I just bought some Barnes XPB's 125gr .357 caliber. I also bought a little Kel-Tech 380 Auto pistol. I am reloading for the 380 and my three 357 revolvers. I noticed that the 380 bullets are .355 and that the 357 bullets are just that, .357 inches. Is it safe to interchange the two provided that the correct powder charge is used? Also, what is the practicality of polishing the feed ramp of the 380 to facilitate smoother feeding, and if it is practical, is it something that I could do myself with possibly a Dremel tool. Hopefully those of you who read this are not cringing at this point. If I was given to doing stupid things before asking people with the proper knowledge, I would be typing this with one hand. Thanks in advance for your help.

Just checked my books and there are no .357 bullets listed for the 380!

The 9mm Luger lists the 125 grain .357 bullet, and all of the other bullets are .355 diameter!

My favorite bullet for the big .380 pistols is the .357" Hornady XTP 158 gr. JHP.
But for the P3AT I use the .357" Honady 110 gr. XTP or .355" Speer Gold Dot 90 gr. JHP

While it may be possible to get performance in excess of 357 mag from some 380 pistols, it is not possible to reach even 9mm performance with a stock P3AT pistol. The P3AT pistol has several things holding it back:
1) Case support. I am measuring the feed ramp to intrude .270" from the breech face on the first generation P3AT and within .005" of that on the second generation. With the .380 brass case web .170" thick, that leaves .100" of thin unsupported case wall on the bottom of the chamber. Other 380s support all the way to the web.
2) The mass of the barrel and slide is low. The whole pistol only 8.3 ounces unloaded and less than 8 ounces in slide and barrel
http://www.kel-tec.com/p3at_pistol.htm
3) The spring volume available is small. Changing springs will get the opening force to 5 pounds and the force with the slide all the way back at 10 pounds. With a slide travel of 1.3", the energy that the recoil spring can absorb is about 10 inch pounds. The Generation 2 pistol has more volume for springs and more can be done there.
4) The chamber walls are thin. We may know the slide is 4140 treated to RC47, but we don't know the composition of the barrel. If we assume the same, we have a steel that is good to 200 ksi. With the thin spots on the Gen 1 at .025", the maximum chamber pressure should be P = 2 thickness Steel Stress /inside diameter = 2 .025" 200k/.380 = 26,315 psi. The Gen 2 with .030" walls should be good to 31,578 psi. The strength of the brass must be added to these numbers. Overloading the P3AT until the case bulges will yield Quickload calculated pressures of about 26,000 psi.
http://www.thehighroad.org/attachment.php?s=&postid=1286012

As as result of these limitations:
1) With 6.2 gr. Power Pistol, 90 gr. Gold Dot, the P3AT will do 1100 fps with it's 2.76" barrel.
2) In contrast, with a 1903 Colt .380, I can get 8.5 gr. Power Pistol to push 158 gr. XTP at 1187 fps with it's 5" barrel. http://www.loni.hr/browning/brow1903/brow1903.htm

Even with these limitations, the P3AT at 10 ounces loaded, is my favorite carry piece.

It's never a good idea to run bullets down a bore that's .002 smaller, but it could be done w/ light loads. But why, good 124grjhp are available for the 9mm, use those. AS to loading a 158grjhp, I'll cry on this one. The bullet seats so deep you could not get enough powder in to get anywhere close to 900fps & w/ the bullet being .002 larger, definetly a problem in a blowback pistol. Be carefull of taking anyones advice on a forum w/o looking into it yourself (8.5gr of PP fills a .380 case).