It's difficult to cast a bullet too soft for a single shot target rifle. As long as velocity is reasonable -under 1400 fps or so it's not going to cause any problems at all.  Only real reason tin is added to the mix is to help mold fill out.

Go fast what will happen ? I don't know, never saw the reason to push one fast.   I have fired thousands of .30 cals at 1600 fps with Gas checks and 1/25 tin-lead and no problems. Go faster than that I use jacketed bullets.

Boats

The following is my rule of thumb without gas checks:
For 800-1000 fps range the alloy should have a BHN of 12 or below.
For 1000-1400 fps, a BHN of 12 to 16 is needed.
For velocities of 1400 to 1700 fps the BHN needs to be in the range of 14 to 20.
Above 1700 requires a BHN of around 22 - linotype or heat treated bullets are required.
Remember, the above is without gas checks.  If gas checks are used to help mitigate leading, my rule of thumb changes and softer lead can be used at higher velocities.

One final thing to keep in mind, at high velocities soft lead bullets can reach a point where they literally "blow up" (disintegrate) once they leave the barrel due to centrifugal force.  Of course that also depends on the twist rate.  If I remember correctly, at around 2000 to 2200 fps pure lead is reported to disintegrate in a 1:10 twist barrel such as a .30-30, .30-06 or .270.

Wayne

Hi Joe,

You could be right.  As I said, "If I remember correctly...".  No doubt I've lost a few million brain cells since storing away the data some time ago.  By the way, is your calculation of 2528 fps based on pure lead or the harder stuff typically fired in .30 caliber rifles?

Wayne

Wednesday, 11/24/10, I shot ~ 30 each Borton-Darr 185 gr., 311299 no g.c.., 314299 no g.c. from a Martini 12/15 30-30 bench gun. Bullets were WW, load Rem 2 1/2, 14.5 IMR 4227, breech seated, 1/16" plastic wad in the case neck. No leading.
joe b.

1/19/11 45-70 C. Sharps 1875, I shot 84 WW bullets, 2 different bullets, 3 loads. No leading except one extremely small dot of lead.
All plain based bullets, 457193 and Ohaus 45-405. 
Does antimony = leading for plain based bul;lets at lower velocities??
The answer seems to be NO.
Can one get antimony-containing PB bullets to lead??
Yes, if too small or going too fast or a combination.
I believe too small is the most frequent cause.
joe b.

2/3/11 C. Sharps 45/70 457193 and Wolf No-Grease-Groove WW bullets, Buffalo Arms plastic wad,  24/SR4759, Rem 2 1/2. No lead. 100 yard 5 shot groups averaged 1.75" and 1.7".   
The antimony isn't a problem yet. Maybe the Maynard 32-35 will lead with WW bullets, thou8gh it hasn't in 20 years.
joe b.

I have been having very good luck with Lyman Super Moly lube in several guns, and wanted to try it in a 45-70. Not wanting to try to pan-lube with it, I bought a .457" Lyman sizing die and had it honed out to .460". The 457193 WW and lead bullet only kissed the die here and there, and the bottom groove only was lubed. A set of 25 of these were loaded with Rem. 2 1/2 primers and 24/SR4759, and shot on Wednesday, March 30, 2011.
There was unburned powder in the bore every time I looked.
There was no leading.
The second patch came out clean.
My normal loads of SR4759 for this gun/bullet-Darr-lubed leave no unburned powder in the barrel. The presence of unburned powder with the moly lube suggests that pressure is lower.   

Summary
At least I, in my guns, can shoot plain-based antimony-containing bullets at lower = <1500fps velocities without leading. Bullets that are too small will lead. 
I suggest that the myth of antimony=leading has to do with breech-seating, where groove diameter/hardness/breech-seating- force/required together operate toward a tendency to too-small bullets.
I would add here that the alleged mathematical relationship between pressure, BHN of the bullet and accuracy; where "obturation" is often mentioned, is untrue.    
joe b.