Complex sentence: However the Army became interested in a much more capable weapon instead, and on 9 June, 1938 they issued a development contract calling for a 90 mm caliber gun, which they felt was the largest possible while still allowing for manual feed at high elevations.
Split into simpler sentences: However the Army was interested in developing a new weapon as well, and on 9 June, 1938 they issued a development contract for a much improved weapon. It called for a 90 mm calibre, which they felt was the largest possible while still allowing for manual feed at high elevations.