Battery AMTB 921

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Battery AMTB 921 (1943-1946) - Battery AMTB 921 was a reinforced concrete World War II AMTB rapid fire 90mm gun battery on Fort Varnum, Washington County, Rhode Island. Battery construction started 1 Apr 1943, was completed 2 May 1943 and transferred to the Coast Artillery for use on 22 Jul 1943. Deactivated in 1946.


Part of the Harbor Defense of Narragansett Bay.

The AMTB battery at Boston Neck consisted of two 90mm M1 guns mounted on 90mm M3 fixed gun mounts and two 90mm M1 guns mounted on 90mm M1A1 mobile gun mounts. The battery had a dual mission of defense against fast enemy motor torpedo boats and enemy aircraft. The effective range of the guns was about 8,000 yards. Each weapon required a crew of 15, 9 men in the gun squad and 6 men in the ammunition squad.

AMTB Front View
AMTB Interior View
AMTB Side View
AMTB Back View

The above photos were taken at Fort Monroe, Battery Parrott

From FM 4-91: "This weapon consists of the 90-mm gun M1 and top carriage M1A1 on the 90-mm gun mount M3. The mount M3 is of the turret type, the shielding being constructed of boiler plates. This boiler plate shield provide fragmentation protection only. No shielding is provided at the rear due to the requirements for service of the piece. A sighting port in the shielding in front permits sighting in direction by the gun pointer."


  • FM 4-91 - 90-MM Gun, Fixed Mount Service of the Piece
  • FM 4-126 - Antiaircraft Artillery, Service of the Piece, 90-mm Antiaircraft Gun
  • TM 9-373 - 90-mm Gun M1 and 90-mm Gun Mount T3 (M3)
Electrical power was furnished by two M-7 Mobile generators.

Battery AMTB 921 Armament (edit list)
Model Serial
Manufacturer Carriage Service
1 90mm Rifle 186.15" M1 15339 GM Pedestal Mount, M3, #171, GM 1943-1946
2 90mm Rifle 186.15" M1 15384 GM Pedestal Mount, M3, #155, GM 1943-1946
3 90mm Rifle 186.15" M1 ? Unknown Mobile Mount, M1A1, #?, Unknown 1943-1946
4 90mm Rifle 186.15" M1 ? Unknown Mobile Mount, M1A1, #?, Unknown 1943-1946
Source: RCW Form 1, Aug 1944, Coast Defense Study Group, Berhow, Mark A. ed, American Seacoast Defenses: A Reference Guide, 2nd Edition, CDSG Press, McLean, VA, 2004, ISBN 0-9748167-0-1, pages 80-81, 205
Battery AMTB 921 Plan

Current Status

No period guns or mounts in place. Two concrete mounting pads mark the location of the two fixed guns.

Location: Fort Varnum, Washington County, Rhode Island

Maps & Images

Lat: 41.447302 Long: -71.432226

  • Multi Maps from ACME
  • Maps from Bing
  • Maps from Google
  • Elevation: 17.92'


  • U.S.Army, Supplement to the Harbor Defense Project of Narragansett Bay, (HDNARB-AP-45), 1 Feb 1945, CDSG


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