I am going to be taking my Ruger Gp-100 out shooting in the next couple of weeks. I think I am just going to buy a bunch of .38 ammo since its on sale around here. When I have the gun sighted in for .357 and shoot .38's if I remember correctly the used to shoot low. Has anyone else notice this. If I recall they used to be alomost 4-5 inches low at 25 feet. How much would I have to move the rear sight just to get my started.

I was shooting a mixed cylinder of .357, .38+p, and .38 wadcutter at a 25 yard bullseye target as a recoil exercise. I was able to hold the black with all the rounds, but the wadcutters were hitting about 4" lower than the .357's.

When shooting within 21 feet at least I notice no drop.

  I have a GP-100 and I don't notice the drop also.  Even at more than 25 ft.  But then again I reload my ammo.  The .38 SPL. ammo I reload is at +P level and sometimes +P+.  With Rugers robust frame, the +P and +P+ level are like a mild .357 Mag. load.  Sorry but I have not shot commercial loads in my Ruger for at least several years now.  The only thing I can think of is that the .38 SPL load you are using could be watered down.  Meaning lower in velocity hence the bullet is already dipping.  

In my experience, most all handguns will shoot different ammo to different point of impact---whether its different bullet weight within the same caliber or switching caliber, like 38's in a 357, or 44 special in a 44 mag--------velocity and recoil impulse all play into this-----thats why shooting 110 grain 357's have a different POI than 158 grainers.
With some firearms, the change between POI seems much less than with other--but with magnum (357)compared to non-mag(38)--the change can be pretty dramatic.

Come to think about it was wadcutters I was shooting.

once i shot a cylinder threw my 6 inch gp 100 with 8 special,i was board to tears.shoot hot 357 mags man.But moma,thats were the fun is.tap tap tap tap tap tap.dot dot dot dot ndot,tee tee tee tee tee do da  dey!