What difference would it make though, other than possibly mistakes? There is a massive difference between arming and using anyhow
Crime figures wouldn't really go down with arming police. it's like harsher sentences which people call for so much, the crime will more than likely still be committed. most crimes are emotive or situational so the sentence doesn't get thought of when the crime is being committed, and the same will happen if police carry guns (which is different to using them)
With more 'hardened' criminals (I don't particularly agree with that terms - maybe determined or experienced is better suited) a custodial is an accepted risk.
How many people are involved in serious transportation and dealing of drugs? The answer is thousands, even with the risk of lengthy periods in custody.
The reason being that the potential rewards exceeds - in their eyes - the risk of time spent in prison. Arming officers will have little to no impact on that, at all.
If anything, it would only drive criminal gangs to further explore the use of firearms, knives et al. For me, there is very little reason for routine carrying of firearms.