Was doing some reading. So my question is -- Does the bolt stop it rearward travel via the buffer striking the rear of extension tube. OR does the spring stop via 100 compression going solid?

In theory what is the difference between adding weight to carrier versus a heavy buffer?

Thanks in advance.

Buffer has a rear urethane bumper, and the back of the buffer to the back of the receiver extension is the items that control the total distance of how far the bolt-carrier-buffer moves rearward.  This is the reason that if you use a carbine buffer in a full length a-2 stock, the carrier can move back too far and the carrier key can strike the back of the receiver and crack the receiver. Simple, if the correct spring is used, then it should never fully compress out and bind the travel in any way.

In regards to weight, if you are trying to control bolt bounce, then you want the weight in the buffer.  If you are just trying to slow down the unlocking of the action, then the carrier weight works better than just a standard heaver buffer since the inner weights do move freely.
Note: buffers that have the dead blow effect weights/mass spring loaded/hydraulically forward would be the same as the fixed carrier weights.

Thanks

That's what I thought.

I was reading in another column about people switching buffers lengths without changing extension lengths and that didn't sound right to me.