The new version of my freeware, Rad Pro Calculator will be able to give you a quick visit with this question. I am currently programming in buildup factors and am weeks away from release of version 2.0. It does now, however, calculate your answer without buildup factors. The linear coefficient, mu, tends to overestimate the effects of shielding. The mass absorption coefficient, mu sub en, tends to underestimate the effects of shielding. Since the pipe is so small, I would tend to use mu sub en and overestimate to take in account scatter and sky shine.

My current software is in good agreement with cincinnatinuke's rule of thumb answer for unshielded Ir-192.

At 15 feet, unshielded, 1438 mR/hr. At 2 inches, 1.16E4 R/hr.

If the source is inside of the pipe, passing through 1" of steel, using mu sub en, at 15 feet, 764 mR/hr, at 2 inches, 6186 R/hr.

If the source is behind the pipe, passing through 2" of steel, at 15 feet, 406 mR/hr, at 2 inches, 3292 R/hr.

Using the linear coefficient (overestimating the effectiveness of the shield and not accounting for any Compton scatter or sky shine over the pipe).

At 15 feet through 2" of steel, 33 mR/hr, at 2 inches 264 R/hr. Through 1" of steel: 209 mR/hr at 15 feet and 1694 R/hr at 2 inches.

I hope this helps. I think version 2.0 of my freeware will be of use to many and your question has inspired me to keep programming in all these buildup factors (painful to make it accurate). When you click "Use buildup factors" it will get you closer to predictable numbers, or that is my wish.