Some recruits drop out, fearing how they could be treated following a police shooting,
Concerns over the way armed police officers are often treated in the aftermath of a police shooting are hampering recruitment efforts.
A planned increase in the number of elite counter-terrorist firearms officers has yet to hit its target, partly due to the difficulties in getting enough volunteers.
Police chiefs revealed a 70% increase in the ranks of the most highly trained armed officers over the last two years, but the pool of counter-terrorist specialist firearms officers (CTSFOs) still needs about 100 recruits.
The government made an additional £143m available to increase the overall number of armed officers in the wake of the terrorist attacks in Paris in November 2015.
The latest figures show that Home Office forces which were given funding have seen an increase of 874 armed officers, up from 640 in April 2017.
When other Home Office forces without funding and non-Home Office forces, such as the nuclear and defence police, are included, the increase rises to 1,351.
The number of Armed Response Vehicles (ARVs), driven by regular armed officers, has also risen by about 27%, in line with plans announced in 2016 to increase the number to 150.
This means that in any 24-hour period there are 55 more ARVs on patrol
in England and Wales than there were in April 2016.
Simon Chesterman, the National Police Chiefs' Council lead for armed policing, said one of the challenges in recruitment was that those selected for the elite role were from a pool of regular armed officers, who then had to be replaced to meet the target.
"It's made the ARV uplift even more challenging because we are recruiting from the ranks of ARVs to become CTSFOs and therefore we are constantly having to backfill," he said.
"Delivering the uplift has been a bit like filling a bath with the plug out."
Mr Chesterman added that recruiting rank and file officers to the ranks of firearms policing was often difficult because of the perception many officers have about the treatment of those who have shot suspects.
"We've got enough volunteers coming forward at the moment, but when we speak to those officers, on perhaps why they don't follow through on an application in the first place, quite often the main reason they give is that they're concerned about what will happen to them in the event that they have to discharge a firearm.
"Not just about whether there will be some sort of proceedings against them, but they worry about their livelihoods, their families and the impact it'll have, so some choose a different career.
"They deploy to around 14,000 armed incidents every year. They discharge firearms on about four of those incidents, it's a tiny fraction.
"These are not trigger happy people and they're quite happy to be held to account after the event, but their perception is they'll be treated as suspects of wrongdoing, as opposed to the professional witnesses that they clearly are."