9mm bullets in 38

A guy gave me a bag of 9mm 147 gr flat point bullets sized .356. I loaded some in 38 cases with 3.0 WST and they shot tighter groups than my 148 gr HBWC's sized .358 with 2.7 of Bullseye. Why does this 9mm bullet shoot so good out of my Model 19 ?

Can I suggest a few measurements be taken?  What do plug gauges say your cylinder throat diameters are?  Slugging your barrel, what is your groove diameter?  

I have a lot of sizer dies that don't always produce a bullet of the marked diameter, particularly with a hard bullet.  What does a decent micrometer say your bullet diameter is?  

I have a lot of oversize 9mm Beretta and Walther barrels, and I shoot .358" and .359" bullets in them.  

I would not be surprised if your bullets were actually larger than .356", and were about your throat diameters.

Are these cast or jacketed?

3.0 wst is fairly stout...

NuJudge, The bullets measure .356 with my digital caliper and they will fit tightly into the front of the cylinders and have to be pushed back out with a wooden dowel. They do not go all the way in, just a little. I need to get a soft piece of lead to slug the bore. I only have one of these bullets left, I should have saved more of them.

Jim, These are cast bullets. With 3.0 of WST they hold a 1 1/2 inch group at 50 feet indoor range. Maybe using 2.7 or 2.8 of WST might tighten the groups up some more ? What charge would you recommend ?

I have not used a plug gauge on a lot of S&W .38 cylinder throats, but my impression has been that they typically are .357" to .358".  

Can I suggest you use your digital calipers on a known quantity, such as a commercial .358" bullet?  My experience is they are frequently off by up to .002".

I checked some Penn bullets and they measured .358

50 feet isn't really a valid measure of long-line accuracy.  Some 50 yard testing will really let you know if you have something special.

Your right 50 yds will tell how well they shoot.
First I wanted to see if I could identify the bullet and be able to get more. Then when the weather warms some I will get outdoors for 25 and 50 yd testing.

2.7 wst

Thank you Jim