I recommend using the Butler Creek Flip Up scope covers for the Sig Sauer Sierra6BDX 2-12x40mm Rifle Scope. Those scope caps can be found by clicking the link below. Use the sizing chart on the left side of the product page under the product images to determine the best size scope caps to use.
https://www.opticsplanet.com/butler-creek-flip-open-scope-covers.html
The Sig Sauer Sierra6BDX 2-12x40mm Riflescope will work on your crossbow, however I do not believe the Sig Ballistic Calculator when paired with the BDX rangefinder will work with a crossbow. I would recommend the Vortex Viper PST Gen II 2-10x32mm Riflescope for a TenPoint Crossbow.
The SIG SAUER Sierra6BDX 2-12x40mm Rifle Scope will be compatible with a Sig Cross in .308. You would need to purchase 30mm scope rings or a 30mm one-piece mount in order to install it. If you need assistance in selecting scope rings or a mount, email us at Techsales@opticsplanet.com.
Yes, the SIG SAUER Sierra6BDX 2-12x40mm Rifle Scope can be used on an FN SCAR.
The SIG SAUER Sierra6 BDX 2-12x40mm Rifle Scope is 10.8" in length.
The provided link will be the mounts that you will need.
https://www.opticsplanet.com/trybe-optics-advanced-scope-rings.html?_iv_parent-tube-diameter=30-mm&_iv_tube-diameter=30-mm
Yes, you'll want a 30mm scope mount such as this one.
https://www.opticsplanet.com/v/soa20001-sig-sauer-alpha-scope-mount-ring.html