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It would appear that the screws used to mount the base are too long & causing the bolt to be held back.
Try loosening the screws, if this allows the bolt to go into battery you have located the problem.
Try filing the screws to make them shorter. Go slow & check often.
They're right - screws too long. I know, because it happened to me too at one point.
Does your receiver have a casting 'bump' on the top? If so, you might be best to even out the top before mouting the base. Check rimfirecentral.com for tips on how.