Why the 30/1 mix for serious plain base cast shooters?

[fecmech]

I have noticed in various posts over time about fellows shooting Schutzen and other high accuracy games using PB cast bullets with smokeless powders. They seem to favor using a PB/SN mix of 30/1 and I'm curious as to why. Some of the fellows shooting stuff like the .32 Miller short are running pretty stiff charges of AA9 behind those boolits. My guess is maybe a better, flatter base with 30/1 as opposed to something with SB in it but it's just a guess. Can someone please enlighten me on the reasons for the popularity of 30/1?

[JeffinNZ]

I suspect it is a factor of certified pure alloy casting perfect boolits and the correct BHN for what they are doing. Given how those guys can shoot they seem to have it sorted.

[Doc Highwall]

Remember a lot of these shooters breach seat their bullets into the bore and are not shooting fixed ammo.

[fecmech]

Remember a lot of these shooters breach seat their bullets into the bore and are not shooting fixed ammo.
I understand that but am wondering if it might have some benefits with fixed PB loads. I plan on making some up in the near future to try but was hoping for some insight as to the "why" of that particular mix.

[Doc Highwall]

With plain base bullet you are limited to a velocity of about 1400 fps which does not require too much pressure. A 30:1 alloy will be more ductal then a alloy that contains antimony, even though they are same hardness, which will help in sealing the bore.

I am sure some people are shooting original rifles with softer barrel steel, and the absence of antimony might be less abrasive on them.

[country gent]

Im currently using 20-1 and am going to test 30-1 in my sharps caliber 45-70. The lead tine mix is somewhat soft the tin helps bullet fillout alot. These softer more ductile bullets seem to swell and fit the bore much better than a harder bullet with Black powder. My paper patch are swaged lead so pretty much pure lead there. Another issue with the schutezen breech seating is the amount od force to seat the bullet to depth in the rifling. ( havent tried that yet). Some rifles like diffrent hardnesses of bullets also, testingis the only way to find out what your gun likes best.

[MT Chambers]

a) for ease of breech seating
b) obturation