Bullet size for throat erosion?

I have owned my Savage Enfield mk1* since I was 2, bought back in the early 90s. My grandpa purchased it for me to enjoy when I grew up. 

Anyways, I did a pound cast on the throat and here's the measurements. 

THIS IS A 2 GROOVE GUN

.200 in front of the case mouth measures .318

.400 in front of the case mouth measures 315/316

.750 in front of the case mouth measures 310

The groves in these areas seem to be 318/320 but the rifling on this gun is really dirty, like, after a few days with penetrating oil in it and suddenly patches come out black again. At the muzzle I get a consistent measurement of 319 groove.

The muzzle is 308.5/309 bore and I can personally tell you that section of the barrel is very clean. 

I'm paper patching for this gun, and I'd like a bit of guidance as to how thick a bullet I can swage into the rifling. I am optimistic that the paper patched bullets will help clean out those grooves to get a better measurement and obviously better performance. This gun has never been shot by me so I have no idea what it's capable of doing. This barrel is not pitted in any way shape or form. It's actually very very smooth

A local cast bullet guru here in town told me to start with 316 and see how things go. Work up from there essentially. His knowledge was slightly limited as far as enfields go and I know they're kind of their own unique monsters as far as bullet size.  

So what do the experts out there think about the erosion on this barrel? Will I be more than likely be going up in bullet size? I guess in general I've always thought you cast a bullet to fill the grooves in the barrel. That's what I do for 3006 & 3030. Where things get weird is knowing you have a bullet compressing in your barrel .010 by the time it reaches the muzzle. 

Thank you all for your time Hope to hear from you all soon. For what it's worth I would like to attempt to make this gun accurate enough for hunting out to 200 yards maybe three? All of my lead bullets are essentially hunting bullets except for one or two molds

After a day at the range with paper patched ammo, my barrel has nothing left in it besides a mirror polish. So, I see your point about not shooting over a foul barrel but for whatever reason paper seems to do the trick on the stuff you can't get out with a brush or jag.

My new throatcast has the exact same dimensions as my old one did. The barrel was not dirty in that area so I knew this wasn't going to change. 

The muzzle of the gun was also clean. So my muzzle dimensions are the same as well. He used his calipers and got the exact same dimensions I did. 

For every other rifle that I cast bullets for I always go 001-002 over the groove diameter. 

So the reason for this post is if I base that directly off of my throatcast I would be putting .320 bullets in this rifle. I'm having a hard time processing that as this is only a two-groove barrel and It seems like an awful lot of metal to swage down. If it's the right thing to do I will work up to that point. 

Right this minute I am loading 317 paper patch bullets to sit over 21 grains of 3031 powder. 

So am I the only one out there that thinks a 320 bullet being sized down to 309 is crazy? That bullet is going to swage down .010 within 1 1/2 inches in the barrel.

Gun managed 4moa shooting off the hood of my ram charger ( 2" @50 yards )

I have 50+ properly patched bullets waiting to be loaded up and load developing 4895 powder. Bullet diameter was 316. 

3095---310 were consistent 

I have a set of ridiculously accurate calipers that have longer measuring edges on them and 1 in below the muzzle surface measures 309.5

I stopped half way down the barrel, from beech to mid barrel, and it was 310

OP, when you mention pound casting, are you describing the filling of a cartridge case with lead alloy and then half the neck with a long lead bullet and upsetting it with a steel rod to the throat and ball seat or simply slugging the bore by driving a lead ball through the length of the barrel?

I'm just curious to your method that your using if you would describe.

Thanks