I don't exactly understand your question, but the design of the M1A is a slightly modified version of the M14 which was a full auto main battle rifle. I do not think the act of shooting, either extended or otherwise will harm the front sights.
The issue you will have with shortening a barrel on an M1A is managing gas pressure. The system uses the back pressure of the gasses behind the bullet to operate the action. If you shorten a standard barrel, you need to increase the diameter of the gas port to allow more volume of gas into the gas cylinder. How much you enlarge the gas port is related to how much you shorten the barrel.
If you rebarrel your M1A, you run the risk of increasing the cost to the same or more than a Scout Squad or other factory option. Rebarreling M1As is not easy. Because of the barrel timing, most aftermarket barrels are short chambered and a pull through reamer is required to get the headspace correct. In other words, very high labor costs, not to mention that chrome lined barrels for this application run a lot more than the average rifle barrel.
My advice? Get the rifle you want the way you want it with out having to work on it.
Or, get a quality FAL. Pros for the FAL:
- Shoots any ammo without any muss or fuss.
- Can be cleaned from the breach end.
- Light years ahead of the M1A on field repair.
- Can be had in almost any practical barrel length.
- Superior ergonomics for 99.44% of humanity.
- At the time of this posting, quality magazines are slightly cheaper than quality magazines for the M1A.
Cons for the FAL:
- Not quite as accurate as the M1A.
- Iron sights are not as precise as those on the M1A.
As for twist rate, 1:11 is pretty standard for NATO rifles. It will handle anything up to and including 175 grain lead core bullets with aplomb.