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Good morning Brian
Caliber 38 rifles are right at the top of my favorites... only caliber .41 and .40's (slim .41's) beat them out.
The measurement more important than groove is the throat diameter. Groove is always less. Fill the throat and you have the groove filled.
Your lead bullets.. are they Hard cast ? Can you easily scratch it with your fingernail ? If you can good. If theses are hard then find a lead sinker, some split shot or some soft lead of bore diameter to tap down the barrel. Lightly oil first.
Take a dowel (wood or brass) place it in an empty case and cut it so it is case length. Chamber the doweled case. Close the chamber. That bullet if it is soft or the soft lead tap it down the barrel all the way. You will know when it reaches the "filled case". Set rifle butt on your foot. You need a sturdy rod on top of the bullet or lead now. Wack the rod with a plastic or wood mallet. No metal hammers. Did you feel a solid impact on your foot.. you should. Do this at least 3 wacks. You are filling the throat with lead to discover throat diameter. Eject case. Tap rod and "throat slug" should come out. Is it round and filled out. If the lead did not "expand" and form a good round solid shape do it again. This is the ideal bullet diameter for that rifle. This is the diameter I shoot in my rifles(if possible). Some tight chambers force us to use maybe .001 + less diameter but this is the best diameter to use. Fill the throat and the lead bullets start down the barrel straight and seal the hot pressure behind the bullet.
It is work... but do the home work first (20 minutes) and your range time is far more productive.
You already have your bullets and .380 is not real bad. If the throat is only .381 you are probably OK. But if your throat measures .383 (common) and more ( I have a .384 Marlin BP barrel) there are issues to address but solutions are available.
Your rifle can be a shooter. I have one that has pitted bore and interrupted rifling that I shot groundhogs with for years. But it took some fine tuning to get there.
 

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Greetings Brian
That is a good photo of a fine rifle. Looks like with a little tweeking that Trail boss load will be a keeper.
I would think that rifle's throat is not to much more than .382 by the way it is shooting. Hard to tell without a throat slug. But it is nice when a new to you barrel will print on paper.

Now a task... Using one of your fired cases but not resized yet.. how loose in the fired case neck is one of your cast bullets your are shooting ?
Would you call it a snug fit ? Does the bullet easily drop into the fired case ?
This will give you an idea how fat the chamber neck and throat is. Once again the closer we can match those diameters especially using smokeless the better the accuracy will be.
I have a 44WCF that demands a .434 diameter bullet with smokeless. With 3F BP and a soft cast bullet .432 works OK.
 

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Howdy Brian
If the bullets just spin in a fired case (but not loose) that is very good. I would continue loading that powder increasing it a few tenths, fire 10 and so forth. If you get a group with all shots touching I would smile a bunch and enjoy some shooting.
If I had that rifle it would become a life long keeper. People will offer the moon but hang on to it. You may never get another one with a tight chamber.
If you have not cleaned the barrel or muzzle see if there is evidence of lube at the muzzle. Some bullets do not carry enough for some particular barrels.
 
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