S&W 629-6

Howdy, Mark
Where do you read that they're not as strong?
Al

Some ammo  companies do not recommend it to be used like Buffalo Bore and Garrett.  Once could assume that it is not the same strength as others.
Marc

Just the 629-6 or any 629?

How hot do you need to go?

I loaded some  240 Penn bullets over 25 gr. of 2400 powder.  Set the bullet up , &  light crease, on the 1st grease groove...... just enough to clear the cylinder. 
Don't ever shoot those in a 29-2 from the  80's.      I had to drive the cases out with a dowel rod & small mallet.
ROTATED ...the cylinder , backwards....too. (  dissengaged the  cylinder's  locking block )  Prolly' ... accelerated aging on  my gun...a few years. 

I should start this by saying that while I'm not a dyed in the wool Smith man, I enjoy and admire the N frame, and I have enough experience to shed some light on the topic.
First of all, the N frame-model 29/629 in this instance-is plenty strong to handle any 44 magnum cartridge generating standard industry pressures of 35,000 psi. The dash 6 has the heat treated yoke, and the cylinder stop to prevent reverse rotation, only problem is most of 'em have that infernal round butt grip frame. It is a better and stronger gun than the beloved p & r guns of a bygone era. The problem arises when folks get the strange idea that they want to make a mini 454 Casull, and load her up to above industry pressures. Standard pressures drive 240 grain jacketed bullets to 1300+fps., depending on barrel length, and really, its a mystery to me why anyone would want a faster round than that. 
I can load up a 300 grain cast pill at a bit over 1,000 fps in a 4" bbl, and be perfectly safe. For those who think this isn't rip snorting enough, my question is-have you ever fired this load in desperation? It'll penetrate forever, especially at white of the eyes distances.
Folks tend to get all balled up over numbers, but then,what the Hell do these numbers really mean? 1325fps vs 1450fps, 36,000psi vs 45,000psi? In the real world, it doesn't mean horsefeathers. Follow solid reloading practice, and then get a manual, hell, get a few manuals, and stick to their recommendations. With a little work, which also should be fun, you will find the load your gun really likes.
Good luck and good shootin'!