Yes, it can be legal to make a silencer. For everything Form-1, from silencer designs that are easily made, to filing forms with the BATF, to 3D modeling. Remember, you must have an approved BATF Form-1 to make a silencer. All NFA laws apply.
Since I got my Kahr P45 I have wanted to suppress it, I went so far as to get a 2nd barrel for it just incase something went wrong. Well I had spoken to several people about extending the factory barrel and threading it, I was told by several people that I would have to get a custom barrel made that was extended and then thread it. I tried to see if a P45 barrel would work but no go. then one day I was looking on this forum and ran across a thread about extending and threading a sig p238 barrel and posted to see if the same could be done to the p45, kc0myg replied that it could be done but he would have to see the barrel/slide to see how the best way to do it would be. He looked @ pics of the p45 and let me know he thought it could be done so I shipped him the p45 top end and spare barrel. He then looked @ it and suggested a fixed extension that was threaded. I gave him the green light and within a week or so I had it back and could not be happier. I got a chance over the weekend to run about 20 rounds thru it (blazer 230gr) and a couple handloads with 230gr fmj and N320 powder at a local indoor range. The scilencer I have for it is a Thompson Machine Poseidon 45 and with both rounds was quite and I must say it was better then I thought it would be. I ran several rounds thru it with out the poseidon on to verify it was shooting as well as it did before I sent it, I am happy to say it is. I then ran a few rounds thru it dry and with out the wipe and it was quieter then with out the poseidon but not close to hearing safe. Next I added the wipe which made a big improvement and then I added a bit of wire pulling gel (a little to much and the first shot looked like I had been sprayed with oil ) but the next few shots were the quietest and what I would call hearing ok. Over the next few months I want to work up some handloads that should make it very quiet. I am very happy with the work kc0myg did and will be sending my p380 to him for the same work. Here is a pic, I will post more pics of kc0myg's work this weekend.
http://www.youtube.com/watch?v=BDtd2jNIwAU MUSAFAR!
http://www.youtube.com/watch?v=8CrOL-ydFMI This is Water DavidW
Complete Form 1s http://www.silencertalk.com/forum/viewtopic.php?f=10&t=79895
I had a Kahr TP45 barrel chamber area turned down on an engine lathe to the exact length of the chamber area/lockup area on the PM45 barrel and the modified TP45 is an exact drop-in fit. This leaves almost an inch to thread with a .578x28 die and square the mating surface on the threaded end to seat snug against the suppressor. EZ-PZ - a perfect fit.
If you put the PM45 and TP45 barrels side by side with the breach ends lined up and the top surfaces clamped to a smooth flat surface, you can see they are identical. The TP45 is an inches longer. And chamber lockup area is longer. But this can be turned down to drop into the PM45.
then one day I was looking on this forum and ran across a thread about extending and threading a sig p238 barrel and posted to see if the same could be done to the p45, kc0myg replied that it could be done but he would have to see the barrel/slide to see how the best way to do it would be. He looked @ pics of the p45 and let me know he thought it could be done so I shipped him the p45 top end and spare barrel. He then looked @ it and suggested a fixed extension that was threaded. I gave him the green light and within a week or so I had it back and could not be happier. 
I got a chance over the weekend to run about 20 rounds thru it (blazer 230gr) and a couple handloads with 230gr fmj and N320 powder at a local indoor range. The scilencer I have for it is a Thompson Machine Poseidon 45 and with both rounds was quite and I must say it was better then I thought it would be. I ran several rounds thru it with out the poseidon on to verify it was shooting as well as it did before I sent it, I am happy to say it is. I then ran a few rounds thru it dry and with out the wipe and it was quieter then with out the poseidon but not close to hearing safe. Next I added the wipe which made a big improvement and then I added a bit of wire pulling gel (a little to much and the first shot looked like I had been sprayed with oil 
) but the next few shots were the quietest and what I would call hearing ok. Over the next few months I want to work up some handloads that should make it very quiet. I am very happy with the work kc0myg did and will be sending my p380 to him for the same work. Here is a pic, I will post more pics of kc0myg's work this weekend.
