MP5A3 incorrect offsets for underbarrel

### Testmerges

N/A

### Round ID

N/A

### Description of the bug

It floats, same with the gyro, bipod and BFA.

### What's the difference with what should have happened?

Should be offset more.
Or something.
Things like the Vertical grip and flashlight grip and laser sight look fine.

### How do we reproduce this bug?

1. Spawn a MP5A3
2. Attach a bipod or something
3. Weep
...

### Issue Bingo

- [x] Issue could be reproduced at least once
- [x] Issue happened in a recent (less than 7 days ago) round
- [ ] Couldn't find an existing issue about this (https://github.com/cmss13-devs/cmss13/issues)

Reproduce the issue by spawning an MP5A3 and attaching the bipod, gyro, or BFA, then compare those attachments with the vertical grip, flashlight grip, and laser sight. Trace the MP5A3 attachment-offset definitions from the weapon and attachment implementation. Done means the affected attachments no longer float and the existing correctly positioned attachments remain unchanged.

Written by the indexing model from the issue text.