Monolithic bullet size to groove diameter

[big5ifty]

I did a sulphur cast of the three inches of barrel up to the muzzle.

The caliber is .375 .

I measure 0.3733" on the sulphur cast as the groove diameter. I understand that someone else might measure the same thing and get a slightly different measurement, and I will get someone else to verify for me.

What is the right diameter of a monolithic for that, both across the drive bands, and the between the drive
bands ?

My best load is 7 ES and just under half MOA at 200 meters. That's as good as I can shoot right now, as good as I'm able to control the reloading metrics, and I'm happy with the result.

I'm trying to understand why these particular monolithics shoot well in this barrel, compared to why I could never get good results with monos in my .243 and .308 barrels. I switched to thin jacketed lead cores in those two rifles, and without even tuning the loads, they shot well within the best results I could ever get with the monos I tried.

Is there a particular dimension that the monolithic shank has to be in relation to the groove diameter ?

 

[Alan Warner]

.0017 is a lot of disparity. I'd question first, how much does sulpher cast shrink??
We have found that solids of exact same size to + .0002 works best overall
Push a few tenths more and pressure starts to arrive early.

 

[big5ifty]

Thanks for that.

The sulphur cast does shrink, at least here it did a little, because I sized a soft lead plug to .377 that I tapped into the muzzle about one inch, then out, and it measures 0.3734 across the grooves.

Everything is working great, there is no problem. I just want to understand why it's working.

 

[big5ifty]

I tried one more way, but the result is not consistent. I tried a few times, the largest measurement I got was 3.748 . That indicates that the barrel may be a true .375 .

Measure groove diameter.

My attempts to do this seem to distort the measurement because if the bullet does not go in perfectly perpendicular in one go, each tap against the bullet shifts it a small amount from side to side, making it smaller than the true diameter.

I think the only way to I have to really see what it is without proper gauges is to recover a fired bullet.

 

[big5ifty]

There is carbon buildup in the grooves towards the muzzle. I realised this when tapping a soft lead slug from the muzzle to the chamber. There is a resistance for one third of the barrel, then the resistance is much less the rest of the way.

The borescope is showing hard carbon buildup, which it always did - I just didn't know what I was looking at. I thought that no copper meant it was clean.

Been cleaning for 2 days now, and the buildup is visibly reduced.