You should be able to do this. Obviously you have your original AMD65 tube to use as a guide. BTW a Bulgarian tube works fine on the FEG too. Anyway, hacksaw the new gas tube but leave it about 1 or 2 mm longer than the old tube; get it cut square and keep it square later when fitting. Now you'll need to flatten out the flutes around the end. The good military gas tubes are hardened so you might have to use heat to keep from cracking it and temper it when you're finished (red hot, oil quench). You might find one of your sockets in your tool drawer will be just right for very slightly expanding and rounding the end. Keep checking the old one for dimensions and appearance. Finally file it down to the same length, temper it and check fit. Paint as required. A gas tube should fit fairly tight and there may be some fitting on the latch end - tapping it into position with a rubber hammer helps . Be careful with this fitting and make sure the gas tube has fully snapped into place before filing on the slanted face so you can press down the latch lever. So, there are three fitting areas; the gas block end, the lower lug and the latching cam surface (on the tube head, never touch the cam on the latch). Normally a well-fitted gas tube will stay in place after you raise the latch lever and have to be gently pried up with a piece of wood. It will 'snap' in and out of place.