First step is taking a side profile measurement.
Using a standard measuring tape or ruler (if the length will work) place the zero-mark at the receiver end of the rail.
Pull the tape down the rail keeping the tape parallel to the top rail and trying to not cover any openings in the rail ( mlok, keymod, lightning holes or any other openings).
Once the measuring tape is positioned correctly, focus in on an area that you will want the cut and ensure it will not interfere with any openings and is at least .75 or 3/4 inches shorter than the length of the barrel you wish to use.
Second, do the same thing along the bottom.
Ensuring that the measuring tape is in the same starting location as the side profile pull the tape down the rail in the same manner.
Make sure that the same measurement from the side will work on the bottom as well with no interference with openings.
Lastly, make sure that the location you are wanting to cut will work in a straight line cut.
Using a straight edge or item with a straight edge to verify the cut will work all around the rail. This will help with any opening on the 45-degree areas of the rail.
If the straight cut does not seem to work, use tape or a marker to find a line that will be aesthetic for you. This could be an angled bottom, top, or both to accommodate an oddly placed opening or mirror an existing profile of the rail or similar.
If an opening will be cut through the rail chop can still work without any structural issues most of the time. If you are unsure if the opening is workable or not, simply send an email with a picture similar to the one on the left and we will try to help.