Where's the documentary evidence for the rifle against the window?
Is WPC Jeapes the only officer to make the claim?
What time was it seen?
It's frustrating not having measurements for WHF. Surely unless the windows are low she would struggle to make out anything that might resemble the barrel of a rifle at the window?
Sounds like another 'trick of the light' 
Jeapes is the only officer who claims to have seen what "appears to be a rifle barrel" leaning against the window sill in the boxroom window.
She did not make this claim contemporaneously. At the time she did not tell other officers at the White location that she sees what appears to be a rifle leaning against the window sill. A month after the incident after viewing the photo she made this claim in her written statement.
There are several issues to consider:
1) Since she said it appears to have bee the barrel of a rifle at most that means she wasn't sure what it was and just saw a stick like object that could be the barrel of a rifle or something else leaning there.
2) Maybe she didn't think anything of it at the time and only in retrospect did she think she might have seen a rifle barrel. She saw the photo and incorrectly thought that was the window she had been looking at though it wasn't. Maybe this gave her some false memory of seeing a stick like object in the window. This false memory would explain why she didn't tell anyone at the time. Just think about it. The raid team was going inside wouldn't they want to know there might be a rifle leaning against the window so that in that room they could be extra careful of someone potentially trying to grab it? So her failure to tell anyone casts doubt on her claim that she saw something.
The only excuse she could really make for not telling anyone if she actually saw something at the time is that she didn't think it was a rifle barrel but rather thought it was something else.