o’clock: Pre-2006 Walker, Timney 510, Timney 517, XST Mod 22

Once we realized the SIP built 700 wasn’t going to fit, we grabbed the only stock 700 left in the inventory. This was our AAC-SD “Budget Precision” Project. The budget went away long ago and it was sitting in a Cadex Strike 30 Chassis and equipped with an X-Treme Shooting Products Mod-22 trigger. The AAC-SD still wears its factory barrel and recoil lug. The lug slid into the chassis just fine. Then the trigger bottomed out. To be fair, the Mod-22 is a tank of a trigger. It has the largest housing of any trigger in the shop. It only took a minute to knock out the pins and replace the X-treme Mod-22 with a Timney 510. The Timney 510 and 517 (flat trigger) have been our go-to replacement for the Remington X-Mark. Unfortunately, neither Timney trigger would fit the inlet. A few more minutes work and I swapped on a pre-2006 Remington “Walker” style trigger. This trigger and the Remington 40X are extremely common on “factory” Remington 700 precision models (700P, 700PSS, M-24, etc). The pre-2006 trigger was also a no-go. As a last-ditch effort, I grabbed a discarded X-Mark trigger off the bench. The X-mark is Remington’s current 700 trigger. Thankfully the X-Mark allowed the action to drop in without issue.

IMG_8154Once we had the action in the center section, I realized another potential issue. The rear action screw hole is under the pistol grip. When the grip is mounted, the screw hole is covered. This prevents checking the action screw torque without removing the grip. This may not be an issue for the majority of shooters, but checking the torque on those screws is something I do when I am prepping for a big match. It is also something that LE Snipers should be doing on their monthly training day. Having to repeatedly remove the grip runs the risk of cross-threading or wearing out the grip screw hole.

AB_Arms_ModX_20150930_GH4_006The Mod*X chassis included an AB Arms “SBR P*Grip” and LuthAR Buttstock. Installation of those was a straightforward affair. Both install exactly like they would on an AR15.

After the rig was fully assembled we dropped our Bushnell XRS and Spuhr ISMS on top. I went about the task of adjusting the stock for my length of pull and comb height. Here we noticed another issue. The comb would not rise enough to get a correct cheek weld. Even with the LuthAR stock topped out, it was too short to put my eye where it needs to be. The simple solution would be to use a lower scope mount. This proves impossible with a 56mm objective and a one piece top rail.

The next issue we ran into with the LuthAR stock on the Mod*X is that the length of pull is far too long for the average shooter. I am not a small guy but medium to small shooters will find it very difficult to use this setup even with the LuthAR at its shortest adjustment. Normally, a slightly long length of pull can be overcome on a precision rifle. However, in this case the scope cannot reach back far enough to achieve proper eye relief. The Bushnell XRS is a fairly long scope. Shorter scopes with less eye relief requirements will be a larger problem.

Luther stock with the comb fully extended.

The last ergonomic issue we encountered was with the pistol grip. The AB Arms “P*Grip” would be perfectly at home on an AR15. The small profile and angle would work great with gloves and a “squared up” shooting stance. With the Mod*X Chassis the grip is positioned directly over the rear action screw. This, combined with the small diameter of the grip will place most shooters’ hands too close to the trigger. In my case, I could not get a proper trigger finger placement without floating my hand far off the grip.

Mod*X Target
Five shots at 100 yards from a factory Remington 700 AAC-SD.

Despite the ergonomic issues, once we got the Mod*X to the range, it shot well. We did not experience any accuracy issues and the system worked well prone and on the barricade.

I did find one feature that I thought was pretty neat. AB Arms chose to place the magazine release inside the trigger guard in the form of a button. This placement makes it pretty difficult to accidentally release your magazine. While I am usually hesitant to reach my finger inside the trigger guard to do anything other than press the trigger, there is little danger of a negligent discharge with a bolt action rifle. In most cases where you would release the magazine, the bolt would be open.


The magazine well of the Mod*X also shows some thought. The opening is cut out in a manner that allows for loading of a ten round AICS magazine, without having to lift the butt of the rifle.

AB_Arms_ModX_20150930_GH4_022The staff of AB Arms were very responsive and interested in the issues we found. They asked for dimensions on our Mod 22 trigger in order to improve their inlet. They also mentioned that many of the issues we discovered will be resolved in the Generation 3 of the Mod*X.

AB_Arms_ModX_20150930_GH4_016The overall fit and finish of the Mod*X chassis was excellent. We have no complaints there. We are anxiously awaiting the release of the Gen 3 version to see if the ergonomic issues improve.

Thanks to American Built Arms for sending out the Mod*X for review.


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