[Island made]

.5 steel at 156 is right around 7 foot pounds, although technically that is "enough" I like to see at least a minimum of 12 foot pounds. Something to consider on the quest for power, if you double your projectile weight, you double your power, but if you double your speed you quadruple your power. So sometimes dropping your shot size and getting some more speed you'll get more power.

I hunt with 7/16" steel and In the summer my speeds are usually 280-290, that's with long draw. but anything over 220 with 7/16 I would say your golden.

 

[Island made]

With .5 steel and a head shot at that speed I'm sure you'll be fine, but around here game is skittish so at 160 FPS there gonna be long gone before your ammo reaches them. If you plan on hunting with glass I would say your gonna need 5/8 glass and 250 FPS and headshots. Marbles don't have much mass, so it needs to be travelling!!

As for terrible handslap, I think you need to play around with tapers, you don't need a heavy draw with .5 steel. From personal experience I know that .7 elastic cut 1 1/4 x 3/4 with a 10" active will send a 1/2 steel ball at 250 FPS no problem at all with no handslap.

Play around with it!! You'll get it. And it's ALOT of fun trying!!