Col On HF raised the example of the 25mm gun - a good example of how things go wrong, but possibly deserves a bit more detail.
Original 25mm Aden was developed from the 30mm design by RO and was not right - but accepted and paid for by the MoD.
Job was taken off RO and given to a small company who did a great job of fixing it - result (in well under 10 years) was the best 25mm cannon ever built - fast, accurate required no external power. The original 30mm shell was not up to the requirements for ground attack performance.
Problem - MoD had insisted that BAES design the pod and ammo feeds - ALL other gun teams design the gun and the pod. Result - jams and problems with spent links.
Project was cancelled mainly because of the RAF dogma that guns were inaccurate and of no potential use. This was in 1997. Right.
The 25mm failed due to MoD failure to use basic systems project management and lack of funds, but primarily because of user ignorance over what a modern gun could do.
Damn shame, actually.
Best Regards
Engines