Swiftman,
I have some very limited experience shooting 240 and 300 gr bullets in my new 44mag that I can share with you. Unfortunately, I can't report velocities because I havn't had a chance to shoot over the chronograph, and my gun is currently at H&R for a check-up and trigger job (and fitting a second barrel. Yeah, I got the bug). Anyhow, the following are average group sizes for 3-shot groups at 50 yards. All factory ammo.
300 gr Hornady XTP ------------------- 1.22 inches (average of 2 groups)
250 gr Winchester Partition Gold --- 1.16 inches (average of 2 groups)
240 gr Winchester JSP --------------- 1.59 inches (average of 3 groups)
180 gr Hornady 44spcl XTP --------- 1.21 inches (average of 3 groups)
As you can see, based on very limited testing, the 300 grain bullets seem to shoot just as well as the smaller bullets. My barrel slugs out at about .429 inches, and the rate of twist in indeed about 1 in 38. I've read a lot that 1 in 38 is too slow to stabilize 300 grain bullets, but my initial testing suggests the 300gr Hornadys should shoot well in my gun. The winchester soft points were the least accurate, but also very much the cheapest. I will probably be hunting with them this fall unless their groups really fall apart at 100 yards.
Duane