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Propelled,122mm, Model 1944... [PDF] The US/UK zones are only half that of the Ger/Rus of the same percents. This is why converting from a 90% UK zone to a 50% German zone the dimensions are divided by 1.22 rather than 2.44. For mean deviation and standard deviations this may apply as well. But, I am not convinced yet. Maybe next revision I'll have more information on this point.
Next I will turn the Ballistic Accuracy Calculator on some of the accuracy data in order to find the dispersion that it is using. 12. WO 291/751 AORG Memo No.427, 24th Nov 1944, "Comparative Dispersion of Tank Guns"
















Probability of a hit when firing for effect on a target 2' high by 5' wide (M.P.I. assumed on centre of target) at 500yds; 800yds; 1000yds; 1500yds: Calc 50% Disp. at 1000 yds Sherman 75mm M3 M61 150rnds 100% 96% 90% 73% 0.26 Sherman 76mm APC M62 40rnds 100% 100% 96% 92% 0.21
13. WO 291/238, The Importance of Gun Dispersion in AP Shooting AORG report number 256. Theoretical chances of a hit on a 2ft high target representing a hulldown tank at 1,000 yards. This also says the standard average range estimation error is 250 yds at 1000. Does that mean std. Deviation error is 25% ?
Second shot corrected: 1000yds Calc 50% Disp. at 1000 yds. 75mm M3 M61 70% 0.41 76mm APCBC 82% 0.329
14. Table "D." 28 Dec. '44 No. Q. 2.908 Range At Which There is a 50 Per Cent. Chance of Obtaining Hits. Hulldown Q.F. 75mm APCBC Hulldown target = 2' x 5' Calc 50% Disp. at 1000 yds First shot 400 yds 17% mean ranging error and dispersion 0.65 yd. Subsequent hits 900 yds. Dispersion only 0.625 yd. (Note. This also includes jump and throw off.) If you thought things were clear on the subject of percentage definition differences between US/UK and Ger/Rus I remembered a magazine article.










15. In Warship International , No. 3, 1991, Evolution of Battleship Gunnery in the US Navy, 1920  1945 by W. J. Jurens The Dispersion Rectangle appears. It seems the US Navy defines the 50% dispersion zone center at mean point of impact different than the Army.
From Russian Archive the US 76mm on the M18 Hellcat had a dispersion "At a range of 1200 meters, average horizontal deviation is 0.16 meters















The Dispersion Rectangle (reproduced from article)










and vertical deviation is 0.1 meters." As this is Soviet radius multiply by 2 to find the 50% zone i.e diameter Giving .32m horizontal by .2m vertical. Or at 1000m 0.267m x 0.167m
It may seem the dispersion of the US 75mm and 76mm guns is much better than that of other nations. And that may be true of pure dispersion from the center point of a collection of hits at 1000 yds. However, other factors will cause this center to be off center the target. In particular the M4 linkage on the periscope sight has slack such that after a some firing it produces an additional dispersion of up to 4 mils in both planes. Source: War Department Report of the New Weapons board, April 1944 [PDF] page 58 f.
Aiming differences of HVAP for vehicles based on jump, muzzle brake and other factors of 76mm and 3" guns. link













