Customizing the 1911: round butt Officer’s ACP conversion

Perhaps the biggest disadvantage of 1911 pistol is the shape of it’s butt.  The square edge can print when the pistol is carried concealed.  To address this, Ed Brown designed a Bobtail conversion mainspring housing for full and Commander sized guns.  This conversion replaces the factory main spring housing (MSH) with a rounded model that uses an Officer’s length mainspring and mainspring pin location.  The frame is then ground and blended to match the new profile.  The Bobtailed guns look good and carry well making them quite popular.


The smallest of the three conventional 1911 sizes, the Officers ACP starts with a relatively short MSH.  While a factory Ed Brown Bobtail MSH can be used and modified to round off the rear of the pistol, the effects aren’t as apparent as with the full sized guns.  A few manufacturers, notably Wilson Combat, manufacture a round butt MSH housing for the compact Officer’s sized guns.

On this project gun I’ll be converting the butt on a Colt Officer’s ACP to a round butt.  I’ve had this gun since the mid-1990s and figured it was time to give it a new lease on life.

For reference purposes, a schematic of a 1911 pistol can be found here.

Before we get working, take a few minutes to read’s disclaimer:

Warning:  The contents of are produced for informational purposes only and should be performed by competent gunsmiths only. and its authors, do not assume any responsibility, directly or indirectly for the safety of the readers attempting to follow any instructions or perform any of the tasks shown, or the use or misuse of any information contained herein, on this website.

Any modifications made to a firearm should be made by a licensed gunsmith. Failure to do so may void warranties and result in an unsafe firearm and may cause injury or death.

Modifications to a firearm may result in personal injury or death, cause the firearm to not function properly, or malfunction, and cause the firearm to become unsafe.

I ordered the following tools from Brownells to complete this project:


I selected this smooth aluminum MSH for this project.  The frame is stainless steel, however, the gun will be coated in Cerakote, so the aluminum will match when it is complete.


In case you were wondering why I just didn’t use an Ed Brown Bobtail MSH, take a look at what happens when it is installed in the frame.  Note the lip on the bottom of the MSH.  This doesn’t meet the frame and would leave a small opening in the bottom of the pistol if it was simply cut down.  I can use it as a template though and trace the approximate shape with a marker.


If you take another look at the smooth aluminum MSH I am using (temporarily pulled out from the frame), the lip is in the right spot and will not leave a gap when installation is complete.


I like to blend my main spring housings.  I begin with a file.  Since I am filing aluminum, which is relatively soft, file chalk prevents the tool from fouling.


Coming along nicely.


Finally I finish the surfaces with some abrasive cloth.


Grinding down the shape can be accomplished a number of different ways, perhaps the fastest, and riskiest is to use a belt grinder.  If you aren’t careful you can grind too much and ruin the frame.


Another view of the MSH off the grinder.  Note the cut isn’t flat and has a slight radius.


Back to the bench with some files and abrasive cloth.  Looks pretty good so far, doesn’t it?


This MSH will have a golf ball texture to match the rest of the gun.  I coat it in Dykem and mount it in the mill vise.


I use a 1/4″ 2 flute end mill to machine a pattern.  Each cut is .025″ deep and spaced .150″ apart.  Rows are staggered .075″ on both the x and y axis.


The finished design looks pretty good!


The newly modified MSH looks and handles well.


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