You can determine your twist rate by using a ramrod that rotates freely as it is inserted in the bore. Put a TIGHT patched jag on the bore, then insert it to the plug. Mark a spot on the rod to use as an index. Then slowly remove the rod, allowing it to turn freely. Just as the patched jag clears the rifling, make another mark on the rod. Measure the distance the rod traveled in the bore, then make your best estimate of how much of a revolution the rod turned during that travel. From there you should be able to calculate the distance the rod would have needed to travel one full revolution. That will correspond to your twist.
Example: your index marks are separated by ~90% of one full revolution. The rod moved 26" while being withdrawn. 26 divided by .9 = 28.8 so your twist is very likely 1:28"
The other thing you need to do is drive a slightly oversized slug of soft lead into your bore a few inches, then gently tap it out. Use a caliper to determe minimum diameter measurement where the lead is showing marks from you rifling's lands. That will tell you the bore size you need to try to cast or size. I would go no more than .001" greater than that measurement and 0 below that measurement.
Both of your rifles are probably either 1:24 or 1:28" twist and that is not such a critical difference in making your choice of bullet. Now the bad news, there just isn't much by way of current production molds available (aside from the mini's and maxi's) for the .50 caliber. If you get lucky and your land to land turns out to be .501", a Lee mold for the .500 S&W handgun ( C501-440-RF ) might work quite well.
http://leeprecision.com/mold-dc-c501-440-rf.html Lee also sells a .501 sizing kit.
Making bullets that are right at bore size has the advantage of providing a "slip fit" in your rifle, making a short starter unnecessary.
Aside from that one mold... the reality is that you will probably have to go custom - if you don't want to shoot the maxi/mini types and hammer your palm. I WISH Lee had made the UC Short mold a production item, but they did not. There are some few older molds by Lyman that could work well and you might find something on eBay that would work.
http://www.ebay.com/sch/Hunting-/7301/i.html?_nkw=%28mold%29%2C+%28mould%29&_armrs=1&_from=R10&_ipg=&_mPrRngCbx=1&_pcats=159043%2C382&_sop=1&_udhi=&_udlo=Use the purest lead you can lay your hands on for casting any such bullet.
