Gun selection for a n00b

[Steven]

I want a nice accurate rifle, preferably in 6.5 creedmore. My two current options are the following.

American Tact 6.5cr Silver 18
https://large.shootingsportsmedia.com/540864.jpg

OR

6.5 creedmore Ruger prec rifle RPR

Here’s how I will be using it.
-Always suppressed.
-Max 500 yds (sorry if wrong armory forum!)
-I will never be using it for competition.
-No plans on ever using it for hunting. It’s almost exclusively for steel plates and tannerite.

Questions:
Would I be better suited with a short barrel like the 18” posted above if I’m using a heavy long form 1 suppressor?

Will creedmore stay fast enough to ignite tannerite out to 400 yds ?

Any other suggestions? I’m heavily into AR’s so buying a creedmore barrel is also an option. Other caliber suggestions are welcome as well.

Thanks!