Not from Browning .50 cal, they do not use an ejector as the cartriage stays within the bolt lips and is ejected downwards by the next round.
I suspect some sort of AT rifle.
I can't find any Churchill numbers that end in 3427...
Going through my lists I can only find 22 Churchill IV ending in 27, 10 of which are five figure and 12 which are six figure.
Could it be T172427?
Were IVs always built as IVs?
I have T32427 as part of T32246 - T32470 on contract T301 with Vauxhall for Churchill I, II, III and IV but I have another listing as T32397 - T32470 on T755 with Gloucester Railway for Churchill I and II.
All of the M2 series eject the cases in the same way, by pushing it through the bolt slots with the next round. Are they Browning .50 cal, 12.7 x 99 or Vickers .50 cal 12.7 x 81 or even Vickers .50 HV, 12.7 x 120?
Actually, I don't remember anything on Salisbury Plain having concrete in. It was/ is primarily a range for artillery, the concrete filling being more used on anti tank weapons ranges.
As to when it started, I have no idea!
I'm afraid not...... The manufacturers issued a serail code but I've never found it anywhere other than the plate, though to be fair, there are a lot of places to look! The Army were only concerned with the T number.