...and measure the strap width between the lugs of your watch (where the strap attaches to the case). It should probably be 18mm, 20mm, or 22mm.
Next, buy the appropriate size strap.
Insert a small pointed object, like a safety pin or springbar tool, between the stap and case, angling the point toward the case. Apply pressure toward the center of the strap. If done correctly, you should feel the springbar compress, and come out of its hole in the case lug. This should allow you to remove the band. You can probably get someone at WalMart or any other jewelry counter to remove the strap for you.
If you go NATO or Rhino, put the springbars back in, and install the strap following the directions on this webpage:
Thanks for the replies everyone. The reason I ask this question is that this model of G-Shock has a 16mm width band, but some people have still been able to install 20mm Rhino or 3/4" Waterborne straps by doing some kind of alteration. Has anyone done this modification? Thanks again for any replies.
I think you're bumming on the DW-6600. It is possible on certain models of smaller G-Shocks, such us the G100-1BV, by using the Casio band adapters and some creative restitching of the Rhino band. I'm not sure about the Waterborne strap. But the DW-6600 just looks too big, even if the band adapters fit. It's not the width of the band that kills you, but the length of the band between the rings and stitching.
If I were you, I'd buy something like a G-2300B-1V, which is available with a VERY good cloth/velcro band at Wal-Mart. It's smaller than the 6600, but superior in pretty much every way. I have almost 20 G-Shocks, and almost always use my 2300.
Good point about the interlug distance, Buzzbait. I hadn't thought of that. :footinmou
My Rhino strap (original Nato G10 style) is pretty much a perfect fit for the 41mm interlug distance of a Seiko SKX007. The stock strap might still be workable on something about 5mm longer, but any more than that and you'll have to remove the stitching for one of the rings.
A better solution would be a BR-2 Rhino, which is still a one-piece but lacks the secondary flap on a regular Nato strap. Another possibility is a two-piece Rhino.
A Rhino or Waterborne is generally too thick to fit well on a G-Shock. The Milshock has been custom milled to accept a 20mm strap, but there is only so much plastic that can safely be removed. Trying to squeeze in a Waterborne might work, but understand that it will be a very tight fit.