My new (to me) pistol almost fits in my pretty nice leather holster. The stretch is mostly needed in the barrel area, due to the bull barrel. I’d like to mold the holster with the pistol, which has (and will keep) iron sights.
I don’t want to ruin the holster by experimenting, so some good advice would be greatly appreciated.
In case you’re curious, the gun is a used but pristine Ruger Mark II target model, and the holster is a Brauer Brothers Model H.
Peace,
mangorge
Several years ago I made holsters for my Colt (Uberti) 1851 Navy revolvers. I started by folding a manila folder around the gun and stapling it together as close to it as I could. Then I cut off the excess so that I had a paper holster. Next, I took the staples out and traced the outline of the holster-shaped manila folder onto leather about 3/8" thick. Then I cut it out, used a leather grooving tool around the edges, used a sprocket-shaped tool to mark out stiching holes, drilled the holes with a Dremel tool, and finally dyed the leather with brown dye. When it was dry I stitched holster together, starting with the flap I left at the top for use as a belt loop.
Now, this holster was too small for the gun. I soaked it in hot water until it was completely soaked. I liberally oiled the gun and wrapped it in plastic wrap, then pushed it into the wet holster and left it until it dried. When the leather dried, it was form-fitting to the gun.
I used the same process to make a shoulder holster for my then-girlfriend’s Ruger Mk.II.
So you might soak the holster, oil the gun, wrap it, force it in, and let it dry. It worked when I made holsters from scratch.
Of course, I can’t be responsible for any bad results you may have if you try this. 
I make knives and knife sheaths and always wet mold my leather.
Johnny L.A. has it right, except for the hot water. Hot water will ruin leather, making it hard and very likely to crack!
Soak the holster in cold water until it’s very soft and flexible, at least 20 to 30 minutes, the longer the soak the easier the stretching will be. Then what Johnny L.A. said. You should be able to stretch the leather a good 1/4 to 3/8 inch.
Work the gun in the holster, the same way you would if you were putting on a tight glove, then every hour or so till it’s dry (if you put it on the belt your going to use it on, you can mold the belt loop at the same time). Don’t be surprised if it’s still tight when your done, it should be, and it will get looser with time. My sheaths were always tight for the first month or so, it’s like breaking in a new pair of shoes.
I’d try to put a plastic bag in the holster first, then the wrapped gun, so you decrease the chance of a tear (wet leather and gun = instant rust!)
After it’s dry you could oil the inside of the holster, less chance of rust and a slicker draw.
Important note; If the holster is natural colored (i.e. not dyed) iron/steel will react with the tannins in the leather causing permanent black stains!
CMC fnord
Actually, come to think of it, I used warm water. The holsters are stiff, but then I didn’t want them to be floppy. These guys are a dozen years old, and they are still in perfect condition.
You might want to check out the stuff baseball players use to break in their gloves. I think its some sort of foam/cream that softens up the leather, and allows you to form fit it.
I wasn’t going to post this, since it doesn’t relate to the OP. But I’ve already posted how I made holsters, so what the heck.
To finish the edges of the leather I used the wooden handle of a mallet (you could use a handle-sized dowel) to rub the edges vigorously. This gave the edges a nice smooth finish.
You mean Neatsfoot Oil? I used it on a leather holster I had, and it worked well, but it tended to leave my revolver a little slick in the hand. But then again, you just gotta take care to wipe off your weapon before you fire. 
Usually, you can find a bottle of it for less than $3 in any good hardware store. . .
Tripler
Whoo! Recoil could be a bitch with a slippery .357 mag. :eek:
Its sad but I haven’t had the oppurtunity to work in a glove in a long time.