Some noob questions

That is correct. Missouri Bullet's sells slugging kits by caliber online, and you might prefer one of those for your first try at the practice. An OD thimble micrometer will measure more accurately than calipers (which some YouTube video authors use) and also usually have a vernier scale to resolve tenths of a thousandth of an inch, which let's you compare slugs to spot a constricted places in the bore other issues.

Bullet data is usually interchangeable as long as you stay with the same weight and general shape and, most importantly, type of construction. But ignoring any of those can result in up to 30% difference in pressure.

Military rifles, especially those made in wartime, can have more generous tolerances than we expect. The US 30 Cal bores during WWII had a tolerance span of three thousandths, with bore diameters from 0.2985" to 0.3015", and groove diameters from 0.3075" to 0.3095". Most are closer to the middle of the range, but you do have to slug to be sure of what you've got.
 
Another point about slugging the bore, is the "feel" of driving the slug through the barrel. Sometimes barrels have "loose spots" or "tight spots" which an experienced person can feel but a new one might not recognize.

And, of course, the slug will only tell you the tightest diameter of the barrel, which, of course, is worth knowing, but so is knowing if the entire barrel is uniform, or not.

Knowing the exact diameter of your bore is a "nice to know" thing, but not a "must know" thing it may save you a little $ by keeping you from buying stuff that isn't likely to be a good fit. But that's about it. And, sometimes, the "wrong" stuff still works acceptably well.
 
Here’s another entirely different thought. Since your a new reloader, why not just start with something easier to load for? The British .303 rifles are a bit tricky, but a newer commercial rifle might be a simpler place to start until you gain some experience. A good place to start would be something in a bolt action common caliber such as a .308 or a .223, plenty of supplies to choose from and pretty straight forward loading wise to start out with.
 
I would assume the OP wants to reload .303 British in order to be able to shoot his .303 British. No mention of other guns was made, so perhaps he doesn't have any, and the .303 he has is where he is going to start.

The .303 has more "quirks" than many other rounds, but it is something one can learn to do easily enough.
 
I would assume the OP wants to reload .303 British in order to be able to shoot his .303 British. No mention of other guns was made, so perhaps he doesn't have any, and the .303 he has is where he is going to start.
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I've been thinking about this. I do have other stuff. This caliber was the main motivation to get started, since ammo is hard to get for it. But then I thought, given how hard it is to get, I don't want to waste any brass learning the basics. So I am going to try some other caliber until I get the hang of it. I think 223 will be a good start. Long term, I'd rather buy 223 than spend my time reloading it, but at least I can practice using the basic reloading tools without fear of running out of brass.
 
OK I slugged the barrel. I tried it on a few milsurp rifles I have to help me get the hang of it and compare. They all had variations in resistance, but the Lee Enfield was the most consistent.

The trouble is, it has 5 grooves so it's very difficult to measure the slug with any certainty. Here's what I did based on some tips I read:

* I wrapped a .003 feeler gauge tightly around the slug and measured that with a micrometer, then subtracted twice the thickness of the feeler gauge.

* I measured the slug, then rotated it a little, and measured again, repeatedly until I had turned it around several times. This way I hoped I would occasionally catch the almost-opposite corners.

With both methods the highest measurement I get is consistently .311". I'm not totally confident in this. Obviously it's not like .308 or .317, but it's possible that it's really .312" or .313" and I just haven't measured it the right way. Any tips on what else I can do to improve my measurement?

I also got a bullet puller and pulled one of the Prvi bullets from the ammo that has been shooting really well. It's .312.

I got the Lyman reloading manual and they give loadings for both .311 and .312 bullets, tested in a rifle with a .313 groove depth. Would I be safe enough copying that?
 
I also got a bullet puller and pulled one of the Prvi bullets from the ammo that has been shooting really well. It's .312.
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You answered your own question.

Buy .312" diam bullets if they are shooting well in your rifle.
 
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