I'm a holster maker so.......
I DON'T recommend putting leather in an oven, that's very bad for leather and can cause it to crack and dry the leather out.
To loosen a too tight holster, get a can of automotive silicone spray. Spray the inside of the holster with a good blast, and allow to stand until the spray's carrier evaporates.
Push the gun into the holster and gently twist it side to side. Don't twist more than a very small amount. This will loosen it up, and use will finish the break in process.
Before using the holster spray it again with the silicone spray, which will allow it to slip out better.
Of course, a good idea is to call Galco. It's their holster, so they should have info for you.
For those of you who are interested, here's how to wet mould a holster for a perfect fit:
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I've been a holster maker for about 40 years, and this is how I do it.
First.....wash your hands. Any thing that touches the leather will either stain it or leave marks in the softened leather. If you don't want dye stained hands, wear plastic gloves.
Fill a sink with cool water and a couple of drops of liquid soap as a wetting agent.
Hold the holster under water for around 20-30 seconds, until it's damp, but not soaking wet.
OR:
Some holsters have enough oils in them that the holster may not wet properly. In this case, spray it with alcohol until damp, but not soaking.
Lay it on a clean cloth or paper towel, not newspaper, this will leave ink on it. Allow the leather to absorbe the moisture. This is known as "casing" the leather.
The leather is ready for molding when the leather feels dry but cool to the touch. Tan or brown leather will have returned to it's original color. If when you press on the leather, moisture appears on the surface it's too wet, allow it to case longer.
If you try to mold when the leather is too wet, it will not hold the molding properly, and the leather will stretch too much. If it's too dry, spray a thin coat of water on it and allow to stand for a few minutes.
Coat the gun with a thin coat of some rustproof lube and insert it straight into the holster without twisting. DON'T put the gun in plastic bags or wrap, this will leave the holster slightly oversized.
With clean hands, gently massage the leather around the prominent features like the cylinder, trigger guard, slide, and frame, pressing the leather into the hollows of the gun.
If you want the "detail" look where the leather is so closely molded you can see cylinder flutes or slide stops, use rounded hard plastic or metal tools to press the leather around the features as though you were outlining them with a pencil. My favorite tool is a 1/2" ball bearing attached to a handle. This "detail" molding usually isn't necessary, or even disireable on most holsters, but works well on open, no safety strap concealment rigs where a very tight fit is necessary.
After molding the leather, gently remove the gun, without twisting. The gun will be bone dry, but will need another coat of lube.
DO NOT leave the gun in the holster. The gun will rust, and the holster will stretch too much, ruining it.
Allow the leather to dry overnight. Keep it away from sunlight and heat. After it's completely dry, apply your favorite leather dressing. For a finish coat, I recommend Fiebing's Resolene. This is an water-based finish coat that won't crack, peel, or flake off. It's nominally waterproof, and seals dyed holsters.
When moldiing, be careful not to stretch safey straps or thumb breaks until they won't hold the gun properly.
Don't put too much leather dressing or oils into the leather, and NEVER use gun oil. If you want real leather oil, get a small can of 100% Neatsfoot oil, NOT neatsfoot Compound, which is fish oil with just enough neatsfoot oil to allow the use of the name.
You cannot completely waterproof leather, but I've had good luck on heavy duty field gear with THIN coats of Thompson's Waterseal. Although sold for wood decks, this works well for canvas and stiff leather. Thats it!!!!!