Yes the Sherman swing arms had a cast-in assymetric bumpstop underneath to limit the travel. I have photos of the upturned suspension units belonging to the "cut and shut" Grizzly featured in the original Tank Overhaul series. If you want to post them for me!
My advice is to run the tracks tight but if you have very pebbly terrain to replace the steel M3 bolt that locks the idler adjustment, with one made of brass. It should fail before the track snaps if the worst happens. Fiddly to drill out and replace but cheap at least.
Best to avoid gravel, sadly the real world does not scale down with our models and nor do the laws of physics.

cheers, Fabrice