The very similar R8 and TRR8 revolvers have left a few folks, including myself, wondering what the difference between the two is. I recently acquired an M&P R8, so I'm very interested in knowing the difference.
Based on my research (summarized here), I have concluded that the differences are nearly negligible.
These revolvers were supposedly designed for the shield bearer on NYPD's SWAT entry teams. Should the shield bearer need to shoot a semi-automatic handgun, the slide could get hung up on the shield as it cycles and cause a failure to fire. Also, many stoppages on semi-automatic handguns require the availability of two functional hands to clear. With a revolver though, there are far fewer complications. The R8 and TRR8's upgraded 8-round capacity also reduce the revolver user's capacity deficit vs a semi-automatic shooter.
Based upon the spec sheets on Smith&Wesson's pages, the only apparent difference is that the one "comes with rails" and the other one doesn't. The reality however, is that my R8 actually came with the detachable top rail just like the TRR8.
One difference between the two is that the R8's lower rail is actually an integral part of the frame, while the TRR8's lower rail area has a couple of threaded holes for you to attach the optional lower rail. The upper rail segment on both revolvers is a detachable piece with four threaded screw holes.
Both revolvers are Performance Center pieces. The R8 weighs 1oz less, but the difference between a 35.3oz gun and a 36.3oz gun is not likely to be noticeable.
The R8 has a V-shaped rear sight, while the TRR8 has a squared-off rear sight.
The slot for the TRR8's extractor rod is cut all the way through the frame. The R8's is not.
In some photos, the forcing cone area seems to be blued on the TRR8 and stainless on the R8.
Purported Common Issues
Light hammer strikes: Purportedly, there's a tension spring which is frequently not set correctly, and produces light hammer strikes. This is an easy fix.
"DJ Niner" asserted on 04-29-2009 that the "R8 specs state it has a two-piece barrel; the TRR8 does not say, and the photo seems to show a blued-steel forcing cone area vs. stainless on the other two-piece-barrel gun, so it may be an older style one-piece barrel. The TRR8 has standard notch-and-square-post sights; the R8 comes with the V-notch-rear and white dot front. The R8 has the bottom-mounted light rail milled directly into the barrel shroud; on the TRR8, the rail is mounted with screws, and can be removed. This added metal for the rail, and the lack of a lightening cut around the extractor rod could also account for the 1-ounce weight difference between the two guns."
"Old Fashioned Six Shooter" asserted on 01-15-2012 that the "...327 TRR8 has a skeletonized ejector rod shroud (ejector rod visible from left and right side of gun) where it is not on the M&P R8 and the 327 TRR8 has a removable accessory rail under the barrel where on the M&P R8 it is integral to the barrel."