As I shop for a new espresso tamper, I see that there are a dizzying array of options for the *shape of the base* (i.e., the shape of the part that contacts the grounds). Some shapes include convex ("Euro" more curved than "American"), flat, rippled, or hybrids of those -- see some examples at [Reg Barber](http://coffeetamper.com/store/pc/viewContent.asp?idpage=7). (I'm also aware that there are many *shapes and sizes of the handle*, and differing *sizes* for the base; I'm not asking about those.)

In part spurred by [this recent question on the essentials of tamping](http://coffee.stackexchange.com/questions/1583/what-is-essential-for-a-proper-tamp), and other questions tagged [tag:tamping] (which are mostly about pressure), I got to thinking that the shape of the tamper could have a significant impact on the result.

There also seems to be a fairly large amount of *personal preference or opinion* in this realm. I can't find any proper comparisons or trials; at best I can find conversations about various preferences (e.g., [home-barista](http://www.home-barista.com/tips/flat-or-curved-tamper-base-t7752.html) including [this poll](http://www.home-barista.com/espresso-machines/tamper-bottoms-flat-or-convex-t24.html), [coffeegeek](http://coffeegeek.com/forums/espresso/questions/38331), [coffee forums](http://www.coffeeforums.com/forum/coffee-espresso-machines/1234-tamper-flat-curved.html), [barista exchange](http://www.baristaexchange.com/forum/topics/tampers-flat-or-convex-which)). A few patterns emerge:

* curved bottom provides a greater *margin of error* for avoiding the "donut" effect of water channeling around the perimeter of the portafilter;
* curved bottom could over-compact the center;
* ripple shape is pretty, is used by some pros including winner of a competition in 2009, but has no demonstrable benefit, and is marginally harder to clean.

I hope I'm not spurring a sectarian flame-war by asking this; so I try to ask as objectively as possible: <br>**Is there any *objective* benefit for one tamper base shape over another?**

For example, side-by-side trials; experienced use beyond preferences; ...