Got a 336...Does anyone know if these can be converted to fire pistol rounds?

Go check a reloading manual for similar size cases with the same base dimensions.

A better way to go is to reload really wimp loads in 45-70.  Lighter bullets, different powder and Lots
of filler to take up the empty space in the case.

The 336 kicks at both ends with full factory loads?

I wasn't looking to take the balls out of it!!, just interested in increasing ammo capacity. I'm going to see if 45 LC will feed, that might give me two extra rounds. Any thoughts on the 45 LC ?

I have a Winchester 94 and a S & W Mountain Gun in 45 Colt.

I really like them both.  The 45 Colt Cowboy commercial loads is a real wimp load in both guns.

Just checked one of my reloading manuals.
Nope, ain't gonna work.  Case head is larger on the 45-70.
Definately a NO-NO.  The 45 Colt will be Very Loose in the 45-70 chamber.
   Case head on the Colt is large enough to most likely be held in the chamber.
   The case body will be waay loose.  You will get major Split case and gas back thru the action.
DO NOT DO IT

If you want more capacity, cut down enough 45-70 cases to keep you happy.

BUT, before you go cutting down cases, experiment by cutting down 1 or 3 45/70 cases to the length you
   want, load to what ever 45 Colt load you want and function fire them in your 336.

You May or may not have feeding or loading problems.

Take photos, if it works, you can make a How I did it. 

Good info, I gonna give this a shot.

Another problem, 45-70 bullets are .457-.458 dia and weights start at 300+ grains to 500 grains in weight

45 Colt is .456 and weight Normally ends at 250 grains.

There are Special 45 Colt bullets up to 300 gr.  You have to have the correct reloading top punch.

If you cast your own, get the right top punch and loose bullet lube die and Maybe you can stretch a 45 Colt bullet to 458.  Maybe if you know how.  Otherwise you're stuck using Custom cast extra light 458 bullets.

Be careful when you cut and shorten the longer case.

Have fun, keep us informed on your progress.