NOTE - Added info at the end of the original post
There have been a few questions recently concerning the use of other grips on a URX, UDX and Challenge frames. Unfortunately, I only have pistols with the standard and URX frames for comparison. Here are some pics showing what standard sized grips look like on the URX frame.
Standard grips "fit" a URX frame in that they bolt right on. Since there are front-to-rear dimensional differences between a standard and URX frame, they certainly don't fit in the true sense of the word. There are overhangs and gaps. The Nill grips look like they fit better than standard grips on the URX frame. That's because the rear of the Nill grips is closed except for the very top.
The Nills on the Contour URX in the one photo are comfortable. I don't notice the gap at the top rear, and the overhang at the lower front of the frame isn't distracting. I plan on shooting this gun as-is in our league matches.
Here are some pics to show you what I mean:
This is a shot of a standard Buck Mark grip on the URX frame showing the overhang at the rear of the frame.
Here is one showing the same grip (front overhang)
Here's one with a Nill grip with the front overhang
A rear view of the Nill on the URX with right grip removed to see the gap
A better shot of the top rear
Finally, a side view of the Nill on the URX
Now you can see what I mean when I said "fit" at the beginning of the post. Grips made for a standard grip frame will fasten to the URX frame. Because of the way the Nills are made, they fit better than regular grips. If I didn't want to use the Nills on other pistols, I'd consider removing a little material from the front edges and they'd look like they were made for the URX frame...................chim
EDIT 12/4/2013 to add info:
Questions keep arising on the Buck Mark grips and frames. Here's a rundown of what I am aware of at this date.
1. Standard Frame (Standard and UFX Grips) - This was the original Buck Mark frame. It is essentially a Challenger II/III frame with a hole added for a 1911 style mag release when it transitioned to a Buck Mark. For some time after the introduction this was the only frame available. It didn't matter whether the grips were the original black slabs, fancy rosewood or wraparound walnut. Everything fit this frame. This frame nearly went extinct a couple years back when different grip styles were introduced. For a brief period this frame was only used on a couple models. It is now being used for the newer UFX grips.
2. Challenge (Challenge, URX and Buck Mark Rifle Grips) - This frame was introduced for shooters with smaller hands. It is the same width as all other frames but is (A) shorter in the front-to-back dimension and (B) has a slightly different shape. See the pic below. The left frame is a Challenge/URX frame. The one on the right is a Standard/UFX. Note the different curve on the backstrap. It doesn't taper out wider on the bottom as the Standard frame on the right. It has a little forward curl at the bottom of the front strap. Its smaller front-to rear dimension made it the go-to frame when Browning wanted to have the wraparound rubber grip and keep it small enough so it fit more shooters. I have some wraparound wood grips for the Standard frame and thank goodness I have hands like King Kong so they fit.
3. URX - See#2 Above
4. UDX - Unfortunately I don't own one of these for a grips-off picture. The only grips that fit this frame are UDX. It has scalloped wood grips and the frame has small corresponding scallops in the front of the frame. It COULD be a URX frame with the front strap whittled away but I'm not certain. If that is the case, you may be able to get a URX grip on it.
5. UFX - See #1 above
Please note that the grip info is based on factory grips available for the frames. Some aftermarket offerings are made to fit the different frames.
Yes, I know that all Buck Mark frames have the same holes for screws and the mag release. What that means is you will be able to fasten grips I didn't mention to different frames. Of course it also means that they will look - as Flys would say - "like Charlie Romeo Alpha Poppa" because they'll hang out some places and be short others as in the pictures included in the original post avove.