11.5" 556 AR intended to shoot 100% suppressed. Any downside to developing load with the suppressor on? I figure if this is how I'm shooting it, may as well develop that way. Seems like a no brainer to me but I was searching around for something else and ran into an old thread on a different board with a couple people saying you should NOT develop with suppressor on. fwiw I'm not competing, just having fun trying to find a good load for a new upper.

I always used to do it that way...

Then learned the hard way that sometimes you'll hit a load that isn't quite as stable as expected. In my case it resulted in a nasty baffle strike and the can having to take a trip back to the manufacturer. Now I always test a few for stability prior to putting the can on.

Could have been a freak incident, but it's definitely a lesson I only had to learn once.

I only take my cans off if the bullet is going to be borderline unstable.  I run some very heavy bullets because I cast my own and like to experiment.  I have only ever had one not fully stabilize which was a 260gr bullet for my 357 mag.

I started testing with my 5.56 can on.

I don't venture above max and watch bullet weight to twist rate.

In another caliber, or pushing upper or lower limits or casting...I would be more careful.

I would be a lot more likely to leave a can on with a middle of the road .223 load development than I am with a .300 BLK load using cast bullets where I was not sure how stable they were going to end up.

That being said- if it is a QD can, I would absolutely remove it first.  Too easy not to, serious pain in the neck if you get a baffle strike…