Dave,

  You're probably getting about as good as you can expect with your set-up and cheap ammo.

  With a good gun and good ammo...... the $14 a box stuff...... that shoots well in your gun you should expect to consistently shoot 1/2" to 3/4" regularly at 100 yds. I've got a BSA that will shoot that good, but they don't make that brand of ammo anymore and I'm still looking.

PETE

 
I would think that using the cheap bulk type ammo with iron sights off cross sticks and getting 1.5 inch groups at 100 yds is extremely good. 

I have shot the better target ammo in two different target rifles at 100 yds. and like tenx Pete have gotten 1/2 to 3/4 inch groups, but this was on a nice day, off a good bench, and with a 24 X scope. 

Have you tried shooting it with better ammo, off a good bench with a scope, it seems to me that it would shoot as good as most target rifles.
Shooter

With this type of ammo you are doing very well and you could cut the size in half if you used top end ammo like Eley Tenex, Lapua Midas, etc.

But then to get less than .75 inch groups you still need to weight and rim thickness sort even the top end ammo.

I'm not sure you need to change your barrel if you are getting 1.5 inch groups with the low end ammo. The only improvement for you is most likely some top end ammo that you weight sort and check rim thickness (if you wish to be ultimately competitive).

But if you do wish to change the barrel a Ron Smith RKS gain twist 1:16 twist works great for .22 rimfire.