Indeed, you are right, the .223 is recoilless... I was more or less just starting the Inquisition.

OK, now the bull-barrelled thing shoots OK.

As you say, I need to test my hunting rifles from sled and from sitting. Note, I had no added weight in the "shelves" or whatever they are called.

As I mentioned earlier, for a 50-meter rifle, I don't think it would matter much. At 200+ yards? Maybe so. We must rezero or fine-tune many of our rifles from sitting after rough zeroing from the bench.

I am going to guess that every rifle is a law unto its own. SOME no doubt will comply with Sled Zeroing, and many others, not. As for stock cracking, I don't see why many guns wouldn't break after many rounds were fired. I'm no engineer, but it seems to me that a buttstock backed up by something that doesn't give at all would tend to splinter, just like if you hold a wooden tentpeg in your hand and beat it with a mallet, it does nothing but strain your wrist. Place the tent peg against a concrete shop floor and whack it and the thing blurs over or cracks.

Xausa, how many rounds have you fired FROM YOUR SLED with your .505? I'm guessing here that the zeroing process calls for fairly limited rounds fired? The rest then fired off your for-"giving" shoulder? Did you fine tune and zero the .505 off your shoulder after getting it "on paper" with your device? Got me curious, now!

If so, this thing could come in awfully helpful for me with my coming shotgun round ball project.

PS: I'm probably over-ANALysing this, but if you look here you see the deck is mounted on a vehicle suspension. There is a bit of "squish" but I doubt it allows a significant difference in vibration from a solid bench. But maybe so?