I was looking to upgrade the Vortex Venom on my 34.5 MOS with a Holosun. I have a 507/C-GR X2 on my 45.5 MOS with the factory optic plate. Of course I was convinced the 508 is better because it sits lower and doesn't require upgraded iron sights to co-witness. Then I found out the 508's battery isn't user replaceable. I'm really down on built-in batteries and think I'll go with a 507 instead solely because the battery is user replaceable.