What it is
The Ames Colt C400 is a reduced pressure zone assembly in 2½" through 10" on a 304 stainless grooved-end body, with a bottom relief valve between the check modules and a flood sensor on current units. The C500 is the RPDA version with a metered bypass; the C400N and C400Z stand the body vertically on elbows. The Colt shares its housing dimensions with the Watts 957 and the Ames C200 double check.
Length is the shortest of the large Ames RPs: 30¾" at 2½", 33¾" at 4", 43½" at 6". The relief valve hangs 6½" below the pipe at 2½", 7" at 4", 8½" at 6", 9⅞" at 8". Open OS&Y stems set the height, 16⅜" to 45¾" above centerline.
What matters for the cover
Do not size it like a 4000SS. A "4-inch Ames RP" could be a 33¾" Colt or a 40" 4000SS. Both take the 85 × 40 by the price list, but they sit differently and the 4000SS's relief valve is 3½" deeper. Read the tag.
Relief discharge. The relief valve under the body has to drain. Set the cover so the base closes to the pad around the assembly but never seals under the relief port. Ames specifies a 12" drain clearance; the cover's base respects the same gap on that side.
BFG builds are shorter still. With butterfly gear valves the Colt drops to 27¾" at 2½". Still the 70 × 40.
C400N and C400Z. Vertical body on elbows: an N-shape cover install. Pick the N-shape SKU for the pipe size.
C500 bypass. Roughly 4" of added width on one side; the stock cover usually clears it through 4". Photos on 6" and up.
Stocking
The Colt is common on new commercial work in Ames territory because it fits smaller pads and vaults being converted to above-ground installs. Plumbing houses see 2½" through 4"; fire protection distributors see the C500 at 4" and 6". The 70 × 40 and 85 × 40 cover most calls, and the same covers fit the C200 double check and the Watts 957 size for size.